The place for technical discussions about GA and flying.
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By Shoestring Flyer
#1821193
Dino wrote:Weight is indeed the main reason for considering the change: the current Concorde battery is 22lbs, Something like an Earthx 680 is 4lbs.
I'm not sure how to decide on capacity and am wondering if i could get away with something even smaller like a Varley Li5 @ 2.5lbs.


You need to do an electrical load analysis of the total normal operating and potential maximum current draw of your aircraft to determine which battery size/make you need before comitting to purchase.
Things like current draw of pitot heat is one to watch out for ususally taking 10amps or even more.
Have a read of the LAA doc that aerofurb has posted.
By Peter Kelly
#1821298
Shoestring is quite correct. Though for VFR flight in a Permit Aircraft there is no requirement to even have a battery (unless it powers primary flight instruments) and thus no requirement to do a load analysis.

Would a load analysis be sensible for VFR - definitely yes.

IFR is a different story and a load analysis does need to be performed as part of the IFR approval process. This is not so simple to do (or very simple if you have individual battery back ups of all critical avionics). I have done quite a few load analysis and can help you if you wish.

One interesting thing about Lithium versus Lead-Acid battery capacity is that you get more effective load x time out of a lithium battery for a given quoted capacity than you do for Lead Acid.

The Concorde RG25XC (25Ah nominal 22lbs) will give you 30A for 45 minutes (when new). The EarthX ETX900 (nominal capacity 15.6Ah 4.9lbs) will provide 21 A for 45 minutes. The ETX 900 will provide more cranking amps...



[For the real battery-geeks amongst you, Odyssey are the naughty boys with quoted battery capacity - they quote a capacity based on a 20-hour rate and everyone else quotes based on a 1-hour rate. In simple terms the Odyssey battery has less real-world endurance than a similarly sized battery from other manufacturers. They are still really excellent batteries.]