Battery capacity (determined by the airframe manufacturer) is based on two factors. First, you need adequate cranking power for the engine size. Thus assumes the engine is adequately preheated when appropriate. A 35 AH battery is adequate for a 6 cylinder 550 CID engine with pistons the size of 3 lb coffee cans. A 25 AH battery is more than enough for a C85 or anything else you might find in an Ercoupe. Choosing a battery based on some remotely possible maintenance cranking scenario really doesn't make a lot of sense.
The second factor is reserve capacity. Unless you are flying heavy IFR with lots of radios and such (and the generator system to support them), the 25v AH battery is again more than adequate. A 25 AH battery in good condition can supply 25 amps reliably for a half an hour, or 12 amps for an hour. (Or 1.25 amps for 20 hours, which is where the rating comes from.) so, if you have the original 12 amp generator, the 25 ah battery is good for more than an hour of reserve power. -- John Skyport East www.skyportservices.net
