Batteries: Producing the Juice That Makes ‘Em Go

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August 2012

Batteries, like many things in aviation, are unexciting unless they malfunction. Then, they can be annoying, perplexing, or even dangerous. A few tips passed on to the people who own them can save a lot of headache, frustration and possibly, repair cost...

Note: Because nickel cadmium (NiCad) and lead-acid batteries differ in many important respects—and accepted practices for one type may destroy the other!—this article discusses flooded (vented, wet cell) lead-acid batteries. (Lithium-ion batteries, available soon in some new aircraft, have their own full-system requirements and are not covered here.)

Today’s batteries are similar in design to the first voltaic cells of two centuries ago, in that they exploit the chemical reactions that result between dissimilar metals when encouraged by an electrolyte. Each particular pack of metals can generate a given “pressure” of electricity, or voltage; the size of the pack largely determines the “volume” of electricity, or current (amperage), available at that voltage over a given time. Packing several cells into one case can increase voltage, amperage, or both.

Current aviation battery sizes translated directly from other disciplines. The -25 and -35 size batteries, for instance, were based on the 25 and 35 ampere-hour sized light-equipment batteries of the 1940s. (These sizes are still common in lawn tractors today.) They produced sufficient electricity for many airplanes, and they were small enough, so designers used these off-the-shelf components. As technology evolved inside, the case sizes endured. Other sizes were added as designers’ needs dictated.

Dry-charged batteries keep many years in their original, as-shipped condition. Opening the seals on these shortens life; and filling them with electrolyte (only up to the lower indicator; then follow the manufacturer’s specific directions) begins a battery’s service life.

Owner-pilots may service and replace their batteries without supervision by a mechanic (FAR 43, Appendix A). It’s up to mechanics to look for batteries that aren’t installed properly, and double-check the logs. (Resolution of discoveries must follow your mechanic’s own best practices. The key, though, is to first make the aircraft safe.)


Keep the battery charged. It makes starting the engine easier and preserves the battery. Charged batteries fight off internal contaminants. Charged batteries don’t freeze. (Electrolyte in a fully-charged fresh battery may stay liquid to -80 degrees F or even lower; a dead battery’s electrolyte readily freezes at 20 degrees F.)

Battery life is degraded by remaining in a constantly partially-discharged state or by undercharging; by constant ambient temperatures below freezing or above 100 degrees F; or by overcharging, or by fast-charging in extremely low temperatures.

When removing a battery, disconnect the grounded side first; when installing, connect the ground last. This minimizes the opportunity for shorting the battery to the airframe or components through your tool.

For that same reason, and to minimize stress on the battery’s connectors, address the battery’s hold-downs after disconnecting, and before re-connecting the cables. Ensure that the hold-downs are secure, but remember that “too tight” does not add to security, and can cause annoying or even catastrophic problems.

Because batteries can boil or overflow, and because they create hydrogen during charging, it is wise to charge batteries outside the aircraft.

Overfilling a battery (even with deionized distilled water) dilutes the electrolyte, dropping the capacity and making the battery more susceptible to freezing. The good news is that, if you overfill with distilled water, the problem will eventually cure itself—if you do not tax the battery too much in the meantime. (To prevent this, add water after charging rather than before, unless the plates are exposed.) However, you can kill a battery fast if you charge it while the plates are not fully submerged; the exposed plates will oxidize, and that portion of the battery’s capacity will be forever lost.

When filling or topping off a battery, don’t spill any fluid—and remember that distilled water that washes dust and dirt into the cells when you’re filling them is a good conductor, and will drain the battery’s charge. Reinstall the battery only when it is clean and dry. As a general rule, refill with only distilled water. Don’t add electrolyte unless you’ve spilled electrolyte from the battery.

When the battery is cold, it does not produce as much power as when it is warm; and it doesn’t make as much power when it is hot as when it is warm. Charging, too, is most effective when the battery’s temperature is above freezing (and below about 100 degrees F).

When charging a battery, voltage is more critical than amperage. A battery will draw only as many amps as it needs, but too much voltage will ruin the battery. Your charger should never exceed 2.35 volts/cell (14.1 volts for a 12-volt battery; 28.2 for a 24-volt). Below-zero or 100-plus degrees F ambient temperatures may require some special considerations, and the manufacturer is the best source of that information.


Some certified installations employ a battery “sump” between the battery and the outside vent to neutralize the gases and any liquid that may be expelled. The chemicals in the sump (usually, sodium bicarbonate) can become saturated and insipid. They’re cheap; replacement can never be too frequent, and your aircraft’s finish will thank you.

Many aircraft systems become plugged, misrouted, damaged or saturated. All these conditions will enhance corrosion; some degrade the battery life, and some are dangerous.

Confined gases (such as the hydrogen produced during charging) can explode if exposed to sparks. It is not unheard of for a battery to explode when the starter is engaged: gases concentrate in the still air and the motor, a relay, or a loose connection provides the spark. Similarly, if the battery’s vent system is inop in flight, gases will accumulate near the battery, and a loose cable connection may provide the spark.


Myth 1: Batteries must never be placed on concrete, or their charge will be drawn out. That’s bunk. How could concrete draw a charge, when a metal battery box can’t? The myth probably originated when concrete dust was splashed on a battery in a rainstorm, either getting inside and diluting the electrolyte or shorting across the terminals in moisture on the outside. Batteries stored at factories and warehouses are still kept on wood pallets, but not because of any concrete allergies. They’re just easier to move when they are on pallets.

Myth 2: Batteries like to be discharged all the way before they are recharged. Not so. Discharging places stress on batteries, and removal of that stress (through recharging) lengthens the batteries’ lives.

Common Mistake 1: Over-tightening the contacts ruins lots of batteries. Electrons flow better through a snug contact than through one that’s partially broken. Further, a compromised battery post can work loose in flight, causing trouble—including fire.

Common Mistake 2: Topping off with tap water or electrolyte alters the chemistry of the battery. Tap water’s contaminants short the battery internally, and electrolyte changes the optimal pH of the mix, reducing voltage. Use deionized distilled water only, and clean the area around the caps before you remove them, so pollutants do not fall into the case.


Batteries should first be inspected at 600 hours or 12 months; after that initial inspection, the interval is halved. 100-hour inspection procedures also, of course, apply.

Always clean and dry a battery before reinstalling it.

When reinstalling, clean and dry the aircraft’s connections, too—and don’t forget to inspect the grounded end of the cable or strap.

When you see dark (brown or black) electrolyte, that’s a sign of a battery that’s nearing the end of its natural life. Put it out of its misery now, and it won’t fail you later.

Green or white “fur” around the terminals can be brushed and rinsed off using a solution of sodium bicarbonate, with the battery out of the airframe (to avoid shorting with the wire brush, or contaminating the battery compartment or sump). Don’t let any of the solution get into the battery!

Here’s a final tip: FAA AC 43.13-1B warns, “It is extremely dangerous to store or service lead-acid and NiCad batteries in the same area. Introduction of acid electrolytes into alkaline electrolyte will destroy the NiCad and vice versa.”

…And good practice with anything that contains lead (not to mention acid): always wear gloves when working around a battery, rinse your tools and wash your hands immediately after handling batteries, cable ends and posts.


Tim Kern, CAM, MBA, has authored features in over 40 aviation publications. He writes technical, publicity and expository pieces for several companies in the aviation industry, and gives his thanks to the folks at Concorde and Gill for their help in the preparation and proofing of this article. Kern is a private pilot with a seaplane rating, and is listed as the manufacturer (“with a lot of help!”) of an experimental aircraft. Send questions or comments to .





Teledyne Battery Products/Gill 


Concorde Battery Corp.


AC 43-206 (Corrosion)$FILE/AC43-206part1.pdf


AC 43.13-1B (Aircraft Electrical Systems)$FILE/Chapter%2011.pdf


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