BMW 1995 525i (E34) Battery Dead After Sitting – Alternator and Battery Testing – All Chassis
This answer is applicable for many BMWs in addition to the one listed below.
I have a problem with the battery in my 95 525i. Everything seems to be fine, most of the time. However, when I let the car sit for a few days, the battery goes dead. If I get a jump, it will start and be fine until I let it sit again. I bought a new battery a couple of months ago and the problem seemed better for a while, but now appears to be back and it seems to be getting worse, these days. Did I get a faulty battery?
A Your problem is not that uncommon and, to make matters worse, it is often mis-diagnosed. A battery in good shape can sit with no draw on it for a couple of months, minimum. If you have a new battery going dead in the vehicle, either the charging system is not up to par and you are slowly discharging the battery every time you use the vehicle, or there is an excessive static current draw when the engine is not running. The most common causes for the “dead battery syndrome” as you’ve described it, are:
1) old or damaged battery
2) weak alternator/charging system, or
3) a short or other fault in the electrical system that is discharging the battery when the engine is not running. Here are the diagnostic steps to follow to determine which of these possible causes is at fault:
A) The next time that you know the vehicle will sit for a few days (the period that would typically cause the battery to go dead), make sure that the battery is fully charged and disconnect the negative battery cable. If the battery still goes dead, the battery is faulty and must be replaced. Since you have recently replaced the battery, if this happens it is likely that a weak alternator or a static electrical draw has ruined your new battery due to multiple discharges. Once you have a fresh battery, or have determined that your existing battery is OK (i.e. it did not go dead when disconnected), you can continue to the following steps.
B)Make sure that the alternator drive belt is in good shape and is properly tightened. This is one of the most common causes of insufficient charging.
C) Check the voltage at the battery with the engine running. Place the leads from a voltmeter at the positive and negative battery terminals. You should read at least 12.5 volts at idle, (13 volts or more is better). If the voltage is low, you likely have a weak alternator. Go to the next step.
D) Turn on all of the lights, blower motor, rear defroster, wipers (pull the wipers up off the windshield), etc. You should still see 12.5 volts minimum across the battery. If the voltage is lower, rev the engine and then let it come back to idle. Is the voltage now higher? Rev the engine to 1500 RPM or so. You should have at least 13 volts with all the accessories on, (13.5 is better). If the voltage doesn’t come to at least 12.5 at idle (more when the engine is revved a bit), the alternator needs replacing.
If these alternator tests come out OK, you likely have a static current (electrical) draw when the engine is off that is high enough to drain the battery in a few days. If the alternator seems to be fine (after the testing0, then you likely have an errant current draw. See the link below for initial testing procedures in tracking down an errant current draw: