Battery Grief

[Clarksonvalley]

The car gods must be angry. I replaced my 10.5 year old BMW battery with a new BMW battery (this thing weighs a ton). Everything was fine again until the next day - now nothing electrical works. Does the BMW security system not like its power shut off and, thinking thieves are at work, disable the power from the battery? I have not driven the car since replacing the battery as I am reworking the leather on the drivers seat and the seat is out of the car. Need help here.

[Clarksonvalley]

I did not think that a new battery could be bad, but I just checked it - 1.026 volts between positive and negative terminals. Could there be something in the car draining a fresh battery in one day? I have all interior lights switched 'off' at the respective switches and all doors are closed anyway. And yes, the headlight switch is off - key is not in the ignition. Any ideas?

[whiskychaser]

Interior lights left on? Car 'not going to sleep'? Trickle charge the battery and hope you can bring it back to life. If you can, connect up and start looking for the circuit which is dragging it down.

PS you will know it isn't going to sleep if your window switches are still illuminated 20 minutes after you remove the key.

[Clarksonvalley]

I've got the battery tender on it now. I have the center console and the driver's seat out for the Leatherique work, so maybe I did something taking some switches out. When I get it charged, I'll put an amp meter on it and start taking fuses out - there is a drain going somewhere.

[Clarksonvalley]

I have 11.19 volts on the battery now - after recharging with the 'Battery Tender', the current flow is 0.19 amps. Interior lights are not on. Is 0.19 amps enough to cause the battery to drain overnight?

[whiskychaser]

I have 11.19 volts on the battery now - after recharging with the 'Battery Tender', the current flow is 0.19 amps. Interior lights are not on. Is 0.19 amps enough to cause the battery to drain overnight?

11.19v is only 40% charged. So if you have, say, a 70aH battery, it should in theory take 147 hours to go totally flat. I'd still want to know where the drain is though

[Clarksonvalley]

OK - I used my 'clamp-on' amp meter on the positive red lead from the battery and got a 0.45 amp current flow. Removing the fuses one by one from the under hood fuse box did not make any difference in the amp reading.

[Clarksonvalley]

The battery is a 90 AH unit.

[Clarksonvalley]

How do you figure a 11.19 volt battery is 40% charged and how did you arrive at the 147 hours to drain it?

[whiskychaser]

How do you figure a 11.19 volt battery is 40% charged and how did you arrive at the 147 hours to drain it?

You can check what voltage you have against a battery charge table to get the percentage. A fully charged battery is 12.6 volts. Which is handy because the acid in it has an SG of 1.26. I only mention this as I prefer to us a hydrometer if possible. Unless you let the battery stand for a few hours before you took that reading, you will probably have less than 11.19v.

If you have a 90 aH battery that will give 90 amps for an hour. If its only 40% charged you only have 36 amp hours. 36/0.45 = 80 hours. And then it is completely dead. It may not be a linear as that but it gives you a good idea. IIRC, Bentleys says you are OK with a 500mA discharge but I have to say that my old E34 didn't have any. Neither does my E46. Actually, we have two of them and neither of them do.