Battery Drain

Hi folks. I have a Buick with a battery drain. Not sure what’s doing it; I don’t drive it much and it has been sitting. I lost one battery and about four months ago put another one in.

Today I tested it; it standing still and it was at about 12.1 volts. So I pulled it out and brought it to an auto parts place and had them charge it.

Right now its not registered because I’m working on it. In about a week or so, I will register it then get it to a mechanic. In the meantime I put in a manual cut off switch to keep it from draining while sitting around.

I tested it with the switch off and I’m getting about 12.4 volts. I’ve always assumed its supposed to be around 12.6 standing still. Could this have something to do with the switch, in other words you can’t get a true reading when that is hooked up? I don’t want to go have it charged again; the person at the store told me it was fully charged.

check ground on starter.

You are correct that a fully charged car battery should read 12.6 volts. Are you measuring across the battery terminals? Are you certain that your voltmeter is accurate (try it on another car).

If your voltmeter has an ammeter on it, disconnect a battery cable and put the ammeter in series with the cable. You will usually get a several hundred milliamps for a few seconds, and after a few minutes when the computers go into sleep mode, the drain should drop to 50 mA or less. If you are really smooth, you can pull off the cable with the leads touching the post and cable without interrupting the current flow so the computers don’t wake up and you don’t peg out your ammeter.

Assuming your meter and measurement technique is correct, then the battery is showing lower than normal. Of course that could be the batter, the charging system maybe a bad connections) somewhere, or a power drain.

I would suggest measuring power drain with an amp meter Remember that it can take a fair number of minutes for power drain to settle down after turning the engine off.

I would not get excited about 12.4 vs 12.6. How old is that battery? Maybe it is time. BTW many parts stores will do a test of the charging system as well as a load test on the battery.

Its a new battery, just purchased in March manufactured in Jan 2009.

The power wire to the alternator is always “hot”. Disconnect it and put an amp meter between the wire and the alternator terminal. If there is any current flow, the alternator is “leaking”.

Thanks for all the advice folks.

One more opinion about 12.4 vs 12.6 volts: In a perfect world, each cell might produce 2.1 volts. This is how you’d get 12.6 volts. But I’ve measured batteries off the shelf and have seen 12.4 volts and they are fine batteries and will last a long time. Don’t sweat the small stuff. If the battery passes the load test, that is usually an indication it is acceptable. Just having the power go through the battery shut off switch shouldn’t change the voltage unless there was a bad contact on the switch, where you were getting a loss. A voltage drop, if you will. Now if you in fact have a residual leak, an excessive parrasitic (sp?) leakage, you can search on this site and get more hits than you’ll have time to read today.


Really appreciate it. Good point. It occurred to me that when I left the auto parts store one clerk said to the other “Did you test this after the re-charge” and the guy said “Yeah, 12.61”. So it could be my equipment or the fact that now I’m reading it with a cutoff switch installed. Hopefully the mechanic will be able to figure out the cause of the drain.