The 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
RVs 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.
Just 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).
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)
Let’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.
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.
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.
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.
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…
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.