In the typical car or truck the battery box may be nothing more than a tray to support the battery. But there’s more going on in the battery box in most aircraft—and it might not be good. Neglect can lead to unnoticed airframe corrosion, engine starting problems, audio system noise and other electrical system problems.
Part of the problem is that some of the OEM boxes weren’t made to withstand the harsh conditions to which they’re subjected. That makes routine and careful inspection critical. And when it’s time to replace one, hold on to your wallet. The good news is that modern battery boxes are much improved over older ones.
WHAT THEY DO
More than you think, actually. Battery boxes generally house the positive and negative battery terminals. In metal boxes, the terminals are attached to the inside of the box with carry-through studs and insulators. The positive terminal is nothing more than a braided strap that is soldered to the stud on the inside of the box. The negative terminal is simply soldered to the inside of the box, while a ground strap on the outside serves as the main ground. Battery removal and reinstallation, corrosion buildup, overtorquing, airframe vibration and the breakdown of the solder can ultimately stress the connections, causing high resistance and heat that, in worst cases, can melt the positive terminal post off the battery. Corroded terminals can also lead to high resistance and a voltage drop—robbing power to the starter.
Leaking battery acid is an enemy to the box and to the airframe. One job of the battery box is to expel leaking battery acid and vent battery gases. While this isn’t so much of an issue with a healthy battery, a battery with a weak cell or a cell with internal resistance causes the acid to boil and come out of the top of the battery.
Some boxes (including those on Piper models) have external vent tubes to catch the slipstream from the belly of the aircraft and push fresh air into the box, while an exit line sends the vented air back overboard.
A drain fitting on the bottom of the box expels any liquid that gets trapped in the box. Boxes for most Cessna models, on the other hand, don’t have inlet or outlet venting and drainage systems to remove damaging battery gases and liquids. This can lead to faster deterioration of the box, particularly aluminum boxes.
Speaking of deterioration, consider the consequences of battery acid that is leaking from a battery box. The acid can end up between the aluminum skins and between rivets on the airframe—a condition that might not be easily discovered. We’ve seen the resulting corrosion so bad that it required a skin repair on more than one airframe. This generated an invoice north of $5000, not including the cost of the new battery box.
TIME TO GO SHOPPING
Before you do, realize that battery boxes have to be FAA approved, unless your application is an experimental aircraft. We thumbed through catalogs of several mail-order parts suppliers and found plenty of replacement boxes, but few that were approved for certified aircraft.
As one example, mail order supplier Aircraft Spruce sells a fiberglass box for around $110 and an aluminum box for around $190, but these are for experimental aircraft only.
You could, of course, fabricate your own and have it signed off as an owner-fabricated component, but unless you’re real handy with sheet metal work, we think building a system with appropriate venting and drainage would be difficult. Keep in mind that not all models share the same design.
Some don’t even utilize an actual battery box, but instead a battery tray, battery lid and vent tube. If the aircraft is type-certificated with a box, you’ll need to swap it with an appropriate FAA-approved replacement that meets or exceeds the original spec.
We talked with a couple mechanics who told us that sourcing many OEM battery boxes is either ridiculously costly or impossible. For some Cessna models, an OEM replacement box can cost over $3000 and has a long lead time. Unanimously, shops prefer the aftermarket battery boxes made by Bogert Aviation in Pasco, Washington.
Bogert has FAA- and PMA-approved replacement battery boxes for Piper and Cessna applications and is currently working on approved solutions for other aircraft. The company’s latest boxes for older Cessna 180- and 182-series applications have provisions for venting, improving upon the design of the original housing, which doesn’t have venting.
The retrofit includes a venting system that connects to the battery box. The system requires drilling through the belly of the aircraft for mounting the inflow and outflow venting hardware (forward and aft scoops) for fresh air to enter the box and for air to escape. The kit comes with a drilling template and detailed instructions for performing the mod.
On Cessna 150-and 172-series aircraft (where the battery boxes are mounted on the firewall), the installation process is slightly different. The retrofit requires drilling a single hole in the rear pressurized air inlet baffle of the engine. This is to accommodate a small air pickup tube that mounts directly to the baffle, while the other end of the line connects to the battery box.
Both venting modifications are blanketed under the STC of the battery box. It has instructions for continued airworthiness (ICAW) and requires an FAA 337 form, but doesn’t require a field approval.
Bogert Aviation’s Richard Bogert—an A&P, IA and the product designer with over 30 years of practical experience—told us that the modification can generally be accomplished in a couple of hours. Shops we spoke with praised the retrofit for being easy to accomplish.
Aside from venting, we think the Bogert box design is superior to most of the original equipment battery boxes we have worked with, and other technicians we spoke with agree. For example, the main battery enclosure and the drain tube fittings are made of stainless steel, and unlike many OEM boxes that are soldered together, Bogert boxes are assembled using a TIG arc welding process and finished with powder coating. The stainless steel vent tube housing is welded to the battery box and attaches to a threaded, adjustable copper fitting on the outside.
If the battery box is in serviceable condition, Bogert offers an STC-approved rebuilding kit (developed and approved back in the early 1980s) that modifies the existing design of OEM battery boxes for the better. This includes new nylon flange shorting guards, plus replacement battery cables that replace the braided straps and ground terminals known for high-resistance connections.
Given the cost of a new Bogert box, which can range from around $800 to over $1800, depending on the application, rebuilding an existing box could be a better option if the housing isn’t broken or corroded.
The STC’d Bogert replacement for a Piper Warrior (PA-28-161) is $1819, while a model for a PA-24-260 Comanche is $1286. Given the quality, it’s probably the last battery box you’ll ever have to replace. Our advice is to ask your shop or mechanic to put a close eye on the existing box during the next annual. In many cases, proactive replacement is cheaper than dealing with the damage a failed one can cause. It might even solve other issues.
Visit www.bogertaviation.com, 509-736-1513.