Best way to diagnose a battery drain?

96 XJ w/ lots of mods. 4.0 auto trans.

Something is draining my battery.

A week a go, using a multimeter my battery read 12.9 volts. About 12 hours later it read 12.6 volts. Fast forward about a week, the battery is now reading 3.9 volts. Second battery this has happened to.


What's the best way to figure out wtf is causing this or at least narrow it down?

With the engine off disconnect the negative battery terminal. Get a 12 volt test light and connect one side to the negative post and the other to the disconnected battery terminal. If you have a drain the test light will glow. You can then pull fuses to find out which part of the electrical system has the drain (the test light will go out when you pull the fuse with the drain). Just remember when doing this that an open door will show up as a drain. HTH blaise

blaisetd is almost correct.

I would add that a dim glow in the 12 volt test light would indicate a "normal" parasitic draw of 50 mAmps or less (should be under 30 mAmps in a stock XJ). That low draw, which can cause a dim glow in the 12 volt test light, is normal because of the PCM memory and radio presets.

The higher the parasitic draw the brighter the 12 volt test light. So, if the test light is just barely glowing, things are Ok in the draw department. If it is bright, like a taillight, there is too much of a draw.

Most multimeters will have a 10 amp or less circuit that you can use to measure the draw if you want to.

However, using the test light is better when trying to isolate the bad circuit/component. Why, you ask? Well, if you are using the meter, every time you pull a fuse or disconnect a section of harness you will have to run over and look at the display on the MM to see if things have changed. If you leave the bright 12 volt light in series you simply have to look over in the direction of the battery and see if the light level drops.

Good luck.

interesting, as I have a very similar issue with my 96. all stock but the radio.

in 3 days, the battery will be low enough that it won't start.

but the fuse for the radio also powers a lot of other stuff, so pulling fuses is much harder to diagnose.

I cannot find my test light and have been trying to use my multimeter.

I don't really know how to use a multimeter all that well. I have one of those 4.00 harbor freight yellow ones made by Cen Tech. Pretty much all I know how to do with it is measure volts.

But I did some experimenting and plugged the positive terminal into the outlet where it says "10ADC" and then switched the dial to "10A." Is this the correct setting for measuring the draw?

When I place red multimeter probe on the negative battery terminal, and the black multimeter probe on the disconnected ground battery cable, the multimeter reads "-0.68." Would this be my draw?

I have an aftermarket CD player and the face plate is off so it should be drawing nothing.

Yeah, that is how to use the meter Amp test circuit.

That "0.68" would translate to 68 mAmps and will for sure kill your battery.

You could have a bare spot in the wiring harness on a power supply wire that is switched or unswitched or a component that has an internal short causing the draw.

Time to start pulling fuses, if that doesn't locate it try unplugging components or different sections of the harness--connectors are all over the place. Door light switches, bulbs, blower motor, radio, cig/power sockets, the list goes on.

Some scrap wire, a 12 volt bulb, some solder, makes a quick test light.

joe_peters said:

Yeah, that is how to use the meter Amp test circuit.

That "0.68" would translate to 68 mAmps and will for sure kill your battery.

You could have a bare spot in the wiring harness on a power supply wire that is switched or unswitched or a component that has an internal short causing the draw.

Time to start pulling fuses, if that doesn't locate it try unplugging components or different sections of the harness--connectors are all over the place. Door light switches, bulbs, blower motor, radio, cig/power sockets, the list goes on.

Some scrap wire, a 12 volt bulb, some solder, makes a quick test light.

Click to expand...

I'm going to pick up a new test light tomorrow.

Any idea what would draw that much power? Also what should it normally read? I've searched and if I'm correct the multimeter should read 0.03 or there abouts with no parasitic draw. Is that right?

My 96 had a a couple very light power draws. What I found was the glove box light switch would not fully turn off. The rear interior light switch and the last place was a switch on the passenger front power seat.

Good Luck

90Pioneer said:

I'm going to pick up a new test light tomorrow.

Any idea what would draw that much power? Also what should it normally read? I've searched and if I'm correct the multimeter should read 0.03 or there abouts with no parasitic draw. Is that right?

Click to expand...

The question is .03 what. Most meters have both an amps range and a milliamp range. You should read .03 amps or 30 milliamps or less.