On Tuesday, May 17, 2016 at 1:56:12 PM UTC-7, wrote:
I switched to a K2 Energy LiFePO4 battery and charger three years ago when I installed Flarm and updated my flight computer/variometer. I have had no problems whatsoever with either the battery or charger. It seems to have more real world life on a charge than the same sized SLA/AGM batteries I was using and as a bonus it's also lighter. I can only accommodate a single PS1270 size battery in my glider so it was nice to find that the K2 could handle demands of a six or seven hour flight with no problem even with the additional avionics I installed.

As for FAA certification (or EASA, Transport Canada etc.) the only battery I can think of with regulatory authority approval for aircraft use is the MERT PS1270 sized lithium-ion unit which claims to be JAR certified. I think it costs around 500 Euros. The batteries I've seen come with brand new gliders seem to be plain commercial-standard SLA/AGM units anyways.

Been using mine, with the K2 chargers, for over 3 years. No problems. I fly with 2 of them, some flights up to and over 8 hours, never come close to running out of juice between the two powering Flarm, radio, transponder, ClearNav flight computer and vario.