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Corrosion: When Good Metal Goes Bad

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September 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.)

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