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Date: Mon, 16 Jul 2012 21:22:29 -0400
From: Chad Robinson <crobinson@medialantern.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
CC: Stephen Izett <steveizett@me.com>
Subject: Re: [FlyRotary] Battery
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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
>
> --
> Homepage:  http://www.flyrotary.com/
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