Copied from the Brickboard FAQ:
http://www.brickboard.com/FAQ/700-900/ElectricalStarting.htm#Slow Battery% 20Discharge
Slow Battery Discharge. [Query:] I am having an electrical problem with my Volvo. About two months ago the battery went dead--slowly over time. It was an old battery, so I though nothing of it and replaced it. All was good for a month and a half, then it went dead again. With the car running I measured 14volts across the terminals of the battery (12 when stopped) and 14v off the main positive lead of the alternator. What is the problem here? [Battery Drain Diagnostics: Response: Ross Gunn] To check to see if there is something draining the battery while the key is off, remove the pos battery terminal, and with an ammeter, measure to see if there is any current flowing from the battery to the cable. If there is anything more than a couple of milliamps, try removing fuses one at a time to see if you can identify the circuit that is causing the drain. If this pins down a problem, a little more sleuthing through the offending circuit should tell you what needs to be done. If there is no drain showing with the above test, the charging system is suspect. Try measuring the voltage at the alternator output terminal and battery pos terminal with all utilities (headlights, rear window heater, fan etc.) on high. Any difference in reading indicates a poor connection somewhere in the red cable from the alternator. Don't assume that a crimp connection of a terminal on the cable is good. Corrosion can introduce enough resistance to prevent proper (any?) charging when there is a significant load on the system (cold, dark, wet/snowy winter evenings). I have experienced this on a 20 year old Brick.
[Battery Drain Diagnostics: Chris Bowne] I agree with Ross Gunn that the best way to trouble shoot a discharging battery is to find the source with the engine shut down and a multimeter (set to measure DC current) in series with the positive battery terminal lead. Other places to check besides the fuse block for drain paths are the alternator and voltage regulator (if not internal to the alternator). Disconnect/reconnect the connections on them, one at a time, and monitor for drain. I had a problem on a Ford Taurus once where the voltage regulator had shorted, and was the cause of the drain. You may or may not find a source of a drain like this merely by pulling fuses. In fact, you could end up with all the fuses pulled, and still have the drain, like I did! Someone on an earlier posting of this thread mentioned checking to see if his alternator was providing output by lifting the battery + terminal connection WITH THE ENGINE RUNNING. DO NOT DO THIS! Many solid state regulators will be damaged/destroyed by this condition. (And in turn may compound the causes of the battery drain you are troubleshooting!)
Battery Drain Tips. [Tip from JohnB] Check the specific gravity of the cells...if they're accessible. With a fully charged battery, either from your battery charger or the alternator, disconnect the battery and measure the voltage, measure it again 12 hours later and it should be virtually the same, maybe .1v less, no more. Reconnect the battery to the car and turn on the headlights on full bright for 10 minutes and every accessory in the car...if the battery dies in 10 minutes replace the battery. Otherwise, battery voltage should remain above
10.5 volts or so after this test. There are load testers available in auto parts stores for about $30 or so that will do a higher load test (couple hundred amps through a resistance load bank) in about 10-30 seconds against a red/bad yellow/weak green/good voltage scale. If these tests still have you with a good battery, then you just have to trace down the current drain, circuit by circuit.
Good luck,
AJ said:
Still having the same problem. I do have the Green Book, for wiring. And some non-auto electrical knowledge. I guess I can start taking one of the wires off the possitive battery terminal. The 780 has 6 feeds off the terminal. What would one of you professionals do? I know that I have a short to ground, somewhere. I have plenty of time. Just wondering if there is a more systematic way to do this. None of the door switch lights are bad. I have removed the whole radio, equalizer, lighter assembly. So, I have easy access to the fuse/relay panel.
What a sweet car, I had it out today for a while. Runs fine. Battery will last for a couple of days, then dead. Had the battery tested today, it is fine and 6 months old. Of course I had it on the charger for a few hours.
Thanks, Again.
AJ @ Home in Wisconsin
1989 - 765 w/134k Miles
1989 - 780T w/143k Miles
1987 - 245 w/225k Sold
And the Wife's FORD is still going.
