Unless you’re an expert at hand-propping your elderly tube & fabric taildragger, your aircraft has a battery.


Most of the time, as an owner/pilot, you can ignore the battery.  You flip the master switch on, the turn & bank gyro spins up, and the engine spins over when you crank the starter.


However, there is some stuff you need to know about your battery, because it can cause you trouble if you ignore it.


Most (cheap) aircraft batteries are lead-acid.  They have six (for 12V) or twelve (for 24V) caps on the top.  More expensive (better) aircraft batteries are sealed.  These are generally the choices for certified aircraft.  For homebuilt, there are much more intriguing options.


Older aircraft (singles) generally used 12V batteries.  Twins and newer singles often use hideously expensive 24V batteries, which can be annoying to live with.


Anyways, your 12V battery, if it’s fully charged, with the engine turned off, should read around 12.6V.  But with the engine running, the charging system should keep it between 13.5 and 14.5V.  You might think that if some is good, more is better, but keep in mind that above 14.4V the lead-acid battery will start to discharge gas, and that’s not good.


Every once in a while, you should unscrew the caps on your lead-acid battery and check the acid level.  It should never be allowed to run dry.  You should ensure that the plates are covered, and that the acid should be just at the bottom of the ring.  Try hard not to use tap water.  Use distilled water to top it up.


Another thing that’s really hard on the battery is letting it fully discharge.  An easy way to do this is to leave the master switch on.  That will draw about an amp or so, and over a few days will totally drain the battery, which it never really recovers from.  Every time you totally drain a battery like that, it’s always just a bit weaker. 


Note that when the battery is totally drained, it can be difficult to charge it up, using a smart “constant voltage” charger, which doesn’t see anything to charge when it’s connected to a battery with less than a volt.  In this instance, I have some cheap “constant current” chargers which are good at bringing up a totally dead battery.  Another trick you can do, is to put jumper cables from the dead battery to a good battery.  The sum of the two voltages will be non-zero, and will allow a fancy “constant voltage” charger to work.


Not that you cannot leave a cheap “constant current” charger on the battery indefinitely – even a very small one will cook it and destroy it, after a while.  If you are using a cheap “constant current” charger, you must monitor the battery with a voltmeter and take the charger off when it gets over 14V.


Even if you don’t leave the master switch on (or any other current draw), if you leave a battery for a long time (months) it can fully discharge, which is bad for several reasons.


In Canada, it can get bloody cold in the winter.  And often, aircraft don’t fly much.  If you leave your battery in the airplane, what frequently happens is that the battery goes dead, and as it does, the specific gravity of the acid weakens, which raises the freezing temperature of the acid.  And, when it freezes, it expands and can crack the battery, leaking acid all over your aircraft.  This can make one hell of an expensive mess.


So in the winter, if you don’t fly much, consider taking your battery out and put it in your basement.  Charge it every couple of weeks.  When you go out to your airplane, bring your warm and fully-charged battery and put it back in.  It will start much better.


Another option, if you have electrical power, is to put a constant-voltage (more expensive) battery charger on permanently.  One popular brand of these is called the “Battery Tender” and is available at Canadian Tire.  You don’t need a huge, powerful one.  Even an amp or two will do.


Remember that you need to check the acid levels in the battery every now and then, which I might add is perfectly legal under CAR 625 App C “Elementary Work”.


Personally, I hate lead-acid batteries.  Even for non-aerobatic aircraft, I use Concord RG sealed batteries, which don’t need servicing and don’t go flat as fast as a lead-acid battery.  They are FAA-PMA approved for many airframes, probably including yours.


Anyways, if you take care of your battery, it will last much longer, which saves you money, and it will have a charge when you need it, and it won’t leak acid everywhere.


I’ve skipped over a few important things in this note – non-certified batteries for homebuilts, external power plugs, and living with a 24V battery – but I tried to cover the basics of what you need to know.



Sept 2014