Please help me understand....

moonbow · February 11

Hello everyone, I am acquiring a small casita in Baja and need to upgrade the existing solar system. In talking with the previous owner, he states there is a 24v inverter already installed and that power, of course, is coming from a 24v battery system. Now, what I dont really understand is why folks use 24v battery storage vs 12v battery storage? I have two choices here since I am going to eventually replace his old batteries with my own (4) 12v 150aH AGM batteries. Choice #1...I can wire my new batteries in series to make a 24V 300aH bank and continue using his 24V inverter OR I can replace his 24v Inverter with a 12V inverter and then wire my (4) 12V 150ah batteries in parallel to have a 12V 600ah bank. My solar array and charge controller can, according to morningstar's solar calculator, handle the PV array I am going to Install. Since the CC can handle the PV array isnt it in my best interest to have a battery bank w/ higher ah rating? Meaning shouldnt I go with the 12V 600aH battery bank vs the 24V 300aH battery bank?

ELYNN4 · February 11

You have done the math (300 AH @ 12V = 150 AH @ 24 volts) when you double the voltage you cut the amperage in half. The benefit of going with the higher voltage is that you can reduce the wire size because of the lower amperage. Lower amperage means less resistance.

moonbow · February 11

do you mean wire size between batteries and then between batteries and CC?

BB. · February 12

Yes, as Elynn4 typed, and your comment on wiring between battery bank and Charge Controller, AC inverter, and such...

The stored energy in (for example) 4x 12 volt @ 100 AH batteries is the same, no matter how you wire them (done correctly):

4x 12 volt batteries in parallel = 12 volt * 400 AH battery bank = 4,800 Watt*Hours of storage
2x 12 volt batteries in series * 2 parallel strings (4x batteries total) = 24 volts * 200 WH = 4,800 WH storage
4x 12 volt batteries in series = 48 volts * 100 AH = 4,800 WH of storage

More or less--Some back of the envelope numbers... The largest AC inverter (suggested) for a Flooded Cell Lead Acid battery bank:

12 volt: 1,200-1,800 Watt Max "usable" AC inverter
24 volt: 2,400 Watt to 3,600 Watt Max "usable" AC inverter
48 volt: 4,800 Watt or larger inverter

12 volts is higher current, and very little "acceptable voltage drop" in the wiring. It is "difficult" to send "useful amounts" of 12 VDC power much further than 10-20+ feet without using very heavy gauge copper wire.

24 and 48 VDC wiring, more voltage drop allowed, easier to send "useful amounts" of power farther.

And if you have an AC inverter, sending power any distance is much "easier" at 120 or 240 VAC.

There are some other rule of thumbs--Such as (I suggest) a maximum of an 800 AH battery bank (for various reasons)... So if you "need" a 12 volt @ 1,000 AH, it would be better to have 24 volt @ 500 AH, or 48 volt @ 250 AH battery bank.

It used to be difficult to find "smaller AC inverters" at 48 VDC--Today, you can find quite a few 1,000 Watt or smaller 48 VDC input inverters... So typically the choice is between 12 and 48 volts--And skip the 24 volt choice (go "small" or go "big"--Go medium.... Umm).

There are other choices too... If you have 12 volt gear (HAM radio, 12 volt lightning, on a small boat, etc.)--12 volts may be "best". If you are on a larger boat, 24 VDC does have some advantages.

In the end, really like to start at the beginning... What are you expected energy needs... 500 to 1,000 WH per day (or roughly 50-100 AH @ 12 volts per day)--A 12 volt battery system may be perfect.

If you are looking at 3,000+ WH per day--Then 48 volts.

Switching voltage--That is a "pain" with AC inverters--They are typically 12 or 24 or 48 volt units--Change battery voltage, need a new AC inverter.

Solar charge controllers are typically 12-48 VDC ready... Whatever bank voltage they have, they will work with it (there are questions about the "correct" solar array voltage--But we can go down that rabbit hole larger)... In the end, a 40 Amp charge controller will typically work 12/24/48 volts... And will support a 2x large wattage array at 24 volts or a 4x larger array at 48 VDC battery bank... So higher voltage=>"cheaper/smaller" needed solar charge controller(s).

At this point, you have an existing system... Move in and try it out... Does it meet your power needs, or do you need more?

Remember that off grid solar--You need to be very efficient with your energy usage. Solar power is not cheap, and most people overestimate how much they can harvest and can "murder" their battery bank by over discharging. Or--You end up running a backup genset burning fuel/time on genset to make up for "too small" solar system, or too large of loads.

You can tell us what you know about the system (battery bank, solar charge controller, solar array, AC inverter, approximate location) and we can estimate the "useful harvest" from a well running system for you.

-Bill

Photowhit · February 12

There is NO advantage in having a higher Amp Hour rating!
Amps are NOT a measure of power!!!

Watts are a measure of power!
The basic equation used to in doing your calculations is;
Amps x Volts = Watts (or in the case of stored energy Watt hours)

So...
4 - 12 volt 150 amphour batteries, will have the same Watt hours in either 4 parallel 12 volt batteries or 2 parallel strings of 2 - 12 volt batteries.

In addition, other advantages of higher voltage system are;

Wire sizes are based on their ability of carrying amps, so a 12 volt system using 1200watts will need a wire capable of carrying 100 amps, would be 3 gauge wire, much thicker than the 8 gauge wire required to carry 50 amps at 24 volts.

cables - NEC Ampacities Data - Electrical Engineering Stack Exchange

cables - NEC Ampacities Data - Electrical Engineering Stack Exchange

Now consider that it's very hard to have 4 parallel strings of batteries to share charging and discharging. You will likely run into problems as the battery bank ages with some batteries aging faster than other.

Another advantage is that charge controllers are rated one the ampacity. If they can handle both 12 volts and 24 volts, the rated capacity say 30 or 60 amps would handle 2x the amount of solar array at 24 volts compared to 12 volts.

To be honest you might want to consider moving to a 48 volt system rather than a 24 volt system!

I wish I could have when I moved and already had a 660ah 24 volt fork lift battery. When my new power demands made a 48 volt battery bank much more reasonable. Instead I have 2 $600 charge controllers to service the battery bank.

Marc Kurth · February 12

Be careful with your battery selection because there aren't that many AGMs on the market that are suitable for off-grid duty.

littleharbor2 · February 12

Fullriver makes high quality AGM Deep Cycle batteries

moonbow · February 13

Thank you for making sense of all this! I truly appreciate it.

Please help me understand....

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