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Old 02-04-2017, 07:49 AM
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DanH DanH is online now
Join Date: Oct 2005
Location: 08A
Posts: 9,793

Originally Posted by Alan Carroll View Post
I asked Kathy at EarthX about this and she pointed out that their batteries are limited in maximum allowable charging current.
Also true of an ordinary AGM battery. Recently a battery manufacturer told me a good rule of thumb was 10% of the Ah rating for normal charge, perhaps 20% for a fast charge. My secondary ignition battery is 3.8 Ah, so we're talking milliamps.

I noted that in a backup capacity the battery would normally be at full charge and thus would likely never pull anywhere close to 20 amps. Kathy nonetheless felt that the backup battery should have a charging current capacity equal to or greater than output capacity of alternator that was charging it.
True, in normal operation the charge rate is almost non-existent. The risk is an alternator driven recharge after the little battery is discharged by leaving the switch on while parked. That's when discipline will require some controlled charging time before flight.

An alternate plan might be to add current limiting. The ignition pulls a few amps at most (varies with brand), so a quality 15 amp fuse or breaker in the feed to that ETX12A should ensure it can't be whacked too hard. Mine has a fusible link, a really reliable form of short circuit protection, but not real accurate for current limiting.
Dan Horton
Barrett IO-390
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