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umgibso1(at)yahoo.com
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 Posted: Mon Sep 04, 2006 7:23 pm Post subject: Debating on an alternator or two batteries? |
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Hi Group, my name's Tony Gibson, I've been a lurker on the list for almost a year now and am building a Sonerai 2L - read 550lb slightly underpowered two place! 
I'm trying to take as much weight out of the plane as I can by keeping it simple. There's a lot of reasons I'm considering two batteries rather than a battery and an alternator. But saving a bit of weight isn't the main reason, the fact that I can move the weight of battery where ever I want in the plane is a big bonus for servicing it nevermind balancing, and ....the last thing I will do is put lead weight back into it!
I have an ignition system that draws ~1 amp and a single fuel pump that draws another amp. I decided against the starter and the only other amp draws will be two small Stratomaster instruments drawing less than half an amp together. Total draw would be less than 2.5 Amps
With the right warning system to indicate a low primary battery I'm wondering if something like a 3 - 5 Ah battery would be large enough for a backup? What about the primary?
The downside of course is what would I do on a crosscountry trip? Argh!
Thanx a lot, appreciate any help and opinions!
Tony
[quote][b]
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klehman(at)albedo.net
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| Posted: Wed Sep 06, 2006 7:01 pm Post subject: Debating on an alternator or two batteries? |
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Hello Tony
Personally with an electric dependant engine, I'd happily trade 4 lbs
or so of battery for a small permanent magnet alternator and I suspect
the alternator would be the lighter option if you have a few hours of
fuel on board.
Bob just posted some references on battery discharging that you will
want to look at.
For a primary battery with no alternator I'd suggest multiplying your
hours of fuel by 2.5 amps and then picking a battery that can supply
that (from its spec. sheet) and then doubling it. Another battery
characteristic is that the deeper you discharge them, the fewer
discharge - charge cycles they last so planning for a 50% discharge when
the fuel runs out might be reasonable. That would also allow for other
sub optimal conditions such as temperature.
Ken
Tony Gibson wrote:
> Hi Group, my name's Tony Gibson, I've been a lurker on the list for
> almost a year now and am building a Sonerai 2L - read 550lb slightly
> underpowered two place!
>
> I'm trying to take as much weight out of the plane as I can by keeping
> it simple. There's a lot of reasons I'm considering two batteries
> rather than a battery and an alternator. But saving a bit of weight
> isn't the main reason, the fact that I can move the weight of battery
> where ever I want in the plane is a big bonus for servicing it
> nevermind balancing, and ....the last thing I will do is put lead
> weight back into it!
>
> I have an ignition system that draws ~1 amp and a single fuel pump
> that draws another amp. I decided against the starter and the only
> other amp draws will be two small Stratomaster instruments drawing
> less than half an amp together. Total draw would be less than 2.5 Amps
>
> With the right warning system to indicate a low primary battery
> I'm wondering if something like a 3 - 5 Ah battery would be large
> enough for a backup? What about the primary?
>
> The downside of course is what would I do on a crosscountry trip?
> Argh!
>
> Thanx a lot, appreciate any help and opinions!
> Tony
>
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