I have a 1968 PA31-310 Navajo with an aftermarket AC unit that uses what I believe is about a 40 amp motor to drive the compressor. The battery is a 17amp hr wet Gill that's 2.8 yrs old. The alternators are 50 amp. Last friday it was so hot I had to run the AC continuously at cruise altitude of 6500ft. I noticed that my ammeter was showing a constant 20-25 amp charge to the battery which was unusual and the voltage was 27.8V which was normal. When I turned off the AC in cruise the ammeter indicated zero, but as soon as the AC went back on I got the 20-25 charge indication. The airplane started normally twice that day. On Sunday when I went to fly my daughter's family home, the battery was dead. The grand kids swore they had not touched the switch for the aft cabin light which is on the hot bus. I thought of getting an APU to start but remembered there is a note in the AFM to not attempt takeoff with a DEAD BATTERY BECAUSE THE ALTERNATORS NEED A MIN OF 3V TO EXCITE THE FIELD. I suspect that I have a bad battery since the plane often goes 2-3 weeks between flights. I'm getting a new battery tomorrow, but I don't know what to make of the ammeter's berhavior. Any educated thoughts on the matter would be greatly appreciated.

I believe my electric AC pulls closer to 70amps when driving the compressor and circ fan.

An almost 3 year old battery in a turbine is living on borrowed time if you always do battery starts. When they fail they go from good to dead overnight. They don't give any warning when they are about to fail.

Replace your battery every 2 years if you don't want surprises when it fails on you.

I think your AC usage was a coincidence.

Batteries do have Maint Manual and capacity checks performed with specific equipment at called out intervals.
Gives a hard number read of the batteries condition and whether it meets criteria for continued airworthiness.
But the time and labor involved probably makes it more cost effective for low utilization Piston A/C to buy a new one at a whatever you determine is a time.
The situation you describe of a deep draw of amps for continued time while flying will be supplied by the alternators and not the battery. However a typical battery will need about an hour to fully charge after a start. Longer if it is hard to start.
The trickle chargers specified by battery makers are great to keep it up on the ground.

I think Vicente is flying a PA31 (Navajo), not a PA-31T (Cheyenne). Piston aircraft used regularly should get way more than 2 years service from the batteries.

IMO, the reason to not jump a dead battery and go flying is two-fold:
1. It's hard on the battery, as the alternator will be jamming a lot of charge into it at a high rate.
2. The battery is your backup electrical supply. You don't want to launch on a flight and have your electrical system take a dump the moment you select gear up.

Sounds like the AC is connected on the battery side of the ammeter... So that you don't really know if the battery's discharging or not. But, if the AC is drawing 40 amps, and you're only showing 25 A "charge" then the battery is discharging at 15 Amps to keep the AC going.

Time to move the shunt, so that the ammeter really tells you what's happening to the battery. Then replace at least one of those 50 amp alternators with a 70 to 100 amp unit.

Paul

Thank you gentlemen. Paul, I'll check into those things you mentioned.

Paul you were right. The AC motor was connected to the battery side of the ammeter. Thanks again.

Another issue:

If the battery drops below approx. 17 volts, "jumping" it with AUX power will not bring the battery into the buss and it won't get charged. You have to have at least 17 volts at the battery to connect the solenoid, regardless of voltage on the main buss.

Taking off with a low battery, or failed alternator, can cause many unintended issues.