Keeping Batteries Going

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While we hate to shell out money for a new battery, accept that three to four years is the average lifespan.

There will be owners who say they have aircraft batteries that are five or more years old and it still cranks just fine. Chances are they live in a warm climate, donít ever worry about having an alternator failure and have some good luck. Some use proper aviation battery chargers, but probably not. Hopefully, these owners donít fly IFR.

Our staff IA just replaced his Concord AGM in his twin and it was over†five years old. While it never failed, and he knew he was pushing it, he rarely flies IFR any more. To me that is the real decision makeródo you fly IFR? If you fly with an old battery, IFR flying could be risky business. If you have solid partial-panel instrument skills, plus portable nav and comm equipment (and are proficient with†them under stress, with a minimal panel backup of just vacuum instruments or less), that could make a difference on how neglectful you can be of battery age.

If you remember one thing from this article itís that battery voltage is a meaningless indicator of how long a battery will last when it is the only source of power on the airplane. Thatís because two-thirds of the plates in the battery could be sulfated over and the battery voltage would look normal after a chargeóprobably close to 13 volts. The only warning may come if you let the plane sit two to three weeks. It may not start the engine unless it fires one or two turns of the prop, in warm weather.

I was in the ďdonít replace what still works campĒ on batteries 30 years ago, and was at the five-year point when my alternator quit and my five-year-old battery failed five minutes later, while in solid IFR conditions. It had never failed to start the engine (but I had†never asked much of it beyond cranking two or three blades). I lived in southern California and flew IFR nearly every flight, often at night. Then it finally happened, and as luck would have it, I was in solid IFR conditions at night over Los Angeles. I immediately started shutting down nonessential equipment, leaving one navcomm for communications and for flying the approach, plus the transponder. (No handheld GPS then, but the handheld radio did have a VOR and I had three small flashlights).

Getting the high current load off a battery when the alternator lets go is extremely important for keeping electric panel instruments alive as long as possible, even for a good battery.

Since Iím writing this I obviously made it, but the battery (and the panel electronics it was feeding) went out so fast because the battery was way past itís ďreliabilityĒ date. While I made a partial†panel approach with the handheld VOR, the event scared the pants off me. I thought batteries were supposed to last 30 minutesóat a minimum. They often do if you maintain them properly and monitor them with capacity checks, at least after they are a year old.

Part 135 operators have a required schedule of capacity checks to comply with. Today, battery failure is arguably safer and you might get away with pushing your luck if you have a modern portable GPS with a flight instrument display, a handheld transceiver (preferably with an external antenna switch) and you know how to use this†portable equipment without the aid of the autopilot, if it draws any power. Hopefully it wonít happen at night like it did to me. I had three pilot/passengers to help with the flashlights. There were four sets of knocking knees on that Wednesday night sojourn.

Real World

Both of our last two Light Plane Maintenance magazine battery tests, as well as on-line surveys conducted by Aviation Consumer magazine of over 500 owners, show that aircraft batteries commonly last two to three years for reliable operation. The reasons for this short life are straightforward, but†there are things an owner can do to optimize and lengthen useful battery life.

The first problem with aviation batteries is the marginal size and capacity for the job in order to keep battery weight down. Aviation batteries are much smaller with less capacity than an auto battery, yet it is often starting an engine with twice the displacement,†with more oil thatís two to three times as thick to churn through.

As a result, an aircraft battery has to discharge substantially more of its capacity to start an aviation engine than the typical automobile engine. Itís a given fact that the more deeply you discharge a lead acid starting battery beyond a very minimal starting burst, the shorter its life, all other things being equal.

Next, being a chemical beast, a lead acid battery slowly self-destructs from non-use as it spontaneously self-discharges with the simple passage of time. Frequently it is not fully charged from the†typical short flights of todayís aircraft users, so it sits between flights in an already partially discharged state. The more time a battery spends partially discharged the faster it becomes permanently damaged and looses capacity.

It commonly takes two hours to recharge a battery during flight with a†properly adjusted charging system. Charging system voltage that is either too high or too low will slowly and permanently damage a battery and shorten its useful life.

The third factor is improper care of the battery by the owner. The worst thing is to not use a battery charger when the plane is not flown at least once every few weeks. But the next worse thing is to†not use the proper battery charger. Thatís right, using the wrong charger in some cases can be more harmful than not using†a charger at all. How can this be?

First, high charge currents are badóeven the common 10-amp auto charger is too much current for†a healthy charge cycle of an aviation battery. Also, any prolonged time (more than overnight) that a battery charging voltage stays†above 13.2 (or 26.2) volts (in a trickle charge mode) the battery will slowly dry out. Use a charger designed for aircraft batteries and with a trickle charger under 13.2 (or 26.4) volts, and max of 14.6 (29.2) volts.

So, mandatory or not, you should periodically test the battery per the makerís guidelines, also known as a capacity check.

A version of this article appeared in the December 2013 issue of Light Plane Maintenance magazine.