840 ci csi and i batteries
I hope this helps you out there are 2 batteries in this vehicle one is a main the other is a backup or thats how the service manager told me. Either way I know the 840 series has 2 batteries opposite sides of the trunk. There is a red wire on the passenger side battery that is bolted on it's the hot from the backup battery. The bad part is the bad battery, if you have one, kills the other battery by draining it down. I've been victum of this more than a few times including having a dead car after 2 hours of the car being off. So please have both batteries checked seperatly by checking the main battery with the backup disconnected then checking the backup while it's disconnected. They can check for current drain then off the main line which should be near zero; with my interior lights on it nears an amp with the ignition key on and nothing running it nears 10 amps! so be cautious that it might be a fault somewhere thats not seen. This includes a bad or corroded 60 amp fuse as I found out myself lastnight, the alarm went nutts and woulden't disactivate and after I got the car running I had no interior lights/window control. This car has a lot of electronics anything can malfunction at anytime and the central locking system will start disabling functions with a low battery the alarm receiver is the first thing, I found out windows are the second thing. to check for a bad or corroded 60 amp fuse the fuse is located by following the black wire comming off the main batteries positive post it's bolted on as well just unbolt it; but a word of caution nothing will work so don't close the trunk while cleaning it. Another hint is take out the bottom green relay in the trunks drive side it controls all lights and will eliminate a light remaning on somewhere as the culprit. Best of luck, bonze82.