I've got a Wagan iOnBoost v8 (7502, I think, but that's been replaced by the 7503). Noco also makes good ones in the "Genius" line. And there's a lot of cheapies out there that might be pretty crap at spinning a 5l ICE, but are probably fine for providing 45 seconds of 12v to boot up the Volt's computers and trips some relays.