[colinu]

I was cleaning/detailing the inside of my son's e92 yesterday, left the doors open for most of the afternoon (i.e. interior lights on), went to start the car.... nothing. Not even the doors would lock/unlock.
Put a battery charger on it (under the hood)... charger reads 5V, try to charge it but after a few seconds the charger turns off. If I put a voltmeter directly across the battery it's 5V, but if I remove the negative there's 12V across the battery terminals. Sounds like a short to me, but can't think of anything that would have done that while I was cleaning/detailing the interior.
Any ideas what could cause the electrical system to be dragged down to 5V?

Other background:
Battery is 5 years or older, but has had no problems to date.
When I first went to charge the battery I popped the trunk, removed the negative terminal, then remembered "this a BMW, charge it from under the hood". So I reconnected then tried charging from under the hood. Is there anything around the negative terminal that I could have 'messed up' that would cause this 5V issue?
Battery charger is good (I verified it on another vehicle).
Hoping to "steal" my daughter's e82 later today and swap batteries to rule out a "misbehaving battery".

[gbalthrop]

I've never seen a battery do what you describe, but here is what I'd suggest:
1) Disconnect Negative Battery Terminal from Battery Post; That is to protect IBS on (-) Terminal & also isolate Battery;
2) Measure Battery Voltage with Multimeter;
3) Connect charger to Battery (+) post & (-) Post;
4) Do NOT close trunk with battery Terminal disconnected.
5) Charge Battery, or attempt charge. If meter on charger & charger behave normally, there was some issue with car system (such as KL 30g-f);
6) Remove charger if charges normally. Ensure Voltage is ~ 12.5V 15 minutes after removing charger (surface charge dissipates);
7) Reconnect Battery Terminals, (-) Terminal LAST, & now measure Voltage at Jumpstart terminals under Hood, as well as at battery Posts.
Please let us know what you find,
George

[colinu]

OK, problem solved. It was a bad battery. Troubleshooting via swapping in a known good one from an e82... no problems, 12V as expected, car started up up.
Tried charging the e92's battery on the bench... charger measured 41% battery capacity but would not charge any higher even after 2hrs. Bought new battery, fitted it, all is well
Really strange though, I've never seen a battery behave this way before (pegged at 5V when in-circuit, but 12V with the ground cable removed). Got to assume a cell went bad.