diagnosing battery drain

[cwiert]

my battery is draining over night. if i take the neg post off the battery, it will last (no drain). now i need to find out what's draining it. all i can see that's running all the time is the digital clock.

i'm kind of new at this... so can someone walk me through this? tomorrow, (after i recharge the battery) i was planning on taking my digital multi-meter and connecting it directly to the battery posts. i should get around 12 volts, right? then i was planning on pulling fuses and looking for the volts to go up. once i find the fuse that causes the volts to go up, i'll find the culprit that's causing the drain, right?. i realize fixing it will be a different story, but i've got to start somewhere. is this the correct/easiest way to go about doing this?

[JK McDonald]

Hey cwiert, I think most 928 owners will be able to chime in on this one since many of us have experienced the same dreaded “battery drain” to one degree or another. It sounds like your battery is being pulled down fairly quick so you may be looking for a large current draw. Some have found this type of problem was from a bad (shorted) diode bridge inside their alternator.

My battery drain took almost a week of being in the garage before the battery was flat. It turned out to be a corroded door closed switch, which kept the small red door edge lights and interior lights dimly on even when the car doors were closed properly. Don’t forget to check the rear hatch closed switch and the glove compartment closed switch.

Good Luck, Michael

[Landseer]

For mine, exactly what he said ^^^^.

[harborman]

Originally Posted by cwiert

my battery is draining over night. if i take the neg post off the battery, it will last (no drain). now i need to find out what's draining it. all i can see that's running all the time is the digital clock.

i'm kind of new at this... so can someone walk me through this? tomorrow, (after i recharge the battery) i was planning on taking my digital multi-meter and connecting it directly to the battery posts. i should get around 12 volts, right? then i was planning on pulling fuses and looking for the volts to go up. once i find the fuse that causes the volts to go up, i'll find the culprit that's causing the drain, right?. i realize fixing it will be a different story, but i've got to start somewhere. is this the correct/easiest way to go about doing this?

A drain in that short of time leads me to believe a serious short or a bad battery. Take an Amp reading between battery when everything is suppose to be off. Normally you will have some slight draw for clock, alarm, etc., but not enough to drain the battery down that fast. All a process of elimination.

[JhwShark]

Another way to check shorts if it is not resolved by the ones mentioned would be to rig up a test light (solder a lead to 12v bulb and another lead to the other terminal, make sure you have enough lead so the light is visible from when you are at the control panel).

IIRC...you need to have 12 volt bulb between the bat ground post and its lead. if on you can then remove the fuses one at a time. The fuse where the light goes off is the circuit to examine.
If someone know this is the back-wards way please correct.

Jon

[Landseer]

Empirically, my experience, the lights being on in the doors is enough to bring a fairly new battery down below the threshold that it takes to feed the brains and run the car. If one is on, both should be.

The alarm also draws significant power.

[harborman]

Originally Posted by Landseer

Empirically, my experience, the lights being on in the doors is enough to bring a fairly new battery down below the threshold that it takes to feed the brains and run the car. If one is on, both should be.

The alarm also draws significant power.

When the door pins are corroded it affects a series of systems.

Yes, check the obvious first, door switches, anything that that might stick. As a comparison, sometimes I don't drive my Ford Expedition for several weeks and it always starts. Those door switches should hold open and if anything, don't make contact when the door is opened and stick. You would notice lights on when doors all closed. I would think any solid state devise such as the clock is only in the milliamp draw not in the amp range, such as lights. An amp meter should give you some indication of the problem. Sometimes a shorted diode in the alternator will cause a good draw. Just start eliminating circuits, pull fuses, etc.

[cwiert]

ok, thanks. i will def check the door light switches first, as those seem to be common problems.
i'm also running a little experiment... i charged the battery and then checked the voltage... 13.1 I disconnected the neg cable and will see what the voltage is 12 hours later. Hopefully it should be the same. If it goes down... bad battery. If not, then I'll put the neg cable back on and wait another 12 hours and see if and how much it goes down. At least this should tell me if my battery is bad, or if I have a drain (although i'm fairly positive it's a drain).
BTW - I like the test light idea... but I don't quite get it. I'm a little slow sometimes. Solder 2 wires to a test light... to the pos and neg of the light. then connect the neg to the batter neg post. But where is the pos lead from the light going?

[american-kiwi]

you pull the neg cable off the battery...then wire the bulb between them (dont put the neg back on...make the light the full curcuit
sorry for a bad drawing..was in a hurry...but i think you get the picture now?...lol...that hows you test a battery drain without a DMM (meter)

[cwiert]

thank you! i like the drawing! i'll try that. i appreciate the help.

[JhwShark]

AM-Kiwi, Good illustration.
That's what I use to figure it out.
Jon

[american-kiwi]

Where's Allan when we need him...he's the expert on this... ))))you out there Allan?((((
as i dont know what the spec's are on porsche's...there will be a small drain..re:running clock, radio memory etc

[cwiert]

2 more questions:
1. Where's the switch for the int lights on the rear hatch?
2. When I disconnect the battery neg terminal and put the multimeter between the neg post and the neg cable, I get a reading of 12.3ish volts. I guess that's bad, right? It should be 0 or close to it, right? That's why I need to pull fuses until the reading goes to 0 (or until the light goes out). Is that correct?

[dr bob]

The reading you are seeing is normal.

Change the meter to read amps, on the ten amp scale, and connect it where the lamp is in the diagram. The doors must be closed and the hatch must be closed to get a valid reading for at-rest drain. So if you plan t o pull fuses to isolate loads, you'll need to tape the door switch(es) to simulate doors closed. This will also let you see if the red lights on the doors are on with the doors closed. With the meter in the circuit DO NOT TURN ON ANY LOADS.

Be aware that there are circuits that don't have fuses (!), some that don't have fuses in the CE panel (!), so don't give up if you don't find the drain right away just by puling panel fuses.

[JK McDonald]

Hey cwiert, You might try buying one of the inexpensive Walmart/Harbor Freight digital multi-meters – so if you blow it up you’ll only be out $12 - $14. When connecting up a meter, you will want to move the function selector to measure current. Connect the meter in series with the battery - with the red lead (alligator clips?) touching the negative battery cable and the black meter lead to the negative battery post.

As Dr. Bob said above - It sounds like you had selected your meter to read voltage. The +12 volts you are seeing is normal. It helps if you think of voltage as pressure. Like a balloon (+12 volt battery) full of air with a pressure gauge indicating 12 psi.

Think of current as marbles moving down a water hose. An amp meter is like a paddle wheel with a digital counter indicating the number of marbles rotating past the paddles. The faster/greater number of the marbles moving - the higher the current reading on the meter readout. A really high volume of fast moving marbles can make a copper wire hot enough to burn the insulation off. The starter pulls a lot of marbles out of the battery in order to rotate the heavy crankshaft assembly while the dash clock requires an extremely small number of slow moving marbles for operation. A component that handles high current = larger diameter wire. Normally a fully charged battery (plumb full of marbles) can power a static vehicle load – dash clock / stereo memory / travel computer / etc… for many weeks while if you forget to turn off the parking lights, the battery will run out of marbles fairly quickly.

Eliminate the easy stuff first – Is your battery holding a good charge? Are the interior lights/door edge lights staying on with the doors closed? Are there any exterior / parking lights continuously on - headlight selector switch defective? Is the glove compartment light staying on? Does disconnecting the alternator wiring eliminate the problem? (Note: disconnect the battery before removing the alternator wiring – the alternator red lead is electrically a straight shot to the battery)

Reconnect the multi-meter in series with the battery for the following tests - One word of caution – DO NOT turn on any additional loads, ignition key, parking lights etc…. while making these tests because the electrical load of these circuits may pull more current through your small meter than it can handle. If you do accidentally zap your meter - there is usually a replaceable safety fuse inside.

1) With the multi-meter selected to read amps – start with the meters’ highest current scale and then progressively select a smaller scale until you can get your initial current reading. This will probably wind up in the 100 – 150 milli-amp (guessing) range.
2) On the passenger side fuse panel – monitor the meter as you begin to sequentially remove the fuses from each circuit. You will see either no change in the meter reading (no current is being lost through this circuit) or a sudden decrease in the meter reading (current moving from the battery –through the fuse and into this circuit). When a drop in current is found – you have located the potential problem.

The easiest electrical theory to remember is that everything works on smoke. When the smoke escapes from a circuit it quits functioning.

Good Luck, Michael

[cwiert]

Thank you Michael. I appreciate the detailed info.

I did the tests you recommended. I get a reading of 68.0 amps when I connect the meter in series with the battery. I first went to fuse #24...interior lights...as that seems to be the most common culprit. When I pull that fuse, the amps drop to 67.0.

I guess what I'm wondering is, if the doors are shut & key out of the ignition, should the amps read "0" (or close to 0 - since i know the clock will bump it up a few amps)?
And when I pull the fuses, how much of a drop should I be looking for on the meter?

BTW - thank you all again with all your help. This is all new to me and I LOVE learning about new things and how "stuff" works.

[harborman]

Originally Posted by cwiert

Thank you Michael. I appreciate the detailed info.

I did the tests you recommended. I get a reading of 68.0 amps when I connect the meter in series with the battery. I first went to fuse #24...interior lights...as that seems to be the most common culprit. When I pull that fuse, the amps drop to 67.0.

I guess what I'm wondering is, if the doors are shut & key out of the ignition, should the amps read "0" (or close to 0 - since i know the clock will bump it up a few amps)?
And when I pull the fuses, how much of a drop should I be looking for on the meter?

BTW - thank you all again with all your help. This is all new to me and I LOVE learning about new things and how "stuff" works.

68 amps????? Man that is a lot. I doubt that! Maybe 6.8 amps or .68 amps??? Is it possible 68 Milli-amps which is .68 amp?? Unless you have a very big amp meter, 68 amps would fry most wires on small meters.

[cwiert]

Originally Posted by harborman

68 amps????? Man that is a lot. I doubt that! Maybe 6.8 amps or .68 amps??? Is it possible 68 Milli-amps which is .68 amp?? Unless you have a very big amp meter, 68 amps would fry most wires on small meters.

very possible and most likely probable... this is all new to me. on the multimeter, i put it on the amperage section on the "200m" setting. The reading I just got after testing again, was "68.4." Does that mean .68 amps or 6.8 amps???

[JK McDonald]

Hey cwiert, It sounds like your are making some progress. But you are probably mis-reading your meter a bit. An amp is a relatively big ole hard working mama and 68 Amps could probably crank over a Caterpillar Dozer or Bar-B-Q a T-Rex in under a minute…. The DCA (Direct Current Amp) scale selections on your multi-meter are probably something like 200 micro-amps (top right in photo) to 200 milli-amps (shown selected). If you rotate your range selector to 200 milli-amp, this simply means that you will be trying to read a circuit that is expected to pull somewhere from 0 to a max of 200 milli-amps.

Since each milli-amp = .001 of an Amp and if your meter reads 68.0 this is a tiny .068 of an amp. I checked the static current draw on an 84 model 928 last night and found it read 123 milli-amps or .123 of an amp.

If you are actually reading an external current draw of 68.0 milli-amps or .068 of an amp you may have inadvertently disconnected or electrically isolated your problem. Make sure everything is plugged in... I re-read your initial post saying that if the battery cable is removed your battery holds it's charge...so an internally shorted battery would not be your problem.

Good Luck, Michael

[Maleficio]

Originally Posted by JK McDonald

The easiest electrical theory to remember is that everything works on smoke. When the smoke escapes from a circuit it quits functioning.

This is the exact theory the Avionics Technicians utilize when troubleshooting armament gripes on our Hornets. Sometimes they can't get a bomb station to identify correctly, and if it pops a breaker, they just push in the circuit breakers and hold them in until they start seeing smoke. Then me and my guys have to go out and repair 130 wires that were destroyed in an electrical fire.

Hardcore troubleshooting: hold in the breaker (and all the subsequent breakers that begin popping) and look for the smoke. Then have the AE's fix it.