From here
Wanted to fly to Bielefeld to pick up the traditional Christmas mug before the front came through but after climbing to 2400 feet, the backup battery failed – showing 2v, it wouldn’t charge. Tried all different things but realising that if the main battery failed, the engine would stop, I decided to turn back and investigate the issue further whilst on the ground, rather than in the air. Above 2000 feet the air was pretty smooth, below it, it was extremely lumpy, especially in a light aircraft such as mine; plus the wind had increased in anticipation of the front coming through – 14 knots direct cross wind but it was one of my smoother landings – must be getting pretty experienced performing them in the new steed now 
Landed, opened the cowl, the battery is well and truly dead – possibly age, because I’ve not specifically forced it to be overcharged or so… Ordered a new one which hopefully will arrive Tuesday so that I can fit it ready for next weekend where the trip to Bielefeld is still on the agenda. Considering I’d just had the battery relay changed because I believed it wasn’t properly charging the battery, I’m now considering the failure to charge was more likely due to the backup battery being on it’s way out….
Steve6443 wrote:
the backup battery failed – showing 2v, it wouldn’t charge. Tried all different things but realising that if the main battery failed, the engine would stop
Interesting… Steve would you feel comfortable sharing the type of your aircraft?
if the main battery failed, the engine would stop
Same on mine… dual LSE on a 6.9i.
Shouldn’t the alternator keep the electric ignition going even if all batteries are dead?
Yes, if…
Thing is, there is a problem if one’s main battery is dead. Might be the alternator not charging, or that bus having been switched off due to some other electrical unpleasantries like a short, etc.
One will then fly on the backup batt, which in my case is connected directly to one of the ignition systems.
>>if the main battery failed, the engine would stop
Same on mine… dual LSE on a 6.9i.
Wouldn’t you have an alternator still in case the battery failed?
Maybe, but probably not. The question really is, why is the main battery down. It could be because the alternator doesn’t work in the first place, then this non-working alternator couldn’t be used to provide the juice for the system. Another scenario would be some kind of problem, e.g. a short in the system. One would switch the electrical system off, which on my ship would also cut the power to the excitation field of my alternator.
As usual, all of this is very much installation dependent.
Dan, thanks for patiently answering my question twice :) I didn’t realize my original question was put into a separate thread and thought it simply didn’t get submitted :)
Aren’t there magnetos to keep the engine running?
Rarely nowadays, in homebuilt aircraft. And never in any Rotax-powered aircraft.
