How much battery does a day VFR plane need?

[bifft]

So, I've built my 8A as a day VFR only ship with no lights and no gyros. Just about everything I find on sizing batteries seems to be about keeping your all electric panel alive while you fly home on a dark and stormy night. After starting the engine, I all need the electrical system to do is talk on the radio and run the flaps. Could do without either in a pinch. I figure the starter is the only serious draw on the battery.

How many amp-hours does the starter need? I don't find any good numbers for the starting current, everything just says hundreds. 300 amps for 2 minutes (should be enough for several start attempts) give 10AH. Would that be enough? Any one have experience with this kind of minimal system?

I could just get the PC680 everyone recommends in past threads, but wondering if I can save a few pounds and dollars.

[Bevan]

I recently changed to the 18AhEq Shorai Lithium battery to save weight (total battery weight is now about 2.2 lb). I can sit in the plane for well over an hour fiddling with the EFIS settings running on just the battery.

Oh, and this little battery starts the injected O-360 with gusto.

Bevan

[Mich48041]

I figure the starter is the only serious draw on the battery.

You are right about that. Once the engine is running, the alternator supplies aircraft power, not the battery. So for your mission, any battery that will start the engine is adequate. AGM batteries such as the PC680 have lower internal resistance than older flooded cell type. Therefore an AGM battery can be smaller than a flooded cell battery and still start the engine.
It is hard to say exactly what the starter current is because it all depends on many variables like battery size, wire size, resistance of terminals and relay contacts, type of starter motor, engine size and temperature and compression.

[rzbill]

Sounds like this plane is screaming for a single 8 amp vacuum pad PM alternator and no belted unit.

[bifft]

Originally Posted by rzbill

Sounds like this plane is screaming for a single 8 amp vacuum pad PM alternator and no belted unit.

Exactly. Already have that installed.

[Larry DeCamp]

I hope the SD 8 Dynamo is adequate because that is what I chose for the same mission. However, I was recently surprised to learn that a master contractor draws 1+ amps. That said, with duel Pmags and mechanical pump, you can run the tanks dry with the master off.

[Mich48041]

According to Bob Nuckolls' post a Cole-Hersee contactor will draw 1 amp cold, then drop to 0.6 amps when warm.

[bifft]

Thanks for the replies. I hadn't considered lithium, figuring it would be too expensive but the prices aren't bad. Spend a few dollars to save a few pounds. Do worry about fires, but lead-acid isn't explosion free either. Have a few weeks yet to make up my mind. Want to get this last so it is fresh instead of sitting on a shelf for who knows how long.

[jclark]

Originally Posted by bifft

Thanks for the replies. I hadn't considered lithium, figuring it would be too expensive but the prices aren't bad. Spend a few dollars to save a few pounds. Do worry about fires, but lead-acid isn't explosion free either. Have a few weeks yet to make up my mind. Want to get this last so it is fresh instead of sitting on a shelf for who knows how long.

You might want to double check the output voltage of the SD8 and make sure that it is good for the charging of one of the Lithium based offerings.

Lithium offering plus maybe one of the more powerful "alternator" replacements for the SD8 might be better. Clearly not cheaper, but probably better.

James

[Bevan]

When I say Lithium, I mean LiFePo Lithium. It's really the only formula to consider in my opinion and probably the most common now anyhow. Mine is Shorai 18AhEq and am very happy with it so far.

Bevan