Is battery shot or how to detect slow drain of battery?

[Hookmaster]

1996 Sable. Battery is 3 years old but has been drained down a few times. New alternator last fall and battery was still good then as checked by the mechanic. I charged it fully last Sunday and on Thursday night it was down to 12.39 volts. I think the battery is shot but how would you try and measure a slow current flow?

[pinkfloyd4ever]

One way to start would be to set up a test light on the battery with the key in the off position. It should light up. Then pull and replace each fuse one by one until the light goes out.

From there you at least know what circuit is causing the slow drain... then a simple search on the inter web to help you find out what accessories are tied into the fuse you identified.

Learned that trick last year on the gals camry ended up being the trunk light that was causing the drain.

Though with hers with a voltmeter you could actually see the 'countdown' on the battery as it was being drained

Good luck

[Jeremy airjer W]

Just to clarify pinkfloyds post. A test light should not light up but if it does than there may be a draw.

My question is why do you think the battery is discharged with 12.39 volts? Is the car not starting?

[Hookmaster]

Never used a test light before. Describe how it is used in this situation. The voltage was checked it with a multimeter. About 2 weeks ago the car needed a jump. When driving it home after the jump, it was running rough and the either the tach or speedometer went to 0 when he was driving. Charged it up and have been checking it. 13.14 volts after charging at 5 pm Sunday, 12.53 volts at 8 pm Tuesday, 12.39 volts at 8 pm Wednesday, 12.39 volts 9 pm Thursday.

[Jeremy airjer W]

When using a test light to check for a draw remove the positive cable from the battery. Place the test light in series with the positive battery cable and the battery. The test light is now completing the circuit. If it glows dimly there is likely nothing to worry about. If it glows fairly bright there may be a draw. If it is fully bright there is definitely a draw. Remove the fuses one at a time until the light goes out. You just found the circuit causing the draw.

I think the voltages you have recorded aren't giving you an accurate picture of what is happening. A charger puts out more than 12 .volts to charge the battery. The first reading you had is the surface charge which will quickly drop as the normal draw of the vehicle occurs. The fact that you have the same voltage two days in a row after the surface charge has settled seems to tell you there may not be a draw.

The best way to check for a draw is with an amp meter. It will be hooked up the same as the test light. Anything over .050 amps is a draw.

What's the charging voltage of the alternator with the vehicle running? does the vehicle start with this battery?

[Hookmaster]

Using the test light in series with the positive battery cable and the positive terminal, there was a low glow. At times it was pulsing which I thought was the clock. It got much brighter when the parking lights were turned on. Started pulling the fuses in the fuse block under the hood. The one that put the light out was the fuse block fuse which I assume controls power through the whole fuse block so I don't think that's an issue.

I hooked the positive cable back to positive post and the car started fine at 12.38 volts. Charging voltage was 15.00 volts +/- 0.05 volts. Ran on higher idle for a while and slowly decreased. When it got below 1000 rpms, the engine started going up to 1500 rpms then down to 500 rpms. Of course the charging voltage oscillated with the rpms between 15.00 and 14.50 volts. It did this for ~2 minutes and then stayed below 700 rpms and ran rough. I could give it gas and when it got to ~900 rpms or higher it smoothed out. Then ran rough when I took my foot off the gas and the rpms went back down below 700.

I tried using my voltmeter on the mA setting but it did not give any reading when I hooked it up in series like the test light. I also checked the trunk light and it is going off when the latch is closed.

Any more thoughts with this additional info??

[Jeremy airjer W]

was your voltmeter set to amperage and the leads set in the proper terminals of the voltmeter? Most voltmeters that can measure amperage are protected internally by a fuse and that fuse may be blown. At the very minimum you should see less than .050 amps.

That year taurus probably has several items that wont power down for at least 20-30 minutes. This means that you will not get a correct reading until you leave your test equipment hooked up for this period of time.

The unstable idle may have been the vehicle relearning idle after the battery had been disconnected. A rough idle may indicate another issue.

Charging voltage seems a little high. around 14.5 volts is the norm.

[Hookmaster]

The voltmeter was set correctly and the leads were in the right terminal holes. I'll check the fuse and try and test it on a 12 volt fish house light. The first time I tried to measure the amps was before I ran the car. It hadn't been run for a week. Maybe it's time for a new voltmeter. It's only about 25 years old. Thanks for the help so far.

[pinkfloyd4ever]

thanks for straightening me out there Airjer, forgot about having to put them in series.

[walt501]

When you suspect a short, don't rule out that the battery itself may have an internal short. If one or more the plates in a battery cell touch, it creates an internal short in the battery. You can charge the battery and it will appear to hold a charge, but the five good cells will drain themselves into the sixth bad cell. The way to test for this is with a battery hydrometer. Even on so called sealed batteries, you can often still remove the caps to each individual cell and test the actual charge of each battery cell. One cell that will not come up to the correct specific gravity of the other fully charged cells which will indicate an internal battery short. You can purchase a battery hydrometer at any auto parts store for a few bucks. In my experience, a vehicle that appears to have a short is more often a problem with the battery itself than an actual short in the vehicle.

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