How dead is a dead battery

[Andy Alson]

So if a plane has a dead battery, and we know that there must be some charge left or the alternator will not recharge it after a jump start (correct?).

Assume we have a 12 Volt aircraft battery showing 3 Volts on a voltmeter, clearly not enough to start the plane or even to activate the master relay if we turn on the master switch. If we jump start that aircraft will the system recharge the battery or isn't there enough there to activate the alternator and system. If not how much is too little for it to work.

Thanks,

Andy

[Ward Miller]

Andy, my experience is with car batteries, but some of the problems -- and solutions -- are similar.

First, for a battery of questionable charge, a regular voltmeter is useless. I always use a load voltmeter ($25 from Harbor Freight). Next, why is the battery weak? Was it sitting for a year, unused? Or was it relatively new and used last week? If the latter, what could have caused it to discharge? The answers will tend to tell if it can be saved, or is lost.

Given there is hope, I then charge it over night and load test it in the morning. If it works, I'd be very suspicious of what caused it to lose capacity and keep close watch.

[Andy Alson]

What caused the battery to be low was simple. Leaving the master on when putting the plane away. Battery was only a year or so old and was used four days ago and was fine.

[Ward Miller]

Excellent! Charge it overnight and you'll be back on track. Update your post-flight check list to include Master Switch. You do use a post-flight check list don't you? Get a cheap load voltmeter for future use. It is priceless!

[Russell Holton]

I'd suggest biting the bullet and replacing it. The old battery can be used in something else less critical once you get a charge back into it.

Not all battery cells are identical. In the process of discharging, some cells went flat before the others. Continued discharge caused those "first cells" to be reversed charged by the other cells. There is a chance the battery is no longer reliable - even if you should revive it. Furthermore, trying to recharge the battery with the airplane's alternator will subject it to stress beyond normal. In cars with marginal alternators, that has led to alternator failure.

Note that modern electronic chargers will likely refuse to charge a battery that low. You'll probably have to find an old "dumb" charger to get it back somewhere normal before a modern charger will tackle it.

[Dave Siciliano]

You can give the old one away, free of charge :-)

[Ray Tackett]

The alternator gets its power to the rotor via slip rings and brushes. If you did a jump start, the alternator would immediately begin producing output power in the field (stator) winding, which goes through (typically) six rectifiers (three phase AC in the stator) to make a DC output. Once that is established, milliseconds after start, it's self-inducing from there unless the battery goes to dead short. The latter is extremely unlikely and is impossible to jump start.

[Andy Alson]

Thanks, Ray. "dead short" means the battery is really completely dead. When you say it's impossible to jump start, do you mean the alternator is impossible to jump start or the engine itself?

Andy

[Ray Tackett]

Thanks, Ray. "dead short" means the battery is really completely dead. When you say it's impossible to jump start, do you mean the alternator is impossible to jump start or the engine itself?

Andy

If the battery dead-shorted (never seen a lead-acid car or airplane battery do that), it would become a fat chunk of wire instead of a battery. The overall result would be identical to clipping the jumper cable leads to each other instead of the battery -- heat, smoke, and possible destruction of the battery and alternator on the "good" end. The alternator on the "dead" end would be irrelevant. Nothing electrical would happen. and there would be nothing other than hand-propping which could rotate the alternator.

A "dead" battery is a battery with little or no internal chemistry going on, and little or no charge. It may be salvageable by recharging. A dead short is a piece of wire, a metal bar, a screwdriver handle, or some such. Sorry if you were "dead confused".

The alternator converts some of the rotational energy of the engine into electricity. In order for there to be electric power output, the alternator must be turned while some voltage (not necessarily all 12 - 14 volts from normal operation) is applied to the rotor. In the extremely improbable case of a shorted battery, there would be zero volts across it, thus no "excitement" current to the alternator's rotor. If you then spun the alternator via hand-propping, you would either trip the alternator's circuit breaker, or destroy the alternator and/or start the wiring on fire.

[Andy Alson]

Thanks for the explanation, Ray. Now it all makes sense.

Andy

[Paul Millner]

The alternator gets its power to the rotor via slip rings and brushes. If you did a jump start, the alternator would immediately begin producing output power in the field (stator) winding, which goes through (typically) six rectifiers (three phase AC in the stator) to make a DC output. Once that is established, milliseconds after start, it's self-inducing from there unless the battery goes to dead short. The latter is extremely unlikely and is impossible to jump start.

Not so fast... all it takes to make the wheels come off the cart is a momentary load greater than alternator output... like the gear pump spiking for a second to maintain hydraulic system pressure, or the momentary inrush from turning on the landing light. That can drop the voltage sufficiently for the alternator to come off line... and it ain't coming back.

paul