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.