Battery Check!


Remember the old days? Back when batteries had caps you took off to check their fluid level? They had vent lines, and we worried about hydrogen venting and stuff like that – and really worried about having a battery that wouldn’t spill during aerobatics. Checking battery levels was a maintenance item as common as changing the oil, and more often if the weather was particularly hot or cold. Batteries were finicky, and you paid attention to them.

For many years now, we’ve been flying our homebuilts mostly with sealed AGM batteries that you bolt in place, connect up, and go fly. If you leave your master on, you charge them back up. In a few years, you replace them. There is little to check other than voltage – or just judging by how fast they will turn the prop over when you hit the starter.  At condition inspection time, there is usually an inspection item that has you check the battery – but since it doesn’t take any fluid, and you know if it is starting OK or not, you might not climb in an do a visual check.

Well, if that’s true, you might want to reconsider. This AGM battery is typical of those used by many homebuilders in the past few decades – a lawn tractor/motorcycle/jet ski battery that does a good job of swinging the prop when you need it.  It had ben in service and done a fine job for about four years, mounted behind the firewall in a metal box, and not exposed to extreme conditions. It had, very recently, begin to struggle to push the standard-compression O-360 Lycoming over – it would do it, but reluctantly.

We’re not sure what went wrong, but clearly, it was something massive – even though it was still doing its job in this condition. The real question is when it started to look like this!

So lesson learned – even though AGMs are just basic bricks, its worth a good visual inspection every year. Or maybe more often if you have the chance.

Paul Dye

Paul Dye, Kitplanes® Editor in Chief, retired as a Lead Flight Director for NASA’s Human Space Flight program, with 40 years of aerospace experience on everything from Cubs to the space shuttle. An avid homebuilder, he began flying and working on airplanes as a teen, and has experience with a wide range of construction techniques and materials. He flies an RV-8 and a Subsonex jet that he built, an RV-3 that he built with his pilot wife, as well as a Dream Tundra they completed. Currently, they are building a Xenos motorglider. A commercially licensed pilot, he has logged over 5000 hours in many different types of aircraft and is an A&P, EAA Tech Counselor, and Flight Advisor, as well as a member of the Homebuilder’s Council. He consults and collaborates in aerospace operations and flight-testing projects across the country.

5 Replies to “Battery Check!”

  1. I've seen batteries that look like this, and they all had been excessively overcharged. This can happen if the battery has an inexpensive battery charger connected and left for days. The open circuit voltage of these is 15 to 16 volts , which is way too high.
    The other situation is that the aircraft voltage regulator is defective or miss-adjusted.
    A voltmeter in the aircraft is critical to seeing that the battery is not overcharged, or undercharged. The normal alternator voltage should be 14.0 to 14.2 volts.

  2. JimH is right, but on a homebuilt I'm surprised an external component could even get to this state; a healthy paranoia on kit you installed yourself can be useful. In the busy cockpit (I fly weight shift) idiot lights are even more useful than a voltmeter, you can buy a big coloured LED that is easy to hook up to any 12v point and earth or negative, and glows red for undercharge, green for ok and flashing red for over cooking it above 14v. Mine came from an eBay seller in Scotland

  3. I put an Oddysey PC 680 battery in my new RV-9A in 2006 and replaced it in five years when my mechanic said it was weak on the load test. I put it in my motorcycle and it is still going strong. My buddy has had two of them in his RV-9A over the same time and the old one is in his lawn tractor. None of these show the kind of distortion Paul experienced.

  4. Your voltage regulator is not functioning. It is cactus, kaput, broken, not working. You're battery has taken on the function of regulator by absorbing the excess voltage of the generator/alternator. It does this by boiling off the electrolyte in the battery while still keeping the voltage down to about 14.2 volts. This causes excessive heat and pressure which has distorted the battery housing. If you do nothing except replace the battery, it will happen again to the new battery. If you let it go on until there is no longer enough electrolyte to maintain 14.2 volts. The voltage will rise quite quickly ( perhaps in only minutes) to whatever is being generated. Possibly 20 to 25 volts. What do you think that will do to every electrical component in you aircraft that is switched on at the time. What was the price of that glass cockpit again? Ask Jim Weir to explain this to you. The fundamental rule is, "DO NOT RUN ANY EQUIPMENT WHICH HAS HAD A DAMAGED BATTERY UNTIL YOU HAVE ASSERTAINED THE CAUSE OF THE DAMAGE" .Quite often the battery has served the function of a fuse or circuit breaker.

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