Dad of a daughter with an 150,000 mile R56 Mini Cooper 6 speed here. Since buying the car used with 90k, I've replaced the timing chain and guides, a water pump, the thermostat housing (damaged by the used car dealer when steam cleaning the engine I think), changed plugs, all to say I've been able to fix all issues to date. She was driving the 100 miles home when the battery voltage dropped, and the charging indicator lit. We were able to charge the battery and drive it home to the garage. The battery was a little over a year old and holds a charge, so after measuring some voltages, I (probably incorrectly) diagnosed the problem as being the alternator. Ordered a rebuilt Bosch, and have replaced it - with the mileage probably a reasonable thing to do regardless. No joy after the replacement, so further digging found a blown 20a fuse in the cabin. I still haven't completely figured out the fuse symbols, but I'm thinking it was the alternator fuse. Still no joy, and the fuse didn't blow a second time. Checked connections and the serpentine belt, all good. Did some searching here and discovered that some R56s have the IBS battery cables - this one does not. Connected my scanner and cleared several faults related to the low voltage. They haven't come back after several starts so long as the battery voltage stays above 12v (which I've maintained with an external trickle charger). No lights no faults, but no charging either.

Interestingly, if I disconnect the small signal wire, the voltage jumps to 14v at idle. Did some measurements at the end of the disconnected signal wire and it shows 12.3v at idle and actually decreases to 12.1 after turning on headlights.

Does anyone have any clues here - I didn't reset the battery (don't know if I can with my LaunchTech scanner) but given the plain ground cable don't think I should have to.