What Voltage Should My Battery Bank Be? 12, 24, or 48.

August 14, 2015 · 39 comments

Would you do us a huge favor by sharing?

which battery voltage is bestThe most common question I get as a solar consultant is…

What voltage should I use for my battery bank? And Why?

Almost everyone who is new to solar, wind or micro-hydro assumes that 12 volts is the way to go.

It seems obvious. You can buy 12 volt lighting, 12 volt coffee makers, 12 volt fans, and many other 12 volt appliances. You can even buy batteries that are 12 volt. So it would seem that 12 volts is the magic voltage!

NOTHING COULD BE FURTHER FROM THE TRUTH!

 

In the early days the choice was easy, you could only buy 12 volt inverters and 12 volt charge controllers, however 12 volts is likely not the best choice for a renewable energy system today.

The most common battery voltage is now 24 volts and soon 48 volt systems will be the most popular. What are the best choices for your first battery bank?

Homesteaders building renewable energy systems should avoid 12 volt battery banks with the following exceptions:

1. RVs/motor homes
2. Travel trailers
3. Very small systems as for a small cabin or tiny home

rv solarRVs and motor homes already have 12 volt starting batteries and house batteries as well as 12 volt lighting, hot water heater controls, air heating controls and refrigerators. It makes no sense to retrofit systems that are already working fine. The same goes for travel trailers.

ms2012Just limit your inverter to 2000 watts such as Magnum Energy’s MS2012 and you will have a rugged and reliable inverter/battery charger. The MS2012 is a low frequency (big, rugged and heavy) pure sine wave inverter with a massive power factor corrected (PFC) battery charger that works well with fuel powered generators. How to charge your batteries with a generator. The MS2012 has a peak output of 2800 watts for 5 minutes. Divide 2800 watts by 12 volts and the surge amperage is 233.

Use a DC 250 amp circuit breaker for your disconnect and a 300 amp class T fuse with holder for your catastrophe fuse. Add a set of 4/0 cables and you have a setup that will run a microwave, television, radios, toaster, hot plate or even a small air conditioner. Magnum does make an MS2812 that will also work nicely but unless you watch your loads carefully you will need a larger breaker, fuse and cables. Sometimes that extra 800 watts will start even a good sized air conditioner and the A/C unit only needs a lot of current when it starts up. Once it is running, it is not that big of a load (maybe 200-400 watts).

A cabin of this size might get by with a 12 volt battery system.

There are also times when I will recommend 12 volts for a tiny renewable energy system for a cabin, cottage or tiny home. If my fellow off gridder only needs a 2000 watt or smaller inverter, than 12 volts might be the way to go.

The problem with designing a 12 volt system is the inability to grow in the future. But still there are the weekend warriors that only want to operate a small fridge and a few lights or they have a propane fridge and propane lighting. 12 volts might work fine for them.

It is rare that I recommend 12 volts but it does happen or sometimes the homesteader has already bought their equipment by the time we get to talk.


As solar (wind and small hydro) electric systems get larger, the currents we have to deal with are getting huge!

(Remember that VOLTS X AMPS = WATTS) or (WATTS/VOLTS = AMPS)

For example:

4000 watt 12 volt inverterLet’s pretend we have just purchased a new 4000 watt pure sine wave 12 volt inverter.

If it is a decent quality inverter, it will likely be able to surge (for 5 to 30 minutes) at about 6000 watts. When we divide 6000 watts by 12 volts we get a maximum surge current of 500 amps. This is a huge amount of current.

Imagine how large your inverter cable would need to be to handle 500 amps?

Where are you going to find a DC circuit breaker or DC fuse that can handle up to 500 amps?

When you go shopping for those products you will soon realize they don’t exist or are very rare.

A large DC breaker for the solar industry is 250 amps. I have seen two 250 amp DC breakers paralleled together to make a 500 amp breaker. But even if you are lucky enough to find one (and pay up to $800 for it), you still need to get the current from the battery bank to the inverter using copper battery cables.

In this example you would need at least two 4/0 cables in parallel for the positive and two 4/0 cables in parallel for the negative. The cables could add up to another $800 in unplanned expenses.

It just isn’t practical or affordable.

ms4024

Magnum Energy’s high quality MS4024 inverter/battery charger.

If you were to build that same system using 24 volts, the maximum (surge) current would be cut in half or 250 amps. DC 250 amp breakers ($90-$200) are readily available as well as 300 amp DC class T fuses ($70-$120).

Now we can use one 4/0 cable ($80-$200) for the positive and one 4/0 battery cable ($80-$200) for the negative which is a very common size and also readily available.

Now let’s use 48 volts with the same 4000 watt inverter that can surge up to 6000 watts. The maximum surge current is now only a mere 125 amps or 6000 watts divided by 48 volts.

As a general rule you would use a 175 amp DC breaker ($90-$200) as a DC disconnect for this inverter with a 200 amp class T fuse ($60-$90) and 2/0 inverter cables ($40-$100 each) which contain 1/4 the copper that 4/0 cables are made of.

fm60

Outback’s FM60

Just the above mentioned reasons should be enough to make you carefully consider using 24 or 48 volts for your next renewable energy battery bank but there is more.

Let’s look at the specs of one the most popular charge controllers on the planet, Outback Power’s FM60.

It is a 60 amp MPPT (maximum power point tracking) charge controller and the 60 amp rating refers to the battery voltage.

The higer the voltage of your battery bank, the more power output the FM60 can handle.

At 12 volts the Flexmax 60 (FM60 | $400-$600) can only handle up to 750 watts, 24 volts – 1500 watts and at 48 volts -3000 watts of solar modules. You may have to buy multiple charge controllers at 12 volts or even 24 depending on the size of your solar array.

All charge controllers are the same, the amperage rating is based on the battery voltage; the higher the voltage the more power the controller can handle.

You may have noticed in Outback’s specs they even have a wattage limit (3750 watts) for folks with a 60 volt battery bank. Although I have not seen a 60 volt battery bank, it is likely to become common in the future for the same reasons 48 volts is so popular now.

 There is a good reason to choose 24 volts instead of 48 (or 12 volts instead of 24) in certain situations.

If you only have one string of batteries it might make sense to lower the battery voltage so you can have two strings.

In the early planning stages, you make the decision to build your battery bank from eight 6 volt batteries such as Trojan’s L16s. If you wire them all in series you now have a 48 volt battery bank.

Sounds great but what if you have one battery fail? How to find the defective battery in your bank. Since your inverter and charge controller need 48 volts, your system is now down until you get a new battery.

1 string

A better scenario would be to wire your 8 batteries into two groups of 4 and parallel the strings to make a 24 volt system. Now if you have a battery failure you can operate your system on 4 batteries while you wait for a replacement.

Having trouble with a battery or two? Check these guys out…

728x90-3

2 strings

We always recommend two strings no matter what voltage you decide for your battery bank. Then you will always have one string to “limp” with until you can get a replacement. Learn how to make your batteries last as long as possible.

Deciding whether to design and build  a 12, 24 or 48 volt battery bank is one of the bigger (and first) decisions you will make when deciding to live off the grid.

If you have any questions or something to add on this topic please comment below or contact us.

Leave a Comment

{ 39 comments… read them below or add one }

John D Fouts January 29, 2017 at 2:27 am

Off the grid 90% since 2009, all but my 22ov water well. And thats in the process now of being done.
Gonna try to be helpful, not mean.
You and the who argument industry are very wrong about 12 volt systems. The never ending mis information about “wire size savings” and you must use fine stranded welding cable has permeated the solar world like a poison.
I use 12vdc, have none of these issues you describe and my battery bank is 24 – L16 batteries.
I use 2 cheap 3000 watt HEC PEAK 3000 watt SW inverters and 3 Mean Well 3000 watt pure sine inverters and I have 7800 watts of solar and a few small wind turbines of my own build.
Not once did I regret it.
Others i know who live off grid would have one battery failure in a 24 or 48 volt system and all is lost. Whole system crippled. Others I know would have a 24+ volt inverter fail and have to wait weeks to get a new one. I can get a 12vdc one at walmart or a truck stop if I absolutely had to.
I ordered my cabling, 6awg wire for multi panel groups, and 3/0 for inverters from Wireandcable.com for under half the cost of that “solar required wire” and made my interconnects 6v(12vdc) with typle k 3/4″ copper pipe which is capable of over 900 amps and did it for $1.20 a battery. I am also using TRACER MPPT controllers I bought in bulk on Alibaba.com for 1/3 the price of Magnum or outback and in 7 years, not one loss but IF I DID loose a controller, its one 40 amp reduction at $140, not 80 or 100 amp charging loss at $600 a pop.
I used 30 year metal studs to build my array base. What solar installers told me I needed was to pay them $42,000. What I did was build a 300 watt larger system for under $10,000 and in 7 years now, NOT ONE ISSUE running a 102×52′ shop and a 3 bedroom house.
For one thing, Welding cable is the ONLY way to go. LIE, Welding cable is ONLY Fine stranded for flexibility, not superior ampacity. This absolute installer/seller lie that volts travel on the “outside of the strands of wire” is a promotes load of BS from sellers of expensive Wire. Its copper purity, not strands that increase electron flow.
Second, 12 volt is less dependable – LIE, the 12volt industry has over 78% of the market and innovations.
Third, voltage reduction to use 12 volt lighting is not expensive. LIE, its very expensive, converters are always using current and has severe limitations. Many will catch fire or have a Mosfet failure in the case of high input volts from a controller.
On all the other info your have, especially batteries I love it. Keep it up.
I can send you some nice drafts of designs If you had a pic upload option.

Reply

Jody Graham January 31, 2017 at 8:58 pm

Hi John,

Wow. Thank you for your comments and ideas. I don’t feel like you are being mean at all.

First…I would say that “we in the industry who are wrong about 12 volt battery banks” are not wrong. I think they are fine for small RV, street/sign lighting and cottages they are fine as the amperage is usually manageable in smaller systems like that.

Let’s look at my system for example. I have two Outback VFX3648 inverters operating 120/240 split phase sine wave 60hz similar to what the utilities provide in North America. They can each surge at 6000 watts. If I were to operate each inverter (with separate cables for each) at 12 volts I would need a fuse for each inverter at the battery bank rated at 500 amps (6000 watts divided by 12 volts equals 500 amps). I would also need a DC circuit breaker for each inverter rated at 500 amps (if you could buy one or even afford one for each inverter). The cable would have to be 9.5/0 (750KCM) and over .999″ in diameter if using stranded wire. That is for each inverter. I have two.

If we connected one positive and one negative from a 12 volt battery bank to our DC breaker panel large enough to handle both inverters (12,000 watts) we would need cable large enough to handle 1000 amps (with a 1000 amp Class T fuse at the battery bank) and then split 500 amps (with two 500 amp breakers) to each inverter. All super expensive and almost impossible to find.

At 48 volts, each inverter surges at 125 amps (6000 watts divided by 48 volts equals 125 amps). Double the inverters as in our system and the maximum current will be 250 amps which can easily be handled by 4/0 cable going to the DC breaker panel with a 250 or 300 amp Class T fuse, followed by two sets of 2/0 cable (one set per inverter) and 175 amp DC circuit breakers for each inverter. All of these products are easily available and inexpensive.

The main reason we use fine stranded cabling is for the flexibility. Try to bend a 4/0 solid or even large stranded wire. Not going to happen. Welding cable is NOT UL or CSA approved so it is out of the question for an installer as myself. There is really nothing wrong with it but without the certification electricians, electrical inspectors and insurance companies won’t accept it.

You have a good point about being able to find a 12 volt inverter in an emergency at Walmart but I doubt it would be anything but modified sine wave.

Using type “K” copper pipe (the thicker copper pipe for those who don’t know) for your cable terminals (ends) is another idea that is doable (I hope you crimp and solder them) but NOT allowed under any electrical code anywhere and dangerous as far as most installers are concerned. At a company such as http://www.lugsdirect.com I think you can purchase approved terminals for about $1.50-$2.00 each for 2/0-4/0 cable ends but I would have to check on that. They are a fantastic company and we have used them for years.

I am not familiar with solar experts telling their customers they MUST use fine stranded cable because the current travels on the outside diameter of each strand. That is a new one for me? And foolish! Like I said previously fine stranded wire is used mainly for its flexibility and a little because their is more copper in fine stranded wire than large stranded wire. Fine stranded has less air space between strands (allowing more current) than large stranded plus large stranded is almost impossible to route (bend) large stranded wire like 1/0, 2/0, 3/0 or 4/0. I have tried.

I have no evidence or opinion that 12 volt inverters are less reliable/dependable as long as you are using low frequency inverters like Magnum Energy, Outback Power, Xantrex/Schneider, Morningstar and similar products. They have large transformers, lots of copper and are heavy. They can surge up to two or three times their rated wattage. High frequency inverters on the other hand are light, cheap and don’t have the surge capacity or reliability.

I agree…I am 100% against 48-12 and 24-12 volt DC-DC converters. They are horribly inefficient, don’t last and cannot handle any surge (even a small 10-20 watt Ivan Labs circulating pump.

DC-DC converters inverter the 24 or 48 volt DC electricity to AC, reduce the voltage to 12 volts AC and then rectify it back to DC. Hence the inefficiency. They are also high frequency to be cheap making them unreliable.

You have 5 X 3000 watt inverters. Each one would require 3/0-4/0 cable, 300 amp Class T fuses at the battery and 250 amp DC breakers.

I would love to see your setup. Email photos to jody@solarhomestead.com if you get a chance.

I agree with you about battery failure which is not that uncommon. If you have a failure (on your 12 volt DC battery bank) you only have to remove 2 out of the 24 you have bringing you down to 22 barely making a difference. I NEVER recommend my customers build a system with one string of batteries. One failure and you are shut down. We now have 3 strings of 8 x Trojan L16s. If I have a battery failure I have to remove the remove the bad battery and cut my storage to 2/3rds until I can get another battery but I am still operational.

AGAIN, PLEASE DO NOT HAVE ONLY ONE STING OF BATTERIES FOR ANYONE READING THIS. YOU WILL REGRET IT WHEN YOU HAVE YOUR FIRST BATTERY FAILURE.

Thanks Again John,

Please email some photos and feel free to rebut anything I have said by replying to this comment.

Take care…Jody

Reply

John D Fouts January 31, 2017 at 10:00 pm

Thanks for the Reply, Lots of homework in there.
We did the right thing initially in our design by creating a full DC loop in our home according to no opposing code in Idaho. Most states have no DC loop code restrictions. DC LOOP is a Marine Design of running a pair of large diameter ( 2 awg in our case) wires in a continuous loop from and back to battery bank in our attic and dropping UL approved DC lighting wiring into the walls to switches and lighting with numerous 12 volt 30 amp outlets. Some 50 amp. All with fuses.
We use a marine style over amp disconnect which is unheard of in the RE world. Its basically a mouse trap! The sprung loop heats, retracts and pops open. Ours are Soviet made, also are made in Finland and can handle from 50 to 500 amps depending on the snap wire gauge. They are whats used in high power audio amplifiers for concerts, not cars! They look like old mouse traps. Dependability is 100%, they are used in submarines also from 25+ years and back. Cost, about $20 to $50 each.
Secondly none of our inverters use a single fuse, I run 3- 100 amp fuses on a 3000 watt. Each with 2 awg wire and 3 wires in to the fuse from my massive buss, then with 3 lugs to the inverter. Most important factor is NEVER put this material in your home, on your home or next to your home. Our main solar/wind power plant is in the shop near the house.
The Mean Well inverters are absolutely dollar for dollar the best I have tried. After burning up a few $2500 inverters in under 1 year I swapped to Mean Well and have no losses anymore. I replaced one Magnum with 2 mean wells and pocketed $900.
But, the main reason our system works so well is that we have NO large amp appliances. No electric dryer, we use propane. No electric Air conditioner, I have 280 feet of 6″ aluminum irrigation pipe 6 feet in the ground and a air to air unit I built myself which gives up 50 degree air in 100 degree summer heat. Had to put it on a thermostat to keep my wife from wrapping up in blankets in august. Its ran with a 22″ radiator fan pulling 5.90 amps and 6 – 100 mm fans pulling 2.8 amps for the exchangers drafting tubes.
No electric hot water heater either. We heat a 2048 sq foot house with a wood stove as needed and 90% of the winter just a Empire through the wall 35,000 BTU propane heater.
I live in a 105 year old house, I resided it and in the process I ripped off all the outside lap siding, pulled out 18 huge bags of crumpled news paper, insulated with R19, added 2×3″ galvanized foundation ledge, 2×2’s and 2″ closed cell foam to the already R19 2×6 construction then had it sheather with 1/2″ plywood. I installed 2 less windows and used 3x pane windows with Nitrogen/argon combos. I then called in the metal siding company. My roof got the same treatment, they indexed my home in total as average of +R29 radiant loss ratio when they did the siding install. A upgrade from R10 average. Well worth the $7500 it cost me.
The problem as I see it in most cases is peoples lack of common sense. 9 out of 10 people I see on blogs are beating their head over how to run a 300 amp AC load on a solar panel. How to spend $50,ooo to get rid of a $300 a month light bill. Seems NONE think structure first, Amps second!

Reply

John D Fouts January 31, 2017 at 10:24 pm

Jody,
For your readers, please suggest they put up a “POWER HOUSE” like a metal building and that they do not install directly onto their home. a Arrow 10×12 building is about $400 at a Lowes or Home depot.
If they have a shingled roof, skip ALL the solar panel mounting hardware and use 26 gauge Delta style commercial grade metal roofing panels laterally on the rood then with simple clips you can screw to the raised ribs of the roof panel. I am the absolutely only person ever to have done this design, and its insane how it cuts mounting cost by 70% and reduces fire risk. It also aids in cooler panel operation.
Encase your wires in 2.5″ metal framing track, its UL grade grounding listed, no restrictions to its use and its 20% the price of conduit.
I have broken all the rules to cost.
Eventually I will ad links to videos if you can offer that option on your blogs.

Reply

Jody Graham January 31, 2017 at 11:37 pm

Hey John,

I have broken the rules myself to get the job done. What I do in my own home is different from what I do for others and recommend on this website due to the legalistic system our governments have created. It is against the building code in Canada to not connect to the grid if the grid is within reach. Against the law!!! Just a small example of the stupidity created by bureaucrats who have never gotten their hands dirty. Where are you located?

Thank you for your info and I would love to see pictures of your system, building and mounting system….Jody

Reply

Phil December 17, 2016 at 6:50 pm

I’m building my first Solar power system for my garage which is to remote to practically run power to. I plan to run 110 volt hobby grade wood working tools and a 220 volt 15amp air compressor. I’m think about getting a 6000 watt pure sine inverter and am trying to decide what voltage would be best. This is a weekend warrior man cave and is not going to be a heavily used or a critical use system.

Any and all advice is welcome and I won’t be upset if you think I’m off base and have a better suggestion. I really want this to work first time without going broke.

Thanks in advance for any time you spend assisting me.

Reply

Jody Graham January 31, 2017 at 9:18 pm

Hi Phil,

I am so sorry for the delay and I thank you for your comment. You will definitely need a low frequency inverter with a high surge capacity like a Magnum Energy or an Outback Power inverter. The cheap high frequency inverters will never handle the surge of the compressor. I would definitely go with a couple of 48 volt Outback inverters wired 120/240 split phase or their Radian model. Or you could use one or two Magnum Energy 120/240 inverters. Outback is very pretty. Magnum is not so pretty but just as robust and quite a bit cheaper. 48 volt battery bank is the answer unless you can only afford a bank of 8 batteries. If money is tighter go with 24 volt models (8 batteries in two strings) preferably using two strings so if a battery goes bad you will still be able to limp on half your battery bank until you get a replacement battery.

Again sorry for the delay. I had a close friend take his life after a long struggle with opiate addiction around the same time time you wrote this comment.

Take care and please don’t hesitate to ask more questions as you design your system…Jody

Reply

Phil St John January 31, 2017 at 10:37 pm

Thanks for the reply and the information.

I’m very sorry for the loss of your friend.

Reply

Michael November 20, 2016 at 12:45 pm

Hi Jody. I am off grid 100%solar. I’m currently using two separate sets of solar panels with. Flexmax 80 hooked to each set and then both controllers wired to charge a common double bank 48v each. Each bank is made up of the Older Exide IIP 2v batteries in racks. They are getting older so I’m thinking of setting up another bank and dedicating one charge controller to the new batteries. I’m considering lithium iron phosphate LifeP04 3.25v 300amps. If I took 48 of those and split them into 3 paralled banks of 16 ea in series is there a way to connect both this new battery bank and the other to the system so I wouldn’t have to manually switch over to other if one gets low. Also what is your opinion of lithium verses agm? Ty

Reply

Dale Shomette October 31, 2016 at 11:50 pm

Thank you Jody,
Since you have (literally) been around the barn with these considerations for a number of years, do you have a particular MPPT controller you might recommend?
As an aside, I read with interest your struggles and successes over the years, both professional and personal and I hope your endeavors continue to peak your interests, challenge your skills, bring happiness to you and your family and provide you with the feelings of accomplishment that we all seek!
I’ll have to tell you my venture into the diesel-electric hybrid pleasure cruiser business. I had someone else develop the system for me, for a 38 foot retro designed pleasure cruiser that incorporated and utilized all of what you’ve done but for marine propulsion. We had successful sea trials on the craft that utilized two 48 volt DC, 10,500 kw, diesel marine generators, three battery banks of 8 Enersys 12 volt batteries, the Magnum MS448 Inverter/Charger I’m using now, four 48 volt electric pod motors mounted under the hull provide the propulsion. The three battery banks provided the power for propulsion alternately and as each bank dropped down to 60%, one of the generators would switch on and begin charging the rapid-charging Enersy batteries. As that occurred, the next bank switched on and went to the power train. When that bank was depleted t0 60%, generator # 2 jumped in to charge it. We did sea trials that took us 26 miles into the Chesapeake Bay at 6.5 knots on an 8 ton 38 foot boat for 2 hours using only Bank #1. The #1 generator charged it to full capacity very quickly and used less than 2 liters of diesel fuel! A boat this size would use in the range of 8 to 10 gallons.
After our dog n’ pony show with investors (who were mostly not boaters) , our security and exchange lawyer, our Raytheon guy and Composite Yacht Corp, we failed to get the funding to build the marketable craft……SO,
I still have all this stuff and decided to educate MYSELF and power up the shop with solar and the 9 enersys batteries left I haven’t sold. [I still have two Fischer Panda generators (as described) that were $18,000 each and have only two hours on each (from the sea trials). If you know anyone who wants one I’ll sell them for 9 K each and a $500 contribution to Solarhomestead.com]
Sorry, I get carried away with this stuff. Thanks for your help
Dale

Reply

Dale Shomette October 29, 2016 at 11:27 pm

I have 8 Enersys Odyssey 12 volt batteries of 185 amps and a Magnum MS 4448AE Sine wave inverter/ charger.
I personally fabricated two 18 volt solar panels of 3.5 amp each and purchasing two 17.4 volt 5.75 amp Windy Nation panels and a 30 amp charger controller came with one of the panels.
I would like to configure two 48 volt battery banks to alternately feed the inverter. The maximium DC input to the inverter is 68 volts and the inverter’s continuous output is 4400 VA. Not quite sure how to do that or whether I should do a single bank of 48 volts/370 amps? Suggestions?
Secondly, my problem is configuring the solar panels (even if I don’t use the ones I built but buy two more from Windy Nation) to generate enough voltage/amperage combination to charge (alternately/ or singly) each 48 volt bank. Even though I might generate 17.4 volts and 23 amps from 4 panels and run it through the charger controller, how do I configure the wiring to equalize the charge per battery so the lead battery doesn’t get all the charge and the tail end battery gets little?

Reply

Jody Graham October 31, 2016 at 10:33 am

Hi Dale,

Thanks for your questions.

I would definitely make two 48 volt battery banks and parallel them. You could always put a BLUE SEAS DC disconnect switch between them on the positive side that would give you the option of using bank 1, bank 2, both or none. However I would I’d recommend always using both at the same time as you will get more performance and the batteries will last longer.

Next I am assuming the charge controller is 12 volt and will not work for you.

To charge a 48 volt battery bank you will need at least 60-65 volts mpv (mppv) solar modules usually done by wiring solar modules in series.

That would require 4 of the Windy Nation modules you are looking at. Or you could source 2 24VDC nominal voltage modules instead of using 4 12VDC nominal voltage modules.

Then you would use an MPPT (maximum power point tracking) charge controller that you can program if, when and how often the batteries are equalized. That is part of the charge controller’s job.

Please ask if you have anymore questions or I was unclear with this one. Cheers…Jody

Reply

Dale Shomette November 10, 2016 at 11:13 am

Hi Jody,
Sorry I’ve been tardy in reply but we’ve been away for a couple of weeks. What MPPT controller would you recommend to charge the two parallel 48 volt banks with? If you’re running 60-65 volts through it, how would you distribute the current to get an equal charge into each of the batteries? If you run the charge from the controller through one end of each bank, doesn’t that give you uneven charging of the batteries further away (even assuming you charge the two banks separately)?
Also, if you’ve got all four solar modules in series, you get all the voltage mentioned but only produce about 4.5-5 amps/hour, correct? For such battery banks, charging would be very slow. Would not a combination of 2 modules in series with 3 more in parallel give you aprox. 54 volts and 12 to 18 amps/hr when combined? Advantage or disadvantage?
Even though I’m aware of the basics, there’s still some basics that escape me! I know you passed through this stage years ago yourself and appreciate your thoughts. Thanks for your help.

Dale Shomette

Reply

Jody Graham November 15, 2016 at 5:04 am

Hi Dale,

No problem. Sometimes it takes me a month to reply to questions and comments. There are no MPPT charge controllers that will charge two separate battery banks at 48 volts. That is why I highly recommend paralleling the banks together and keeping them together always. You could have a disconnect switch between them if you HAD to shut one bank down due to a defective battery.

When making connections to the battery bank you NEED to connect the positive to one corner of the bank and the negative to the other corner. That will ensure the power is evenly distributed and removed. I can make a drawing if you want.

A 100 watt module will produce 100 watts (really 75 or so in reality) regardless of the voltage, whether they are in parallel or series.

Let’s use 4 X 100 watt solar panels and assume they produce 400 watts.

At 12 volts (really about 14.4) that is 27.8 amps (14.4 volts X 27.8 amps = 400 watts).
At 24 volts (really about 28.8) that is 13.9 amps (28.8 volts X 13.9 amps = 400 watts).
At 48 volts (really about 57.6) that is 6.95 amps (57.6 volts X 6.95 amps = 400 watts).

400 watts of solar produces the same amount no matter the voltage. The current may be higher or lower and the voltage may be higher or lower but the output is the same.

54 volts, as in your example, will NOT be enough voltage to charge a 48 volt battery bank.

If you purchase four 17.4 volt 5.75 amp Windy Nation panels and put them in series you would have 69.6 volts roughly going into your MPPT charge controller at 5.75 amps (69.6 x 5.75 amps = 400 watts). The MPPT controller will reduce that voltage to the battery voltage (57.6 roughly) BUT increase the amperage (6.95 roughly) to keep the output at 400 watts.

Hope that clears things up for you…

Jody

Reply

Dale Shomette November 16, 2016 at 11:42 am

Thanks Jody,
I would still like to know which MPPT you would recommend for the application you described. In the process of developing some experience, I bought a cheap 30 amp controller and installed it to my two, in series, modules that I constructed with cells from Amazon. They put out a maximum of about 18 volts (in series) and 7 amps. My controller worked for a while, charging two batteries with a load of two 50 watt/ 12 volt light bulbs. One day, the controller just stopped. My L.E.D. volt meter (a separate instrument) continued to indicate the same voltage coming in from the modules. I purchased a Windy Nation package of a single 100 watt module and controller, with the controller being a little more sophisticated than the the first but still 30 amp and hooked it up to my two existing modules (not the Windy Nation module yet) and it, too, worked well for about a week. The controller than did what the other one had done, with my L.E.D. monitor still saying I’m getting current. Both controllers were wired correctly (triple checked THAT each time). Took off the load, then the batbattery connections and screen is still black, again. What do you think may be the problem? I know for 4 modules, I’m going to need a better MPPT controller but can’t figure out what happened to these.
Thanks!
Dale

George October 29, 2016 at 1:16 pm

Hi,

I have ebike batteries with a lot of watt hours, but they are basically 48 volts. I’ve been experimenting with a 48 volt to 12 volt DC converter. It seems to work well enough. The wattage is limited to 10 amps, but that’s about what my furnace, the fridge, and some lights draw. The device was cheap and I’m sure there might be more refined or higher wattage versions out there. I am treating it as an emergency system, right now.

Anyway, would using DC converters make 48 volt batteries practical? I realize that 48v lithium solar regulators are fairly expensive, but they exist. The 48v inverters are fairly expensive, but at least 120v is 120v (AC) no matter how you create it.

Thanks.

Reply

Jody Graham October 30, 2016 at 8:55 am

Hi George,

Using 48 V batteries is usually a good idea in an off grid system. However I am assuming you have an RV by the fact you have a 12VDC furnace, fridge and lighting. My experience with DC-DC converters has not been great as they essentially change the DC to AC then change the voltage and then convert back to DC. Reliability and efficiency and have been the main issues. But if you can operate your 12VDC loads with only 10 amps it is viable. You could always keep a spare.

We use a 48VDC inverter system for our home and always recommend 24 or 48 volts for larger and even some smaller systems. It is true you will be limited to larger inverters unless you use something like an Exeltech or similar. We have both a 250 watt and 1100 watt 48VDC Exeltech inverters here as spares in case of catastrophic damage to our main Outback Power System.

I think what you are doing is fine. You can always use a 120VAC to 12VDC converter/charger for your 12 volts loads if you have to. You will find it won’t be much less efficient than your 48-12 converter depending on the model you have. I have seen 48-12 converters with efficiency ratings of 45-85% depending on the model.

Keep up the good work! Jody

Reply

David October 26, 2016 at 1:53 pm

I am looking into a RV Camper which has everything set up at 12 volts. I am also looking at a possible 1000w wind generator that is 48 volts. I think I might get it with your diagram but not totally sure. Would I want to have 4 12 volt batteries hooked up two sets of 2 batteries to equal the 48 volts that the wind generator would put out? Then would need a charge Controller going from generator to batteries? I plan to have Almost everything running DC with exception for preinstalled appliances such as AC/Heater unit or possibly Refrigerator.

Reply

Jody Graham October 28, 2016 at 10:40 am

Hi David,

Thanks for your question. I would definitely avoid a 48V wind turbine if everything on your RV is 12V although I realize 80 amps at 12V could be unmanageable due to long wire runs. The only option if you use the 48V turbine is to use Midnite Solar’s Classic MPPT controller with built in diversion load. That way you could operate a 48V turbine into the controller with a 12 volt output. No other controller is made to go between a turbine and battery bank unless it is built for the specific turbine like a Bergey or Jacobs.

The other option is a 12V turbine connected directly to the battery bank with breaker. Then add a dump controller like a Tristar TS60 or a Xantrex/Schneider C60 connected to a suitable dump load. These dump controllers monitor battery voltage and start to dump power into a water/air heater as the battery bank completes its absorption charge.

Hope that helps. If you need a drawing or I didn’t answer your question please let me know. Jody

Reply

David October 28, 2016 at 11:48 am

Thanks for your help, I will probably go with the 12volt turbine then. but if you do not mind (as I am new to alternative engery) if you could give me a wiring diagram and also how many batteries do you think I would need?

Reply

Jody Graham October 29, 2016 at 11:26 am

Hi David,

What items will you be operating on the battery bank? Please list items like this:

2 led bulbs 7 watts for 4 hours/ day
1 laptop 80 watts for 6 hours / day
etc…

Where are you located?
Is this a weekend RV or used every day?
How many days of storage (no wind,no sun) would you need?

With this info I could give you an idea of the size of battery bank you might need.

I’ll see what I can do for a wind turbine/dump load drawing. Jody

Reply

Trev September 15, 2016 at 3:55 pm

Hey I have a Fm60 and a string of Rolls 1380 to make 12v and I would like to oversize my solar array and run two strings of two 245w Trina solar panels…. Will this arrangement blow up my Fm60… I would like to try and capitalize power production in the lower light conditions…. I all ready have all components…. Just not installed yet
Wouldn’t it just max out at 60a and not let anymore power through or would it smoke
Thanks

Reply

Jerud Crandall August 12, 2016 at 3:32 am

Another reason 12V battery banks are a poor choice for any “serious” sized system is the voltage depression that comes with large loads. Our batteries are 12V and we have the MS2812 inverter you mention from Magnum (an excellent piece of equipment in spite of what follows). The bank is 928Ah. So while we “can” pull up to 2800W at a time, that is 367A…about a “C/3” load. Peukert’s law means that our batteries go away very fast at this rate, and the voltage is pulled way down. Even at lower power consumption levels (like 1500W — water or space heater), if the bank is lower than about 70% SOC the voltage is depressed too far and the inverter cuts out (user-defined @ 11.0V to protect the batteries). That effectively halves our usable capacity.

I wish i had saved some $ and stuck with the MS2012, or sized the whole system up to 24V batteries. A 24V inverter would not be as crippled as the 12V one is by these effects, since it would pull half the current. Unfortunately your post was about a year after i bought that equipment so i wasn’t warned! : )

Reply

Jody Graham August 12, 2016 at 1:04 pm

Hi Jerud,

Thank you for commenting. You are correct that the voltage drop (due to high amperage) is a major problem in 12 volt systems. At full output of 2800 watts that is 367 amps like you said. That is a huge amount of current requiring two runs of 4/0 cable (OOOO cable) to keep the voltage drop to a minimum. That amount of cable is very expensive. You are correct you could reduce the current to half of 367 amps by changing your system to 24 volts or by 4 by switching to 48 volts.

You could likely sell your MS2812 and upgrade to a MS4024 or MS4048 for not that much more money. The inverters cost the same. You would just have the difference in what you can sell your MS2812 and the cost of the new inverter.

Sorry you are having the troubles. I hope you can upgrade to 24 or 48 volts and most of your problems will disappear…Jody

Reply

FastStone Image Viewer August 11, 2016 at 2:33 am

I have a mobility scooter with 3 batteries wired in series to give . I want to go lithium, but have been told that the battery management system won t allow a charger to charge a bank in series, and that I would have to charge each battery independently with a charger. Does that sound right to you? Thanks for all the info in this article, by the way. Very useful indeed.

Reply

Subway Surfers August 4, 2016 at 12:07 pm

I have an electric bike that has 3 12 volt 7 amp batteries and range is an issue. I want to buy 3 12 volt 12 amp batteries for a total of 6 in all, hook them up parallel to keep my 36 volt current. my goal is to extend the range of the motor and bike. Can I use to the two sets of battery s together or should I mix them or what? Also how would it impact my amp rating?

Reply

Recuva July 28, 2016 at 2:58 pm

Currently have two identical LiFE batteries wired in series to power a small transmitter. On the same rig are servos that can run happily off the voltage of one of the batteries, and draw very little current. Would it be dangerous to run a cable in parallel off of the jumper cable connecting the batteries together? It seems like it could lead to a slight voltage imbalance between the two batteries, how significant is that?

Reply

Joe June 13, 2016 at 2:52 am

First time solar need advice, I’m going off grid with 24v system and a 6000w inverter, I haven’t purchased any equipment yet what would you recommend as far as battery ah size and panel watt size and how many going to power a camper completely off grid, will be running a 1500w fridge and a 800w window unit constantly plus other things some of the time, thanks in advance

Reply

Jody Graham June 14, 2016 at 11:19 am

Hi Joe,

I am in Zimbabwe, Africa right now just finishing up training solar installers. I will be home in a couple of days. I need much more info to make such calculations plus it will take me hours to figure out. Where are you in the world? Fridges use approximately 1000 watt hours per day or 1 kWh per day so when you say your fridge uses 1500 watts that would be like running a hair dryer or iron for ironing your clothing. Could your fridge use 1500 watt hours per day? Same with the A/C unit? Does it have an annual usage rating? or BTU rating? What else will you be running for how long such as 10 5 watt LED lights for 6 hours per day equals 10 X 5 X 6 = 300 watt hours per day. You would need to make a list and figure out how many watt hours per day you would consume or how many kilowatt hours per day you would consume. “plus other things” is not enough info to design a proper working solar system. How many days do you want to be able to operate without the use of a backup generator? If you can answer these questions I will do what I can to help you when I get back. Hope you are having a good week…Jody

Reply

Rod Snell June 7, 2016 at 11:01 am

Thank you for the information on your site. I am designing my solar system for the first time. I have an off the grid cabin. I have (12) 12 volt 105 amp hour batteries. I want to create a 48 volt system, 4 banks of 4. I have 12 72 watt, 12 volt solar panels that I want to run 4 in each series and to create 48 volt panels. Will this work? Also, will a 12 volt 4000 watt inverter work or do I need to get a 48 volt inverter?

Thanks much
ROd

Reply

Jody Graham June 7, 2016 at 3:57 pm

Hi Rod,
Thank you so much for your kind words. I would love to answer your questions right now however I an in Zimbabwe training solar installers (http://globalsolar.co.zw/) until Saturday. When I get home I will answer your questions and help you as much as I can. Sorry from the delay….Jody

Reply

Jody Graham June 29, 2016 at 1:24 pm

Hi Rod,

I am sure this is too late. I am so sorry for the delay. Your battery and solar module configuration will work great. However as you have decided on 48 volts, you will need a 48 volt inverter. A 12 volt 4000 watt inverter is not feasible anyway as the amperage would be far too high. 4000 watts/12volts=333 amps. You could not easily source a 350 amp breaker as a disconnect and you would have to use battery cable that is larger than 0000 or 4/0. That size of cable (350kcmil) would be prohibitively expensive.

Take care…Jody

Reply

Rod June 29, 2016 at 8:40 pm

Thank you for the response. I have changed the configuration a bit. I plan to create a 24 volt system with the 12 batteries. If I buy 8 100 watt 24 volt solar panels, will that work?

Reply

Colton Campbell March 19, 2016 at 7:57 pm

First, thank you so much for sharing all this information. It truly helps give me more understanding. I have no experience in solar energy, but given recent events here in the USA, I’m highly motivated to learn. I still would like some clarification on a few concepts if you’re able to help.
1. I do see that there are LOTS of 12v gadgets/appliances readily available. However, if I were to go to a 24v system as you suggest, I do not understand how efficiently any 12v appliance would work? I would have to get some kind of 24 to 12v adapter correct? 1a. And is that even a good idea?

Reply

Jody Graham November 15, 2016 at 6:15 am

Hi Coulton,

I agree with you about the US as well as the rest of the world seeing as all the world’s currencies are fiat and backed by the US dollar which is backed by nothing.

While 12 volt appliances are readily available, the usual way of doing things now in all but the smallest of systems is using 24 or even 48 volt battery based systems and then using an inverter to provide 120/240VAC (North America) or 230VAC (the rest of the world) for your loads. Good quality inverters are efficient, rugged and can provide a better power wave form (sine wave) than our power grid/mains power.

You can use 24VDC to 12VDC converters but these are usually small, inefficient and unreliable.

I only recommend 12 volt systems for RVs and very small cottage systems.

Jody

Reply

JDC January 9, 2016 at 7:29 pm

I have two charge controllers. One is MPPT and the other is not. It is a Trace PWM (pulse wave modulation) CC. 2100 watts of solar array, two systems (1400 and 700). The 700 goes to the PWMCC and that feeds a ‘bank’ of batteries (48v). I have three such banks. The MPPT goes into the front end of the other two. So, in effect, two CCs, one at the front end, one at at the back end. Any variations are ‘buffeted’ by the batteries. So far, I have not encountered any one doing it this way. My batteries seem good but they are just 8Ds and five years old. I am hoping Elon hurries up so I can have something better….(hoping it is better, too). Can’t afford Surettes. If I have to go to lead-acid again, what would you recommend? I think I need more than 600 ah. Maybe 1000 ah?

Reply

Stephen Brecher November 28, 2015 at 6:50 pm

I am in the process of wanting to live off grid. I am going to have a thousand gallon propane tank buried on the property to run the stove and refrigerator. I will be using a compost toilet (no power needed), a Polaris Water Heater and a wood stove as back up. My question – I want to be able to run solar for electrical needs – lights, tv, computers. I’m also going to have a diesel generator for back up. I have been reading and reading and reading and the more I research the more I get confused. I would like someone that is not out to sell me something answer this question. How many 36 cell solar panels do I need, what batteries and how many batteries, and what inverter and any and all other things I might need. I know this is a big/long question to answer but quite frankly I’ve got to try and find someone that is willing to spell it out for me. I’m not handy but I can follow directions and I have a good enough job to buy a good system but I know the packages are way overated. I plan on going off grid before the elections…………………Thank you in advance to anyone that will answer these questions or suggest other things to consider…………..i’m all ears.

Reply

Stephen Brecher November 28, 2015 at 6:59 pm

P.S.

I’m also willing to go totally solar if its feasible………….

Reply

Keith October 8, 2015 at 11:13 pm

Hi all;
Thank you for the great information. Like you said I was about to start collecting 12 volt stuff to set up a system. Now I think I know better. One question I have; Am I able to pull 12 volts off a 48 volt battery bank for simple things like led lighting or an exaust fan for my ekolet composting toilet??

Regards
Keith

Reply