BFL13 wrote:
With 24v charging on a 24v battery is everything the same as 12s except half the amps? Same charging profile, SOC vs time etc? Is 8.3 amps with 24v the same exactly as 16.6 amps on a 12v bank of the same AH capacity? (Do you express the "AH" as half too?--Please, no Watts here! :) )
If so it would be easy to compare results. With my 230w panel and MPPT I was getting 15.5a in the Spring and 13.5a in the Summer. With PWM I would get 14.5a all the time based on my 130w having 8.2 Isc. (which I got all the time no matter Spring or Summer)
The Isc of the 230w panel is 8.3 and in the Summer I often saw 8.4 or so. (Isc goes up with heat) Since I get Isc with a 12v panel I should get Isc with the 24v?
I notice that the 230w and the 130w have about the same Isc, so it is not twice really. Confusing. Panel brands do vary in Isc per Watt though.
I would sure take the 16.6 worth at 12 getting 8.3a on 24v instead of 13.5 on 12 if that is how it works.
So are you planning a 24 volt battery bank to run a inverter? That normally works great, especially if you are installing this on a MCI or other RV with a 24 volt starter and 24 volt alternator to charge the system while driving. If it is a cabin in the woods, then 48 volts will make sense if you have not already bought the 24 volt output controller and inverters.
With a 24 volt system, the battery is still rated in the same 'amp hours'. Just that you can store twice as much energy in a 24 volt 100 amp hour battery bank compared to a 100 amp hour 12 volt battery. (2400 watts VS 1200 watts).
Your wiring size is smaller on a 24 volt system. AS noted a 230 watt panel is only 8.3 amps - so #`10 wire can handle 2 of them in parallel, or many in series. Many controllers can have a 150 volt or 600 volt 'maximum input limit' - so it would be possible to have 8 -10 solar panels of the same amperage capacity installed in series, then using #10 wire run this high voltage to the MPPT controller, with it's output being limited by it's output amperage maximum and voltage that the battery is rated at.
Some MPPT controllers are auto-adjusting for their input and output voltages. Connect them to a 24 volt battery, and they will regulate at 28.8 volts to shut off. Same controller to a 12 volt battery, and it will shut off at 14.4 volts.
Your battery bank weight will be the same no matter the voltage for a given number of watts stored. So 2 each 100 amp hour 12 volt batteries in parallel will store 1,200 watts in each battery. If in series you will have 100 amp hours at 24 volts, if in parallel you will have 200 amp hours at 12 volts. Both will store 2,400 watts. Normally a 100 amp hour 12 volt battery will be 60 pounds, and installing to many batteries will add a lot of extra weight to a RV, so try to avoid that.
If you will be using a lot of 12 or 24 volt power each day, it would be better to install more solar panels than to install larger batteries. You only need to size a RV battery bank to your night time loads, and let the daytime loads run directly from the solar panels. As you know, a 230 watt solar panel is only about 30 pounds, and will produce about 1,000 watts per day, while the battery will only store power, and not 'produce' anything.
The other disadvantage of a larger battery bank is they lose about 10% of their capacity monthly. So a 200 amp hour battery will lose 20 amp hours, or 240 watts a month, about 8 watts per day lost to in-efficiencies in the system. 20 AH at 24 volts is 480 watts, or 16 watts per day.
Good Luck,
Fred.