Battery Losing Charge - BMW Forum

mlg106 · 05-13-2005, 10:24 AM

ghead my battery for some reason is discharging . the battery is new, the alternator test says it is good, ive checked most of the wiring for a short but have not found one yet. has anybody else ran into this problem, could use the help. its a 1995 318is .

Gator · 05-14-2005, 08:15 AM

There are three possibilities. 1. The new battery is no good. I could have an internal short or some other problem even though it's new. It happens. Temporarily replace it with a known good battery from another car or a friend. Or charge the battery and then disconnect it from the car for a couple of days and measure the voltage to see if it holds a charge. 2. The battery is not getting charged by the alternator. With the car running put a voltmeter between the + and - terminals of the battery. If the reading is 13+ volts with the engine reved a little the charging system s working. 3. There is a short which is draining the battery. Disconnect the battery plus terminal. Put an ammeter between the disconnected terminal and the battery + post. The reading should be less than .5 amp. (You have the radio and alarm system that are drawing power and am not sure of the exact reading for your car. ) If you have more draw than that you have a short somewhere. Start pulling fuses one at a time and checking the draw. When the reading drops you have found the bad circuit that needs trouble shooting. When you pull the fuse for the radio and alarm there will be a drop which you should note as a base number for a baseline in the future.

mlg106 · 05-15-2005, 09:09 AM

thanks gator , i will try out those helpful tips. also, do u think the alternator could be starting to go bad or once its fails its time to rebuild? if anybody has had similar problems let me know, thanks.

Gator · 05-15-2005, 10:40 AM

Quote:

Originally posted by mlg106@May 15 2005, 08:09 AM
thanks gator , i will try out those helpful tips. also, do u think the alternator could be starting to go bad or once its fails its time to rebuild? if anybody has had similar problems let me know, thanks.

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Alternators and voltage regulators are either good or bad. There is no middle ground. The voltage between the + and - battery terminal should be 12.6 - 12.9 volts with the car not running if the battery is fully or nearly fully charged. With the car running at a high idle of 1500=2000 rpm the voltage should be between 13 -14.5 volts if the alternator and voltage reg are ok. If not one or the other is bad. (PS This assumes that your battery connections are CLEAN. These should be checked first. Be sure to check where the - terminal connects to the frame of the car. This is often corroded and causes the battery problem and renders everything discussed above invalid.It's also possible but unlikely that the terminals of the alternator are corroded and as a result aren't allowing current to flow to the battery.)