naturist wrote:
As others have said, a fully charged battery is only going to read 12.7 or so, and that's only with an open circuit, i.e., nothing connected to it. Either your battery is bad and not able to deliver power in spite of reading fully charged (I had one do that once), or you have a bad/high resistance connection or broken wire somewhere.
Well said. I would generally tend to suspect the connection first, as even a not very good battery often (in my limited experience, at least) has enough of a surface charge to maintain voltage for a short while, but either one is entirely possible.
Do not neglect to verify that the ground connections are tight and sound, both at the battery and at the furnace (however it connects to ground). Also check that the wires are not corroded within the connectors, particularly at the battery connections; I've seen a few terminals that appear nicely crimped or clamped on the wire, but with the wire turning to dust at the other side within its insulation.
