Recently, in the cold weather, my Warrior’s battery has barely managed one try starting the plane — any more, and it goes flat. Most recently, it happened after I’d just been flying 1.7 hours and tried to restart after a short fuel stop. I had to figure out whether the problem was the battery or the alternator (or regulator).
Battery/alternator diagnosis checklist
I phoned my AME, and he gave me a short checklist that I could run myself using only a multimeter (this is for a plane with a 14 volt electrical system and a single battery):
- With the engine off, a reading at the battery terminals (master off) should give at least 12.5 volts, and a reading at the cigar lighter (master on, radios off) should give at least 11.5 volts.
- With the engine idling, a reading at the cigar lighter should give 12.5–13 volts.
- With the engine running at 1500 rpm, a reading at the cigar lighter should give at least 13.5 volts.
Analysis
I left the plane tied down for extra security (in addition to the brakes), and ran the tests. Here’s what I got:
- Engine off: 12.42 volts at the battery terminals, 11.6 volts at the cigar lighter.
- 600 rpm: fluctuating 12.7–13.1 volts
- 1000 rpm: 13.71 volts
- 1500 rpm: 13.7 volts
- 2000 rpm: 13.7 volts
After running the engine for a few minutes then shutting down, the battery read 13.03 volts at the terminals, but the charge had dropped to 12.61 only 10 minutes after I shut down the plane, and would presumably keep dropping to around 12.4 again.
Diagnosis
My alternator is obviously producing full power even at only 1000 rpm, and the regulator is kicking in to cap it at 13.7 volts. There’s no reason that battery shouldn’t be charged; however, 12.4 volts is fairly low, and more disturbingly, after only one start attempt, the battery drops to 12 volts and can no longer turn the propeller.
I think I’m facing a bad combination of cold weather and a weak battery. I’ve decided to replace my wet cell with a newer, high-cranking-power sealed battery, but I need to wait for a new battery box cover to arrive from Concorde; in the meantime, I’m using a loaner wet Gill battery for an upcoming New York City trip. If you see someone carrying a dead battery into the FBO to be charged while his family waits impatiently in the plane, it’s probably me.
Land and Hold Short 
Not sure what they used at Mr.Lube but they attached both ends to my battery and stated that it was running at 85 percent capacity. They “as in high school kid” said that as your battery looses its charge over time the alternator works harder to charge and run the systems. He showed me this device but how does it work ? I mean you have Voltage or you don’t, can it figure out how many coloumbs of charge you have sitting in it ?
I believe that by using a combination of voltmeter/ammeter and measuring how much the voltage drops at different current loads, one can estimate the power capacity of a battery.
Am I missing something? How is it safe to fly with a battery that won’t hold a charge? What if your alternator failed?
You probably missed the line stating that I borrowed a loaner battery from the shop for the New York trip.
That said, a plane will keep flying fine with no battery and a failed alternator, since the spark plugs are driven by two redundant magnetos; in fact, there are still planes flying that don’t use batteries and have to be started by hand propping. Granted, the loss of the electrical system would cause awkwardness in controlled airspace (I’d have to talk to ATC on my handheld radio or cell phone), and would be a serious risk in IMC (since I’d have no backup to the vacuum pump).
On a nice VFR day, I would not hesitate to fly a plane with a weak battery back home for repair (assuming I could get a boost to start the plane). I wouldn’t knowingly fly outbound in that condition, or fly at night or in IMC or marginal VMC.