Testing for slow battery drain

[renobdarb]

Is there a way to track down the source of a slow battery drain with a voltmeter? I have a ohm meter but not a test light.

The battery reads 12.45 on the meter, and when I disconnect the negative cable and put one connector on the cable and the other on the negative post it reads 12.07.

I'm trying to track down a slow battery drain and wondering if this test is telling me there's a drain somewhere.
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[Vince Waldon]

Reading voltage in this situation can be highly misleading... does your multimeter do milliamps??

You're looking for a current draw... so your best bet is a meter that can read current. A little bit of current draw (10-100 mA) is normal, depending on what your clock and/or stereo draws... hundreds of mA suggests a light on somewhere, 10s of amps suggests a stuck regulator relay (if generator equipped) or a shorted alternator diode.

[telford dorr]

It's not telling you what you want to know. What you want to measure is current, not voltage. Does your meter have a current range (preferably 10 amps)? If so, use the exact same hookup (meter in series with the negative cable.) DO NOT turn on anything. Don't even think of turning the key on. In fact, make sure the front dome light switch is set to the middle (disabled) position, so that it doesn't light up when you open the door.

Now note the reading on your meter. If it's very low, try a lower range. You should read between zero (no stereo) and 0.1 amp (stereo). If you have a stereo, disconnect the "keep alive" full time +12 volt wire (yeah, you'll lose all your settings, but that's life...) Does the meter read zero? No? Start pulling fuses one at a time until the current goes to zero. Whatever is hooked to that fuse needs to be checked (see the schematic in your manual / site Technical section.)

If you still have current flow with all the fuses removed, then pull the big red hot wire going to the voltage regulator, and if you have an alternator, the big red wire going to it (not so easy...)

If that doesn't get it, then you have something strange and non-stock in your wiring. Look for things that look 'modified' or non-stock (they tend to stand out...)

If you don't have a suitable meter, a dome light bulb will work pretty well in it's place.

[Edit: wow - I'm a slow typer...]
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'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
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Same thing happens when you're stupid. - Philippe Geluck
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[Tom Powell]

[static]

With the tutelage of a smart Engineer friend, I did the above exercise awhile ago and it produced two culprits: the dome light (presumably the door switches, not actually the light itself) and the stereo.

Now, I remove the stereo's faceplate when I park for more than a day and I set the dome light to the "middle" position. I am still drawing a very slight amount (I love me my FM presets) but nothing like before.

Oh, and the greatest improvement? Replacing my car's battery with a shiny new one. (The argument "But it's only eight years old" doesn't work)

[ned]

Hey Tom Powell, I like your style. No need for a socket for that lamp, just solder the wires to the lamp. Fast and efficient.

[Mal evolent]

When I had this problem in the Bronco it was a leaky diode in the alternator. not going to find that with a milliamp meter
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[Tom Powell]

[Wildthings]

A Voltmeter will work fine, especially an old analog one. Just remove the negative cable from the battery and hook the meter between the cable end and the battery post. If you have a big drain you will see full battery voltage on the meter, a lesser drain will show less voltage.

Pull all the fuses and see if the drain goes away. If you still have a big drain with the fuses removed it is probably the diodes in the alternator. A smaller drain with the fuses removed may mean that your radio or some other add-on accessory is not fed through a fuse.

Assuming that the drain went away when the fuses were removed, replace the fuses one at a time to see which fuse the drain is on.

You may well have more than one drain, which can make trouble shooting tough. I once worked on an old pickup owned by a rich MF, who had used it on his country estate for years while maintaining nothing. It had five different drains on it when he finally gave up on it and brought it in to have it worked on. PITA to figure them all out.

[telford dorr]

[telford dorr]

[renobdarb]

[SGKent]

it isn't wiring unless one of them is lying in water or some corrison that is dielectric. If you had a short you would see a lot more current. Is there anything else on that #7 fuse besides the light?

#7 is accessories - do you have a refridge, a power amp, a clock, a fan - etc
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[Tom Powell]

[Wildthings]

Does your stereo work with the #7 fuse removed?

[timo78]

Have your assistant watch the voltmeter, then start removing/reinserting fuses. When you get to one that has an effect, then determine what is on that lead. I had a VDO clock back in the day that would drain the battery. Contemporary stereos can have this trouble too.
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[Tcash]

Hi looking at the light bulb tester I started thinking (man it hurts) about the amperage rate of the bulb. That looks like maybe an 1156. Using an 1156 27w as an example the rated amp is 2.10A. That being said it takes 2.10A to fully light the bulb. I don't know how much it takes to make it just glow. But lets compare a test light to it. Most test lights take 500 ma. If I remember right with the introduction of computers they came out with a 200ma. test light so you would not fry the computer. See were I am going. If the Draw is to small you will not see it. I found a great article on Parasitic Draws. If you do not have a meter. (that is another topic) After reading this you will want one. The article also touches on alt. diodes. Good Forum Good Luck

ParasiticBatteryDrains

[telford dorr]

How about the LED DC test light that they used to sell at radio shack? Maybe pulls 20 ma at 14 vdc? Build one from a red LED in series with an 820 ohm resistor and a 1N4001 diode (to protect from a reverse connection.)
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[renobdarb]

[andk5591]

There were some videos on youtube for testing for parasitic battery drain. Go on there and do a search - was informative.
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