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

August 14, 2015 · 20 comments

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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…

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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

{ 20 comments… read them below or add one }

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?

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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

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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

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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

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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.

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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

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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.

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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

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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?

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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

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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

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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! : )

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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

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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

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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

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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

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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

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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

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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?

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Jody Graham November 15, 2016 at 6:15 am

Hi Coulton,

I agree with 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

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