Pilots see their aircraft on every flight, and each preflight represents the last chance to spot impending troubles. We know that many pilots are in a hurry, possibly uninterested or (the likeliest case) not educated or aware of the little things that can impact safe and efficient operation. This article/series addresses a component often overlooked by the pilot who sees his aircraft every day, with the objective to help our pilots understand the importance of vigilance.

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: this article will concern itself with flooded lead-acid batteries, because NiCad Lithium-ion batteries, available soon in some new aircraft, have their own full-system requirements.

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 available at that voltage during 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-amp-hour size light-equipment batteries of the 1940s. 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 the batteries’ lives.

Owner-pilots may service and replace their batteries without supervision by a pro mechanic (FAR 43, Appendix A). It’s up to mechanics to look for batteries that aren’t installed properly and double-check the logs.

Charge!

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 fully charged fresh battery might stay liquid to -80°F or even lower; a dead battery’s electrolyte readily freezes at +20°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°F or by overcharging or fast charging in extremely low temperatures.

It is wise to charge batteries outside the aircraft because batteries can boil or overflow and because they create hydrogen during charging. Before reinstalling, clean and dry the battery and all its connections, including the aircraft’s connections and don’t forget to inspect the grounded end of the cable or strap.

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 reconnecting the cables. Ensure that the hold-downs are secure. Too-tight does not add to security, and can cause annoying or even catastrophic problems.

Overfilling a battery, even with deionized distilled water, dilutes the electrolyte dropping the capacity and making the battery more susceptible to freezing. However, 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 severely 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 a battery, don’t spill any fluid and remember that distilled water contaminated with dust and dirt (as well as any electrolyte spilled when filling) makes a good conductor, and will drain the battery’s charge. Reinstall only when the battery is clean and dry. A good general rule on electrolyte addition: don’t add electrolyte unless you’ve spilled electrolyte.

When the battery is cold, it does not produce as much power as when it is warm. Likewise, charging is most effective when the battery’s temperature is above freezing and below 100°F.

When charging a battery, the important parameter for safety and long battery life is voltage, not amperage. A battery will draw only as much amperage as it needs; too much voltage, however, will ruin the battery. Your charger should never exceed 2.35 volts/cell (14.1 v for a 12-volt battery; 28.2 for a 24-volt). Below-zero or 100°F+ ambient temperatures may require some modification to this advice, and the manufacturer is the best source of that information.

What Can Possibly Go Wrong?

Some certified installations employ a battery sump between the battery and the outside vent. 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 experimental amateur-built installations do not have sufficient ventilation and drainage systems and many certified aircrafts’ systems become plugged, misrouted, damaged or saturated. All these conditions will enhance corrosion, some degrade the battery life and some are dangerous. Confined gasses (hydrogen is 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, gasses will accumulate near the battery and a loose cable connection may provide the spark.

Speaking of experimentals, when an experimental, amateur-built aircraft comes in for any inspection, the battery and its systems should get particular attention. (I once bought an experimental that had the rudder cables running, unsupported, directly above the unshielded battery terminals. It had flown that way — waiting for one good bump — for years.)

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.

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 and 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! Clean and dry the terminals, too, before reinstallation.

If your services involve both NiCad and lead-acid batteries, the FAA has some advice: 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 Ni-Cad and vice-versa."

Myths and Mistakes

Myth 1: Batteries must never be placed on concrete, or their charge will be drawn out. That’s bunk. 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 because they’re easier to move if they are on pallets.

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

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

Common Mistake 2: Filling 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. (Then clean and dry the battery before re-installing.)

And good practice with anything that contains lead or acid: always wear gloves, rinse your tools and wash your hands immediately after handling batteries, cable ends and posts.