Car wonks: Identifying Parasitic Drain.

My commuter car is a 2000 VW Jetta GLX. It has a little over 190,000 km on the odometer. It’s been reliable for most of its life and I’ve taken excellent care of it. Recently I’ve noticed odd electrical gremlins. Something is draining the battery while the car is sitting parked. I’m absolutely anal about turning off everything before I shut off the engine so it’s not like I’m leaving the lights or radio on overnight. I had the battery replaced last year. The car has been starting and operating fine until about a month ago. I notice that it hesitates slightly before starting on a cold morning. Starting through the day after that is not an issue. So the alternator is charging the battery just fine, I assume. Had the car in for service last week and asked them to test the battery… test came back A-OK. Parked the car for the weekend and this morning came out to find the car completely dead. Even the LCD clock was dead. Battery completely drained. Didn’t bother trying to boost it because I didn’t have time. But once I get it started and let it charge the battery, I’m pretty sure it will be drained again a day or two later.

I assume there is parasitic draw that’s killing the battery. Is that a safe assumption? What’s the correct method of identifying the source of the draw? Is a standard handheld electrical multi-meter sufficient to monitor the current?

Check for a draw at the battery (with a multimeter), pull fuses till it goes away, that is your bad circuit.

Keep in mind there can be a draw from radio memory/clock etc, you’re looking for something more than that.

How do I connect the multimeter? In line or parallel with the battery?

As a follow up question… given that my battery seems to be fully drained now, am I looking at having to replace the battery or can I expect it to function normally once it’s fully charged again?

It is a reasonable assumption that there is excessive parasitic drain on the battery. The other possibility is that the battery will no longer hold a charge properly, but that’s unlikely with a one-year-old battery. Note, however, that the battery’s life has been compromised due to its being deeply discharged, and the more times it gets drained like that the shorter its life will be.

The drain can be tested with an ammeter that measures milliamps. Your multimeter may or may not have this function. There might be a workaround way to check it without an ammeter (maybe using a voltmeter or a test light), but if so I don’t know the details of how to do that accurately.

With an ammeter, it is hooked up inline. With the negative battery cable disconnected, the meter’s negative/common test lead is attached to the battery’s negative post and the positive lead to the battery terminal of the negative cable. A reading of (roughly) 60 mA or less would be normal. If there’s a significantly higher reading, the procedure is as Vagabond said. Pull the fuses one at a time until the reading drops to a normal level; the last fuse pulled will be for the affected circuit. Note that the fuses for normal drain items (clock, computer memory, etc.) will have a small affect, but you’re looking for a bigger difference. If pulling fuses doesn’t reveal the drain, disconnect the wires to the alternator – it can have a drain even if it’s charging okay. If the drain is in the alternator, replace it.

If the drain is associated with a particular fuse, reinstall the fuses, then locate and disconnect each component on that circuit until the drain goes away, and replace the relevant item.

The battery could be shorted internally. Even a year old battery can go bad.

I’ve seen batteries with so much gunk on them that they short across the poles. Put a multimeter on the positive pole and probe other spots on the battery to see if you get any readings. Use the amp setting.

Once saw a wrong sized battery that was too tall. The top posts would touch the insulation blanket under the hood which would drain the battery especially when wet.

Does your car have a light under the hood or trunk? Sometimes these don’t turn off. Remove the bulb.

If you can’t find the problem, consider getting an under-hood battery disconnect switch. A PITA but better than a dead battery.

First place I checked was the battery terminals and connections. Battery is relatively new and connections look clean (no gunk). Short across the terminals doesn’t look likely as there is no insulation under the battery cover and sufficient clearance. No engine compartment light, if memory serves. When I parked the car on Friday night it was already dark so I would have noticed if the cabin lights were accidentally left on.

Thanks for the instructions and tips, all.

Found the problem.

Oh, snap.

You are assuming that parasitic draw is the culprit.
I propose that you don’t assume anything.
Did you actually charge the new battery with a charger or just drive around?
My first concern is that the battery isn’t being fully charged.
Do the headlights get brighter when you give it a bit of gas from idle? They should.
Voltage meters can be found for $10-$15.
Measure the voltage of the battery w/ the car off. A reading lower than 12.4 volts means that your battery needs to be charged.
Then measure at the battery with the car at about 20% throttle. A functional charging system will produce between 13.5 and 14.5 volts. If it’s much more than that, you can actually cook the battery which shortens it’s life. Some cars have voltage regulators. Most newer cars have it built into the alternator.
Next; are the starter bearings worn and causing substantial resistance, causing extreme load on the battery. Most any auto-parts place will test your starter if you can remove it and get it to them.

In line. set for ampers

Had the same problem once with a 2002 Passat. Turned out to be the trunk light switch. The trunk would close, and lock, but the light wouldn’t go off. I fixed it. I think (I can’t remember for sure) the light switch was part of the lock/latch mechanism, and I replaced all or part of that mechanism. That took care of the problem.

Battery was fresh off the Interstate truck. It was installed by the shop that services my car. Which isn’t a guarantee that it was fully charged or that it’s not defective in some way.

That had occured to me as well.

They do. Ever so slightly.

Battery was tested when I had the car serviced last week (oil & filter). Checked out. Saw the test readout.

This is food for thought. Yep, starter issue has been an on/off thing for a couple of years now. Every so often, when I start the car, the starter will grind. Like the starter gear (clutch?) doesn’t retract in time and when the engine catches there is a brief but annoying sharp grinding noise. Additionally, the cold starts this winter have sounded like the battery has been under more load than I’d expect at these temps (at or below freezing in the morning). There’s this brief pause when you first turn the key where for a second I’m not sure whether the engine will turn or not. Almost like the dull hum of an electric motor under too heavy of a load. You may very well be right about the starter being the possible cause.

I realize your car is a VW, and I’ll be talking GM, just bear with me.

GM in general had a production problem where the positive (red) ring terminal on the back of the alternator was done improperly, and as the car aged, the copper wire in contact with the terminal crimp would oxidize, creating just enough resistance that the battery would not charge if the headlights were on. I had the problem in both a Chevette and a Pontiac, it was a real bastard to find in the Chevette. When the Pontiac did it to me, I knew what to look for and fixed it myself in about 15 minutes.

Take the red terminal off the alternator, and try to gently flex it. If the terminal breaks off, cut off about an inch of wire and crimp on a new terminal. Done.

No, really. 14 years old, east coast weather & salts, Pontiac of European cars. Just run it into a light pole–it doesn’t have to be a bad wreck to total it. Dude, it could be dozens of things, each more invisible than the next.

Is this your professional advice, Mr. Insurance Adjuster? :slight_smile:

Touche’! Fraudulent claims and intentional acts are, of course, frowned upon and unacceptable in polite society.

This suggests that the starter drive is sticking sometimes.

This could indicate a starter problem, but it could also be due to the battery being a bit low – which could be the effect of excessive drain overnight.

And this could be the effect of excessive drain over a weekend. However, there is no way on God’s green earth that this could be caused by a starter* or by an alternator not charging.

For the battery going dead? No, that cannot be right.

*Unless the starter is stuck engaged, in which case it would make hellacious noise while the engine was running and would be cranking the engine when the car was turned off. You’d know if this was going on.

Well, whatever the cause, I’ll provide an update when I have one. I had a late day at the office yesterday and an early morning today, so I’ve not been able to do a damn thing about the car. If time permits, I’ll boost start it tomorrow morning with my other car and get it over to my local shop. No matter what happens, the starter will probably need replacement. I was going to spend time screwing around with isolating the possible parasitic draw but knowing that there are other issues that will surely need attention, I’m just going to let the experts with all the tools and equipped garage handle it.


Whelp… Had the car in the shop for a week and a half. Interstate battery took the battery back to their shop for testing and the results came back negative for any faults. Meanwhile, the machanic tested the car for any parasitic draw every day (I have no reason to believe they would lie) and results came back negative as well; no draw detected. They handed me the keys and said, “Sorry Mr. QS, we can’t find the fault and frankly we’re baffled. Bring it back if it happens again.”

Is there a way I can record the draw with an A-meter and save the value for me to at least be able to positively confirm the parasitic drain hypothesis?

Take it to another shop?

What say you, oh auto-mechanical mavins?