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Batteries can:
1. Corrode at the terminals, making it look like a higher-resistance circuit. This can cause all sorts of problems if it happens on the negative terminal because it can look like a bad ground, but either way it's a problem. Visual inspection during pre-flight can rule this out although early corrosion under the terminal isn't always visible.
2. Develop dendrites/pitting on the electrodes inside - this is how the battery "ages". This reduces capacity over time. This isn't always obvious and it can be dangerous. It might show 13V at power-up, and have just enough "oomph" to get the engine running, but then take a long time to fully recharge. Terminal corrosion can worsen this. While the engine is running the alternator is too, so everything looks fine. Then you have an alternator failure, the battery may not be charged enough to take over and you may be unable to restart your engine and/or lose other electronics.
3. Be installed incorrectly. You won't experience this in flight. You'll know on the ground when you see the smoke.
4. Come from a bad manufacturing lot, see #2 above.
5. Internally short out one or more electrodes. Usually a function of aging. This is more obvious because it reduces the battery's effective voltage even when fully charged.
6. Crack/leak/vent dangerous fumes. This depends on the type of battery you have. RG "glassmat" batteries don't tend to do this, although it's not impossible if they're overheated.
7. Come lose during an emergency-landing (rough landing?) event if not solidly secured. They're very heavy and this basically makes them a missile. A fiberglass bulkhead probably isn't enough to stop it. Strong securing straps with bulkhead attachment points can prevent this.
There are more things, but not as likely. For my Cozy MKIV, I put a pair of batteries midships, behind the pilot/co-pilots' seats. This gets some of the weight forward and reduces the amount of nose ballast I'd need. It also allowed me to eliminate the relay-style contactors and use a manual battery switch, reducing overall current draw during a failure event (and a bit more weight). I went with a pair of identical Panasonic 20Ah RG-style batteries (Digikey sells them) which have a decent power/weight ratio and the RG technology allows them to be installed in any orientation, and you don't have to worry about them venting acid or fumes (or refilling them). Finally, rotate them at every annual - buy one "new" battery, rotate battery A to battery B, and repurpose/recycle battery B. Take off on "Both", allow to fully charge, then if you really want to be crazy about safety you can fly on just one (B) so an alternator load-dump overvoltage event only has the chance to try to fry one of the two (most batteries can handle this better than electonics, though, and you should have overvoltage protection in your design) and also so you have one known-good battery fully charged and always at the ready in an emergency.
Opinions vary.
Widely.
My opinion: somebody (maybe Al Wick?) once said that lacking any other data about reliability, a decent rule of thumb to see how reliable something will be is to look at the aftermarket for the part. You can find thousands of alternators and batteries online, in stores, etc. You can even find places that rebuild alternators - cheaply. Walmart has racks and racks of batteries. So treat it as something that some day will probably let you down, and plan to deal with it. Redundancy is one option. Maintenance is another. Plans B and C are sometimes used in experimental aviation, so have them. =D
Regards,
Chad
On 7/16/2012 8:14 PM, Stephen Izett wrote:
Hi Guys
I'm about to commence wiring my Glasair Super II RG and was wondering what battery's people are using?
What are the failure modes of batteries and does the risk profile require duplication?
Thanks
Steve Izett
Perth Western Australia
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