Battery draw with nothing on

MOPARMACK69

MEMBER
1997 MERCURY MARQUIS
78,000 MILES

Replaced battery. Test light between neg post and cable. Light on. Goes off with fuses #8 & #4 pulled. All lights off in trunk and glove box. Is this load normal? Will it kill the battery? It is a Grand Marquis. There was a battery in there from 08. It had 4 volts left in it. Thank you much. JIM

6 Replies

CARADIODOC

MECHANIC
33,908 POSTS

You can't test for a current draw with a test light in series due to all of the computer memory circuits. Some computers draw a lot of current for as much as 20 minutes before they go to "sleep" mode. That time-out starts over every time the connection is broken, such as when you take the cable off to insert the light.

What's the history on the old battery? Even a fully discharged battery should read close to 12.0 volts. At only 4 volts, was that battery sitting for a few years?

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Sunday, February 26th, 2012 AT 9:05 PM

MOPARMACK69

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4 POSTS

Thanks for the quick reply. Battery has been okay since 08. My grandad went to go to store and not even a dome light. Took battery out and to Advance Auto Parts. They tested and said 4 volts at rest. Came back bad cell. Car gets used 2 to 3 times a month. We are in SE lower Michigan. Mild winter. I never gave it 20 min. I pulled out the #8 fuse. Light went out. This is what is on that fuse: lighting control module, power mirrors, remote keyless entry, clock & radio memory, electronic automatic temp control, power seats & windows. As I tickled that fuse in and out with the battery fully hooked up, the horn clicked and the rear defrost light just barely flickerd. New battery is fully chargedand there are no other issues. Jim

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Monday, February 27th, 2012 AT 12:48 AM

CARADIODOC

MECHANIC
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Are you having a problem now or are you just concerned with the behavior of the test light? It sounds like you just had a bad battery and there's nothing else to worry about.

One comment though, going back to the mid to late '80s, Chrysler allowed a maximum of a 35 milliamp draw, (.035 amps) due to the computer memory circuits, and at that rate they guaranteed a good battery would start the engine after sitting for three weeks. Most other manufacturers use the same spec. I find my car in storage will start after at least six weeks. If your grandpop is only driving the car a few times per month, be sure it gets a good 15 to 30-minute drive to recharge the battery.

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Monday, February 27th, 2012 AT 1:59 AM

MOPARMACK69

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4 POSTS

No Sir, no problem right now. Yes, just concerned with test light being on so bright. I need to try to obtain an amp reading. I think if I wire a test meter in series from neg post and cable then add a jumper between neg post and cable to complete circuit. Might work.

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Monday, February 27th, 2012 AT 2:18 AM

CARADIODOC

MECHANIC
33,908 POSTS

Yup. That's exactly how we have to do it. You'll need a digital VOM but to get the needed accuracy, you'll want the 200 ma scale but the internal fuse will blow. If you start out on the 10 amp scale, you usually have to move the positive lead, so that breaks the circuit. If you have a 2 amp scale, that's fused, even that can blow because the current can exceed 3 amps for up to 20 minutes. Even if the fuse doesn't blow and you wait for the computers to time out, as soon as you switch to the lower scale, the switch will break the circuit for an instant before it makes the next connection. That's enough to wake the computers up again and start all over. Installing the jumper wire first, then the meter, will solve the blowing fuses. Then wait a half hour, then remove the jumper wire to take the reading. The meter doesn't have to be turned on to prevent breaking the circuit, just connected.