'91 535iA battery keeps dying every 3 months!!

[caposto]

This one has got me baffled. Its my dad's car and I've gone through 3 new batteries from PepBoys. They are Eveready batteries size 49D. Each time it goes dead, I take it in and they do a load test, fully recharge it and it tests out fine. I reinstall the fully charged battery and it lasts a few days then dies again. Jumping it and running the car for an hour seems to charge the battery a little bit but its dead the next day.

The really weird part is that when I take the battery back an exchange it for a new one, it lasts about 3-4 months then its the same thing again.

We've taken the car in and the charging system checks out fine. My only guess is that its an intermittent problem probably with the alternator (brushes, regulator, ??)

Any ideas would be appreciated. My dad is so frustrated he wants to get rid of the car which I can't bear to see happen.

-Chris
'88 535is
'73 2002tii

[632 Regal]

could be a bad diode in the alternator, the diode is a one way gate that allows power to go to the battery when charging. sometimes some damn kids poke holes in the gate and the power pours back through running the battery back down. I would concentrate efforts on the alternator.

[SRR2]

Could be a bad diode, I agree. Could be a bunch of other things too, like a stuck-on light in the trunk or glove box. You can troubleshoot this. Take a fuse and cut the element in it. Solder on two leads and hook them to an ammeter, preferably one that autoranges. One by one, pull out the fuses in the fusebox and substitute the test fuse. One of them will probably show nontrivial (i.e. more than 20 or so milliamps) current draw. This will tell you where to look for the problem.

[KeithW '91 535i]

putting out? I had similar problems every 3 days or so and my alternator was putting out 30 amps at 2000 RPMs, not nearly enough. Keep the car or sell it to me for cheap, I'll put the alternator in it!

[caposto]

Thanks for the feedback so far. I've got the car at my place now along with the Bentley so I'll be focusing on the alternator. Great tip on checking power consumption using a "modified" fuse. I'm excited to try that one.

I'll post any usefull findings, but keep the feedback coming.

Thanks.

-Chris

[winfred]

i do it this way, take the multimeter and set it to amps (10A scale incase you open a door, you don't wanna pop the internal fuse) hold one lead to the -battery post and the other atach to the body, then lift the - terminal off it's post and the meter is now reading the draw for the whole car, anything over about 50ma is bad (1000ma + 1A), start pulling and reinstalling fuses while watching the meter when it drops you are onto the circuit effected by the draw. another fun one can be a intermitant charging alternator, it charges on start up but it craps and the car runs off the battery when it warms up, usually worn brushes loosing contact with the comm, i've had customers with cars hang on for weeks like this, one guy the car would die as he was pulling into his drive way every day going home from work as the battery would get too low to run the injection system, but the car would sit and the battery would build up and when cool the alt would charge enough to go to work

[Robert K]

I had a similar problem with my old 1986 Tbird. The battery kept going down. First thing I did was disconnect a battery cable and put a multimeter between the disconnected cable and the battery post. I set the meter to read DC amps. Sure enough, I had a couple of amps worth of draw on the electrical system with everything in the car off. Once you know you've got a draw somewhere, the next thing is to figure out where it is. I then removed the multimeter and substituted a small taillight bulb between the disconnected battery cable and the battery post. See, if there's anything in the system that's drawing power, the bulb will light up. So I hooked it up and of course the bulb lit up. I then started pulling fuses one at a time while watching the bulb to see if it got dimmer or went out completely when I pulled each fuse out. Usually, there will always be a very slight draw on a battery with the car off due to the engine computer always having power, alarm systems, radios, etc. But I think the amp draw for those items is actually in the tenths of an amp. To make a long story short, even with every fuse out, I still had a serious draw on the system. So I got the wiring diagram out and found out that about the only thing not fused was the alternator. Turns out the voltage regulator inside the alternator was malfunctioning and drawing current even with everything turned off. Installed a new alternator and checked the current draw. It was down to .1 or .2, which is what the clock and radio were pulling. Problem solved!

If you don't understand much about electronics, find a friend or someone who does. Finding a current draw can be tedious, but it certainly can be done. Unless you locate it and rectify the problem, you will continue to go through batteries. Hope this helps some.

Grace and peace,

Robert K
1991 535i

[SRR2]

Why wouldn't the car light up the "Batt" lamp or a Check-Control 'no charge' message? Or doesn't this customer pay attention to warnings? 8-)

[winfred]

a dead battery or draw on the system doesn't usually trigger a warning, the batt light usually informs you of a alt failure but not allways depending on how the alt died

Why wouldn't the car light up the "Batt" lamp or a Check-Control 'no charge' message? Or doesn't this customer pay attention to warnings? 8-)

[caposto]

Here's the stats so far...

I got another free exchange battery from Pep Boys for the third time (they're starting to recognize me.) The old one tested good again and it was 12.50V at rest and 11.57 under test load. The new one is a Bosch Platinum Plus but it looks identical to the Energizer (said Everedy earlier but its Energizer) I traded in so it may be the same manufacturer. Hopefully its a different batch at least from the other three I got.

Anyway, the new battery voltage is 12.70 pre-install, 12.66 after. At idle, the voltage varied from 13.91 - 14.03 which is within spec although I'd like it to be close to 14.4. A rough load test @ 2000RPMs with the lights, wipers, rear-defroster, and fans on showed a battery voltage reading of 13.60 which Bentley suggest should be at least 12V so that's within spec. The battery light comes on when the ignition is switched on and turns off after starting which is right.

As far as warning/check engine lights, my parents say they didn't notice any but my mom did complain she saw a check-control check engine warning but didn't notice it saying anything else. They're old timers, so I'm not sure what they saw or didn't see. Could've been a check-control charging warning as mentioned previously but who knows.

Disconnecting the negative batter lead and jumping it back to the negative post with a multimeter showed 0.000 amps so I don't appear to have a current draw anywhere. Under 0.001 amps (100ma) is within spec according to Bentley. If this was an intermittent problem, I would still be able to charge the dead battery back to normal after jumping it and driving for an hour at good speed so I'm ruling this out.

It must be an intermittent problem with the alternator as suggested. Is the voltage regulator (brushes) replaceable? I'll try to remove and check these tomorrow, but I'm tempted to just swap out the alternator altogether and get on with it. Any good discount parts sources? BavAuto has re-manufactured ones for like $270. Ouch!

Thanks for all your help so far.

-Chris