@changeip: Thanks for that info. That is not what I was told. The Wind and Sun tech support said you can use that MPPT charger to charge a 12v battery from a 24v battery. What could have more "capacitance" than an 85AH lead acid battery?
This is from The Wind and Sun page on that charger:
This controller can also be used with almost any DC source, such as a power supply or higher voltage battery bank, where the max voltage is between 16 and 75 volts. It has built in current limiting*, so may be used to step down a 24 to 60 volt battery system to a 12 volt auxiliary battery for accessories. It is commonly used in tel-com sites to keep a smaller 12 volt battery system charged from a 24 or 36 volt system.
* According to Wind and Sun, this feature is what allows this particular model to perform this function. Without current limiting, I could see a problem with other than solar panels as a power supply.
They also gave me this warning: "Do not try this with any of the other solar chargers".
Edit: I use this feature to overload my charger/battery with solar panels. It is not the good days here that are the problem. If I were in Arizona or New Mexico, I would probably not have this problem. But here in Florida, just yards from the Gulf of Mexico, it can cloud up and rain for a week. Now my almost 20amps of solar panels produce about 5 amps in the overcast conditions, but the charge is still limited to 15 amps on the sunny days.