I have a Mppt 100/20 with 24v pv and 2x 12 volt batteries in serie. I only need to power 2 led lights and one cellphone charger. Is it possible to only use 12 volt? There is no inverter connected. Also the app shows me my pv is 36 volt and my battery is 12 volt by default, is that correct?
No. If you pull off only 1 battery in the string, you will unbalance the bank. You need a step-down transformer like this:
https://www.amazon.com/Nextrox-Converter-Regulator-Transformer-Waterproof/dp/B00BWKXTUU
You should size the wattage to whatever you think you might need to pull on 12V.
Attach the input to the 24V bank and power your 12V loads with the output.
Thank you very much.
Oh if your app shows 12v on the battery (well close to that vs up closer to 24v) you already wired the batteries correctly for "12v" and are good to go and using your MPPT efficiently.
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Also check the particulars for all your "12v" items you want to charge; very VERY few need an exact 12v and in my experience many lower-volt DC devices already have small voltage adjusters (their own built-in bucks converters) within their internals that CAN (or can not) handle 9v to sometimes even 36-42v direct. My LED lights and laptop DC car chargers can handle anything from 6v to 38v and 9v to 42v respectively so work on both 12v and 24v battery wiring without a 2nd buck converter! Electronics devices usually need closer to their exact specs but it's worth checking the tiny print to find out what kind of input things can handle.
If this is all you really use the system for and the wiring runs are all not very long (excepting the solar panel to mppt which can be longer) you'd be better served keeping things "12v" and not using a buck converter but just basic 12v plugs.
If you are going to expand soon and 24v is the plan then the above solution of course makes more sense but ONLY if that is the immediate plan.
Buck converters generate lots of heat and RFI and the cheap ones even more so! Your 100/20 is ALREADY converting everything (as a MPPT) so why do it 2x with yet another device?
Just wire your batteries for 12v and double the amps. Much better than double converting and buying more unnecessary gear and having more energy loss on a small system.
The mppt will easily handle that solar panel and another too if you want to add more things to recharge.
Expand into 24v if you need really long runs (50+ meters) between PV's and batteries or double the solar panels with the same MPPT and such. Once you need 400ah or more batteries or need at least 400w+ in solar panels AND long wire runs to the MPPT it makes more sense to 24v.
