That's like my kitchen light over the sink. Drove to Home Depot and got it a nice new flourescent tube. Drove to Home Depot and got it a new starter thingie. Drove to Home Depot and got it an entire new fixture and fixed the crummy wiring hookup. Then drove to Home depot and got it the new light switch it needed. Seems on humid days, the light switch didn't work...
I'm not sure you have this fixed yet. Do we know how the voltage is being regulated? Another possible failure mode would be the alternator is full on and killing the battery. Or it might be that it has had to start the motor many times and just wore it out. THat's about the smallest battery that could start the motor but it will probably only do it hundreds of times not thousands. Hundreds of times lasts a long time on my formula car, but maybe you've done a lot of debugging with your motor setups? I think I saw a spec for 400 heavy load cycles in the PC680 description.
Here's some more failure modes. If you do not have a "very good" connection from the battery terminals to the cables and the cables to the starter motor it will do this failure where it just clicks when you try to start it. It will run and show the voltages you see but when you ask for a few hundred amps of inrush current when you first hit the starter the voltage will drop enough that it won't start turning. Oddly, this can come and go a bit. After I mounted the battery in the trunk of my '78 Fiesta, I had to buy a new starter motor every year.
You can see the bad connection thing happening with your meter. You will have voltage on the battery terminal, but not on the cables or the starter motor depending on where the problem is. Connections for hundreds of amps must be really good. In the old days with post terminals you would have to clean them every once and awhile because of oxide on the terminals. The spec for OEM ECUs is to ride out a 50 mSec. voltage drop to 5V during initial starting, that's how big the load can be to get the starter motor turning.
Hmm, I forgot the other thing I was thinking of. Keep using your voltmeter!
