> -Voltage at the battery (when new) was ~12.7 , with car running it's
 > ~13.6, with car not running and inverter attached it's ~8.5

That sounds like the battery's had it - you'd expect the voltage to go
down when you attach a load, but that sounds like a lot for that
inverter.  I'm assuming that's without a load on the inverter?

Take your battery to your local FLAPS, they can load test it for you,
and tell you if it needs replacing.  At least, my local AutoZone can.

toodle pip,
--
        Dug Smith         | Terry Pratchett: It's almost
(mailto:dug@dugbert.com) | impossible to ride a rock and roll
(http://www.dugbert.com) | motorbike and stay on for three verses.