Following might explain a few of the electrical "mysteries" that get posted here from time to time.
Stopped at the market after my Sunday "desert romp." Came out and no electricity--nada. Opened the door with the key, then called AAA and got a jump. On the way home, the dash lights & HUD flickered off when I'd hit a pot hole. Voltage was all over the place, varying from 13 to over 15.
Back in the garage it started OK. After supper, tried to start it and move it to the barn for the week, "dead" again. Battery was putting out >12 volts with no load. Any load dropped it to zero but the green "tell-tale" showed battery was OK. Suspected the battery cable. It was OK.
Found a replacement battery (thank you PepBoys). When I pulled the old battery, found that the positive terminal had been severely overtorqued. (Don't know if this happened at BG or maybe at the original dealer if these cars are shipped with the batt disconnected.) Sufficient "wrench" broke the whole case around the positive terminal, exposing an old torsion crack in the lead stud inside the battery case. It had been oxidizing for years. Battery itself was in great shape.
Since I bought the car last year I've had strange dimming and brightening of lights at night (looked like poor voltage regulation.)
Went ahead and commuted with the new battery on Monday and Tuesday. No voltage regulation symptoms.
-- PJ '89 Hookercar '02 e-blu coupe