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Okay folks, this is an electrical issue, but not for a stovebolt, so bear with me please. My son has a '78 BMW that we are trying to get running. For whatever reason,with the ignition off, the battery light is staying on. He has disconnected the positive terminal at night and by morning the battery is dead. Thought it might be a bad battery, so we bought a new one. One night with the positive and negative terminals disconnected at the battery and by the nexy morning it was once again dead. I can't figure out why the battery would drain with no posts connected. It has a new alternator as well, so I don't think it is a charging issue. Besides, he isn;t driving it that much and when he parks it and then tries to start it before disconnecting the battery for the night, it will start right up. This one has me stumped. Anyone out there have any ideas? And what might be causing the battery light to stay on with the ignition turned off? Would this be able to somehow drain the battery when the battery isn;t even connected? I'm guessing not. By the way, the battery light doesn't stay on when the battery is diconnected. Thanks for any insight.
Rick

I was going to say Gremlins are doing it.But it sounds like your hood might be grounding out the battery when you shut it,also try putting a rubber pad under the battery, if thats not it I don't know.

I would think most of us would agree with you...it doesn't seem possible. So, a few questions. Are you sure it is the battery that is discharged? How do you know for sure? Can the engine have a problem that makes you think that the battery is discharged? A clue that something else is amiss may be the light staying on. Keep us posted as we love these electrical puzzles.

Rick,

I agree with the other folks, unless there was some internal construction malfunction, the battery cannot discharge with the cables unhooked.

That statement isn't accurate if there was a battery holdown frame or assembly that somehow could touch or rub up against the posts. Because of the huge amount of current available in a fully charged battery, any short would have to be small and of a high resistance or the fire department would paying you a visit.

As Beltfed indicates, there is more to this story than has been revealed. How much is a collect call to Bavaria?

Stuart

Interesting problem, some scenarios I would explore:
1. Check the battery by putting a decent load on it before reconnecting to be sure it is discharged. If it is I would think about the possibility that the charging system is not actually working (especially since you mentioned a new alternator). What may be happening is that you are running on the battery and at the end of the day there is not enough to start it again. Although though you did mention that it starts if you try to start it right after turning it off, that might mitigate against this scenario.
2. Do you have an alarm system or something that must be reset when the battery is disconnected or discharged?
You could do a Google search on this, with enough patience I have found lots of this type obscure problems/resolutions for my wife's BMW. Good Luck!

Next time try unhooking the power feed wire to the alternator, that should kill any drain it may have. A clock, radio, computer, and alarm all pull power with the key off, but not enough to kill any decent battery, check the dome lights, trunk light, and under hood light.
Alternators are the only items big enough to drain a battery and still not catch fire. A starter might if the relay was stuck in, but it would be very hot to the touch and you would hear something from the starter.
one more thing, crappy cables will fool you, they can act like a dead battery. Check the connections good, at both ends!
Joe

This one would be funny if it wasn't so imperative to get my son's car running so he can work Monday. I learned that someone connected battery cables backwards to jump him a week ago and there was a fried ground wire near the battery (all of the insulation was burned off so it's a wonder he didn't have a fire to deal with). When I say the alternator is new, it's about a year old, but the car has been sitting for almost all of that time. I will run a test on alternator output and also check battery voltage. I'm still stumped about the dash light that is for the battery staying lit with the ignition off. I thought it might be that the wire for that light was connected to a hot lead on the ignition switch rather than an accessory lead, but my son said he checked that already (I'll check it again in the morning). To answer someone's question, there is no battery holddown bracket and no alarm system. No indication of an under-hood light staying on and with the battery disconnected, this couldn't be happening anyway as far as I know. The battery cable inspection is a good point and I'll check that out as well. Something is drawing down the battery somehow. If I had to guess, I'd say it almost has to be in the charging system. I'll keep you all posted. If anyone can think of something that could have caused this as a result of jumping the battery with the cables attached wrong, please let me know. My son says last time he drove it, he wasn't having these problems, so it would seem there could be a relationship between the jump start and this problem. Could it have done damage to the coil or reverse polarized the alternator and would something like that display symptoms like what we are dealing with? Thanks folks.

Could have a fried diode in the alt. Try disconnecting the Alt. and see if the discharge light goes out.

Rick,

The obvious suspect would be the alternator. Attaching the battery cables backwards for the jump start could have damaged the solid state internals of the alternator.

There are diodes groups that change AC to DC that could have been shorted or blown and there are diodes in the solid state regulator that may have been damaged as well.

I think a blown regulator could be responsible for the dash light always staying on, and this would indicate reverse current flow which means the battery is discharging through the alternator.

No polarity was reversed, and the coil is most likely fine. Keep in mind you might get a clean bill of health on the alternator if you take it to a big box parts store. Their tests aren't really sophisticated enough to catch all the fatal flaws.

A good auto-electric shop knows exactly how to give your alternator a comprehensive check, and would probably pin point the problem in moments.

Good luck and keep us up to speed on this.

Stuart

I remember once when i had bad alternator on a old Oldsmobile the battery lite stayed on... therefore it could be the Alt..... Peter

The problem is resolved. Thanks to all for the input. Stuart, you called it. The light circuit in the alternator was shot as a result of hooking the jumper cables up backwards. I was able to track down a used alternator at the local Pick A Part and when we hooked it up, the battery light stayed off when the key was off and the alternator output is 13.69 to the battery, so it is charging the battery as it is supposed to. I took the alternator down to NAPA and their test machine showed a bad light circuit right off the bat. $35 for a used alternator at Pick A Part vs. $120 for a new one. Thanks again for the help guys. Rick

That cousin of mine Stuart knows everything... Peter..