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Thread: How to check the current on a car battery?

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    Flashaholic* offroadcmpr's Avatar
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    Help How to check the current on a car battery?

    So I have had some problems with my car battery dying. So I took it into the shop and had a guy look at it. He said that there is something drawing power away from the battery when it is turned off. One of the door ajar sensors is not working correctly so it says that the door is always open, so I have the dome lights turned permanently off so that it does not draw power. (but maybe it does anyway)

    The mechanic began to play with the fuses and finally got to the one that controls the door ajar sensors and when he took it out the draw on the battery stopped. So now I do not have dome lights in the car which i can deal with, but the fuse is also connected to the car alarm, which he said may also be the problem, and also the CD player. So now I do not have a CD player. I talked so some one and they suggested taking out the car alarm to see if that is the problem, but I do not want to cut the wire and wait a week to see if the car doesn't start when I need it. The mechanic said that he wouldn't be able to fix it, and told me to try to take it to a specially shop.

    Now I want to narrow it down, is it the alarm, or the door ajar sensor that is the problem. So I am thinking of checking out the current on the battery, with and with out the car ajar sensor and also the alarm system to see which one is the problem. I figured out a way to make the car ajar light go off, so I want to do that and then compare the current before and after I change it so see if that is the problem.

    So I do not have tons of experience with electronics, so I was wondering what the best way to check the current is. I would need to go and buy a voltmeter from the store I'm assuming.
    Do any of you have any experience with this kind of stuff? If I buy the voltmeter what settings should I use to check for it, and what should I connect the wires to?
    Thanks.
    oh and the car is a 1996 4Runner.

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    Flashaholic* Hitthespot's Avatar
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    Default Re: How to check the current on a car battery?

    Quote Originally Posted by offroadcmpr View Post
    So I have had some problems with my car battery dying. So I took it into the shop and had a guy look at it. He said that there is something drawing power away from the battery when it is turned off. One of the door ajar sensors is not working correctly so it says that the door is always open, so I have the dome lights turned permanently off so that it does not draw power. (but maybe it does anyway)

    The mechanic began to play with the fuses and finally got to the one that controls the door ajar sensors and when he took it out the draw on the battery stopped. So now I do not have dome lights in the car which i can deal with, but the fuse is also connected to the car alarm, which he said may also be the problem, and also the CD player. So now I do not have a CD player. I talked so some one and they suggested taking out the car alarm to see if that is the problem, but I do not want to cut the wire and wait a week to see if the car doesn't start when I need it. The mechanic said that he wouldn't be able to fix it, and told me to try to take it to a specially shop.

    Now I want to narrow it down, is it the alarm, or the door ajar sensor that is the problem. So I am thinking of checking out the current on the battery, with and with out the car ajar sensor and also the alarm system to see which one is the problem. I figured out a way to make the car ajar light go off, so I want to do that and then compare the current before and after I change it so see if that is the problem.

    So I do not have tons of experience with electronics, so I was wondering what the best way to check the current is. I would need to go and buy a voltmeter from the store I'm assuming.
    Do any of you have any experience with this kind of stuff? If I buy the voltmeter what settings should I use to check for it, and what should I connect the wires to?
    Thanks.
    oh and the car is a 1996 4Runner.
    Current is a very dangerous thing to play with. Your car battery is capable of sending hundreds of amps your way if you accidently short it out. This can cause the battery to EXPODE and should be taken very serious, because I've seen it happen. Current needs to be measured in series with the load which should not be done by an amature in my opinion. IF your lucky enough to have a clamp on amp meter at your disposal then you might be OK. The current draw on your battery is probably a lot less than 10 amps but keep in mind this is probably the maximum a less expensive volt meter is going to measure. Your car battery, again, is capable of pushing much more current.

    Sorry if I scared you. I guess all I'm saying is be very careful. I'm sure those with more experience can give some better opinions than mine.

    Bill
    Last edited by Hitthespot; 11-14-2008 at 04:23 PM.
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    Flashaholic* TooManyGizmos's Avatar
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    Default Re: How to check the current on a car battery?

    You should replace the door ajar switch which you already know is bad.

    That should resolve it .
    ~ "She" says ... ... I have ... TooManyGizmos ~

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    Flashaholic* offroadcmpr's Avatar
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    Default Re: How to check the current on a car battery?

    Thanks for the replies.
    The reason I have not replaced the switch is that it costs money, and I am looking to see if there is a low cost solution before I take it into the dealer. The sensor that is broken is the one in the trunk door latch, so I can not even get to it. So they would probably have to replace the whole latch system.

    hitthespot: Thanks for the advice. I was hoping to be able to just test the main battery load using the battery terminals, but I guess that will not work. It might be hard to isolate the wire that leads to the trunk sensor. Even if I could just disable the sensor I would be fine. And I didn't realize that the cheap sensors do not measure that high amps.

    Thanks, I guess I will just have to take it in and get a price estimate.

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    Flashaholic* matrixshaman's Avatar
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    Default Re: How to check the current on a car battery?

    Sensors might be wired to a relay which will draw current if the sensor is triggered. You might bypass the sensor if you can get to the wires and make your own sensor with a typical car alarm sensor or plunger type sensor. You might find something that would work at a junk yard too. Some car alarms will put a drain on a battery but if the car is used daily it shouldn't be a problem. While you don't want to short out a car battery and potentially have several hundred amps going through something metal if you are careful you might disconnect the negative terminal. Stick your ampmeter (at least 10 amps) between the negative on the battery and the negative cable connector. Have everything turned off. Then you will see if there is a current draw and can try different sensors or disconnecting the alarm etc. It would be good to have a helper to keep an eye on everything while you are probing around in the car. Car electronics can be a PITA but the other option beside doing it yourself is usually a big pain in the wallet.
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    Flashaholic* Wattnot's Avatar
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    Default Re: How to check the current on a car battery?

    Quote Originally Posted by TooManyGizmos View Post
    You should replace the door ajar switch which you already know is bad.

    That should resolve it .
    +1 on that.

    Especially if the car alarm is using that switch too. That switch being dead could be causing the car alarm to draw current for some reason . . . car electronics are funny that way.

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    Flashaholic* TooManyGizmos's Avatar
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    Default Re: How to check the current on a car battery?

    Sometimes those door ajar switches are adjustable.

    See what the dealer tells you.


    It may be a simple adjustment.
    ~ "She" says ... ... I have ... TooManyGizmos ~

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    Default Re: How to check the current on a car battery?

    How long does it take before you battery drains completely? A day, a week, a month? That kinda tells you how bad your current draw is.

    If it's just a small drain, disconnect the battery and put the meter in between the cable and battery. Just don't try and start your car or turn on your headlights while you're doing that. Start with a 10A setting just in case there is a large draw, but most meters should have a fuse in it anyway to prevent any short circuits.

    Newer cars always draw a small amount of current to keep the computer, clock, radio, etc. working. Should be less than a few hundred milliamps though. Hook up the multimeter to read your current draw, then pull each fuse one by one until you see it drop. There might be two fuse boxes, one under the dash, and one in the engine compartment. Once you have the problem circuit isolated, then you can do some more detiled troublshooting. As other people have mentioned, a faulty door switch might be controlling more than just the dome lights, so that should be fixed first.
    Last edited by InTheDark; 11-15-2008 at 12:01 PM.

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    Default Re: How to check the current on a car battery?

    Quote Originally Posted by InTheDark View Post
    How long does it take before you battery drains completely? A day, a week, a month? That kinda tells you how bad your current draw is.

    If it's just a small drain, disconnect the battery and put the meter in between the cable and battery. Just don't try and start your car or turn on your headlights while you're doing that. Start with a 10A setting just in case there is a large draw, but most meters should have a fuse in it anyway to prevent any short circuits.

    Newer cars always draw a small amount of current to keep the computer, clock, radio, etc. working. Should be less than a few hundred milliamps though. Hook up the multimeter to read your current draw, then pull each fuse one by one until you see it drop. There might be two fuse boxes, one under the dash, and one in the engine compartment. Once you have the problem circuit isolated, then you can do some more detiled troublshooting. As other people have mentioned, a faulty door switch might be controlling more than just the dome lights, so that should be fixed first.
    The current limit you are looking for here is 70 milliamps. The door switch could be going to the security system and other things.

  10. #10
    Flashaholic* offroadcmpr's Avatar
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    Default Re: How to check the current on a car battery?

    I usually only drive my car when I get groceries or maybe on the weekend. During the week I just walk to campus.
    It is hard to tell how long the battery lasts because I am not sure when it is fully charged. I think that if I were to charge it completely and drive it once a week for 15 minutes after a couple of weeks it will be dead. But then if I jumpstart it and continue on like normal it will only take a few days before it is dead again. So on a fully charged battery it would probably take a little more than a week for it to die.

    InTheDark: I believe that is what the mechanic did. He said that he tested the current and kept on pulling out fuses until it stopped. The fuse that he pulled was the one that controlled the things I mentioned before.

    jrmcferren: Now that I think about it, that seems to make sense about the door switch going to the security system. Since it is not working, it could be causing the alarm to act up and draw power. So it could be the problem, but the root problem is probably door sensor.

    I don't know when it started because my brother drove it to school everyday before I got it, so it was never sitting around enough for the battery to die.

    Thanks again, I think that I am going to try to take it in and get an estimate. If it comes out to a lot, then I will try something else.

  11. #11
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    Default Re: How to check the current on a car battery?

    you may not have to measure the current amps to see what is drawing the most current.
    it might be possible to attach the meter, set on 20 volts or so, to the plus and minus terminals of the battery and observe the voltage drop when you engage the various circuits..even a 70 milliamp draw could affect the voltage reading by a few milivolts -- the larger the draw the more the drop, this will enable you to judge which device is drawing the most current...without much danger of short circuits, unless of course you forget to set the tester on volts instead of amps, in which case you'll need a new tester, or a new fuse for it.

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    Default Re: How to check the current on a car battery?

    I had a similar problem with my Isuzu Trooper. I went through 3 batteries in the first 3 years believing something was wrong with the battery because it appeared that it couldn't hold a charge and I would get in and try to start it, and the battery was just dead, especially after sitting for a long weekend on the driveway of my home. Then my suspicion centered on the alternator somehow not charging when I replaced the battery and the same thing happened after a few months.

    The problem was that one of the rear side doors was ajar... ever so slightly that you couldn't tell by just looking at it and the dome light would stay on draining the battery. During a bright sunlight daytime, its nearly impossible to tell if the dome light is on from outside. The dome light has a three position switch, on-off-door open. Although the "door open" position was preferable, what I found is often passengers weren't aware of the problem and when they exited the car they failed to give the door a good solid push into the second latch to fully close it. Now anytime I lock up the car, I check the dome light to make sure it is off. BTW, after discovering the problem my last battery has been going for the better part of 5 years now without any repairs to the electrical system.

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    Flashaholic* offroadcmpr's Avatar
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    Default Re: How to check the current on a car battery?

    I figured that the battery was not holding a charge, so I got a new one, only to my dismay to have it dead in a couple of weeks. Now i just got ti figure out when I will have some time to bring it into the dealer.

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    Default Re: How to check the current on a car battery?

    Quote Originally Posted by offroadcmpr View Post
    I figured that the battery was not holding a charge, so I got a new one, only to my dismay to have it dead in a couple of weeks. Now i just got ti figure out when I will have some time to bring it into the dealer.
    Did you mechanic do the basic alternator check? No problems with alternator?

    Bill

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    Flashaholic* offroadcmpr's Avatar
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    Default Re: How to check the current on a car battery?

    I don't know exactly. I just know that he tested the fuses and found the one that was causing the problem. But now that I have taken the fuse out I have not had any problems for the past 2 weeks, so unless the fuse controls something with the alternator I doubt that it is the problem.

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    Default Re: How to check the current on a car battery?

    It might be a good idea to check the alternator anyway. A dead battery can kill an good alternator, and vice versa, so while you might replace one, the other might have already been damaged. Most auto parts stores will check it for free if you bring it in.

    Since you have isolated the problem circuit, just hook up an multimeter and put the fuse back in, then start narrowing it down to which component is causing the problem. Unplug the door switch, the radio, the alarm one by one and check to see if the current draw changed significantly after each one. You've already figured most of it out, just a little more work and you should have the problem solved.

    Quote Originally Posted by jrmcferren View Post
    The current limit you are looking for here is 70 milliamps. The door switch could be going to the security system and other things.
    Good info to know in the future. Does that only apply to this particular model vehicle, or is that universal for all cars? I would think an different ECU's, aftermarket alarm, stereo, or other electronics would change that limit. Either way, the difference between a short and sustaining current should be pretty significant, so you just want to look for something abnormal

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    Default Re: How to check the current on a car battery?

    I think the 70 milliamp limit is to prevent excessive discharge of the battery. I believe that is independent of the car model. I just can't seem to find the source or I would point you to it, but if a 9 volt battery is used to cancel out the effect of removing the main battery, I think the maximum would be in this range.

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