Originally Posted by DanH
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.
That seems like a reasonable way to deal with the problem. Presumably you also have a way to check the backup battery voltage during preflight to make sure it actually has a charge? (the fusible link won't let you know when its blown)