Hi Guys, Seeing as im asking silly electrical questions today here is the scenario: friends battery goes flat when switched off. I took my mutilmeter and measured from the + on the battery to the ground (used the engine block) and got a reading of 12V. My electicaly challenged brain says to me that means the battery is draining and something is causing a ground? Otherwise there wouldnt be a current flow between the battery + and the ground? Danny
Hi Mate, Its an internally reg alternator. The alternator is stuffed, as its not charging the battery. You can measure accross the terminals when it is revving and it shows 12V, when it should should sround 14 or so? Danny
A voltmeter is just showing the difference of potential between the battery and ground. A good battery should show 12 volts. You need to use an ampmeter to look for a current drain. Use a good variable range ampmeter and starting with the meter set on the highest amp range, disconnect the positive lead on the battery and connect an ampmeter, (-) terminal of ampmeter to the positive post on the battery and (+) terminal of ampmeter to the positive lead. Decrease the range, on the ampmeter, if you get a reading then you have a current drain. Pull fuses or disconnect wires one at a time until the current goes to 0, that will indicate the circuit or area of the problem. A voltmeter is good for troubleshooting if you know what your doing, for instance a voltmeter will show voltage across a bad fuse but not a good one on the other hand a voltmeter will show voltage across a de-engerized relay contact but not an engerized one.
Need an ammeter, your multimeter should have that function. Lift the negative terminal on the battery (safest method) and read for D.C. Amps between the cable lifted and the battery terminal. Then you have to chase it down, lifting fuses first, positive terminals second. When the current stops you have the culprit circuit. If the meter deflects the wrong way reverse leads.
It might be the alternator itself. Sometimes when they go bad they can drain the battery. Disconnect the alternator and see if the battery still goes down.
We use a small battery load tester at work to do it also if it is the alt.it will be warm while it is pulling the drain.
If you have a regular 12V test light disconnect the positive terminal on the battery & put the test light clip on the terminal & poke the test light into the pos.post.( leave them seperated)If theres a draw the light will light up.Then do as everyone says & disconnect fuses. Could be the alt.If its a GM & you just hooked the two small slip on plug to the battery to excite the reglator it will drain overnite. jimV
WE HAD ONE IN OUR SHOP DOING THE SAME TYPE OF THING."SACHSE ROD SHOP".THOUGHT IT WAS THE ALT., HAD IT CHECKED OUT BY "DIRTY2" WAS O.K. IT ENDED UP BEING THE CAPACITERS IN THE AMPLIFIER THE CUSTOMER HAD US TO INSTALL. MANUFACTURER SUGGEST TO HOOK UP POWER FOR THE AMP DIRECTLY TO THE BATTERY,SO WE DID. EVEN WITH A BATTERY DISCONNECT IT STILL DRAINED BATTERY."AMP WAS HOOKED TO INLET SIDE OF DISCONNECT.
ummm you just tested the battery voltage and it has 12v like it should. what do you mean by "flat", does it not crank after sitting awhile or just after you turn it off, do the headlights turn on when it's flat (or other things still work). the way to test for a draw is to take off the neg terminal, set your meter to amps (and change the lead if needed), put it in series (red lead to battery, black lead to termial) and see how many amps it's drawing.
there should be 2 wires for the amp. 1 goes to the battery and the other goes to the radio, the wire from the radio (maybe blue) controls the turning on of the amp (when the radio is turned on).....also the wire to the battery should be fused.
Is this an existing car with a new problem or a new build that isn't right? I have seen this problem on a new build. Turns out what was thought was "OFF: was really the "ACCESSARY" pos***ion on the key switch. Sounds dumb, but it can happen to anybody.
Something is staying energized when the key is off and you walk away. Or you never really are charging the battery. To prove the battery crank it down, then recharge it, to prove it's ability. Next you'll need a DC Ammeter, shut the car down, lift the negative cable, place meter in series with cable and negative terminal. Current better be Zero.. Things like car alarms take maybe one milli amp, more than 50 milli amps has to mean something is on all the time.
hey guys, thanks for the help. i replaced the alternator as it was dead (i measured only 12V accross the terminals when it was reving). After replacement it measured 14.5V. The battery is brand new. I was getting 12mA all the time with the multimeter in series. I removed all of the fuses one by one and suddly it went to 1mA (which I think is the for the clock in the radio). When I replaced that fuse it stayed at 1mA. I'm not sure if some how the way the fuse was positioned in the fuse holder was causing it to short accross but it stayed at 1mA even after a few off/on/drive cycles. I dont like it when things suddenly fix with no good reason becuase they usually unfix with no good reason Thanks to everyone who posted, you really helped me out a tonne and saved some money by not having to take this heap to the auto elec. Danny
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