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Discussion Starter · #1 ·
hi guy's, something is killing the battery if the car sits unused for a couple of weeks. if i disconnect the batt. cable and a few weeks go by and reconnect it everything is fine. i suspect the internal voltage regulator is the problem since i have no other accessories in the car except a dome light and radio. how can i test the alternator to see if its drawing current on the battery.
 

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Bama 57 is correct, start with a DVM (digital volt meter). Unhook your negative battery post, set your DVM for DC amps and hook one lead to your battery and one to your negative cable. Your reading should be less than 200MA (.2Amps). If you have ecessive current draw you will need to unhook fuses etc until the current drops. Let me know how you make out. Are you sure the battery is OK ?
 

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That's the process for finding dark current which is often the culprit. It's done, of course, with the car not running! :)

I would suggest getting a fused multimeter (maybe they all are) so you don't fry it! Some would argue that, as with other tools, you get what you pay for and you should pay lots and get a good one (I'm not that kind of guy).

If you have excessive dark current, I'd first start by disconnecting things, if any, that you added to the car (stereo, for example). Do it one at a time and test for dark current (also called parasitic drain) again. You might find the problem quickly that way.

If the problem is more complicated than that, then I'm in over my head! Good luck. Let us know.
 

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Discussion Starter · #5 ·
killing the battery

yes the battery is good, i have a good digital voltmeter but the lowest scale on it is 400 volts dc. ill try it and see thanks
 

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You will need to test Amperage, not volts. With the negative cable disconnected from the battery connect the ammeter to the cable end and the post. Disconnect the alternator and watch to see if the draw stops. If not disconnect fuses one at a time until you find the circuit. I found a few volt meters that would drain 3 - 12 volt batteries in parralell over a 4 day weekend. Once you isolate the circuit then start disconnecting items in the circuit until you find which item it is.
Dave
ASE master heavy truck tech with 35 years mechanicing expirience.
 

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Dave, Great advice! Can you tell us what the amperage should be with the alternator connected if everything is okay? Thanks.
 

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Discussion Starter · #8 ·
killing the battery

thanks, i just borrowed a good meter for the test.i guess i should have .2 amps or less if everything is good. but i know somethings not right. if i have more than .2 ill start with the alternator wires first because i suspect the voltage regulator as of now. i just bought a battery disconnect switch that should do the trick ha! dave
 

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Any draw should be very minute. If you have a clock or a radio with a memory you will have a draw. These new radios with the memories in them hardly draw anything at all. Usually the tip off is when you disconnect the negative cable there is an arc or a small spark. You can disconnect circuits one at a time until you find the culprit.
Dave
 

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I've also heard that you need to discharge the system before taking any readings. This is done by touching the disconnected terminal to the one still connected, right?
 

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Let me see if I can give an example of the difference between voltage and amperage ...........

Volts is like having water behind a dam. The higher the water, the more
"potential" energy is stored. Just like charging a battery to 14 volts, a dam can only hold so much water before it spills over ( batteries boil over ).

Amperage is like having several sized water pipes attached to the bottom of the dam. All of the pipes have the same "potential".... ( height of water in the dam / available voltage in the battery ) , but the different sized pipes
( different size wires) can flow different volumes of water ( amperage ).

The voltage is the "potential" to do work. You can read 12 volts on a volt meter across several batteries, but the volt readings can't tell you how big the lake is behind the dam, or how big the battery is. ( capacity )

I have a 12 volt battery that fits in a walkie-talkie. It has the "potential" of 12 volts , but it doesn't have the "capacity" to crank a car starter.

The available current ( amps or amperage ) is determined by the size of the wire ( pipe ) leaving the battery ( dam ).

A big stream of water will do more "work" ( rotate a water wheel for example ) than a smaller stream even though both have the same potential
( volts or height of water behind the dam ) . Larger wires carry more current and do more work than smaller wires.

It's the "flow" of water ( electron flow measured in amps ) out of the battery that you want to measure.


Most Digital Multi-Meters have a 0-10amp scale designed to measure current flow from zero to ten amps.

For example , a 55 watt headlight draws 4.6 amps out of a 12 volt battery

12volts ( potential ) x 4.6amps ( flow ) = 55 watts ( work ) ( light )

If you tried to measure the current (flow) out of the battery with both headlights on and the running lights on, you would probably blow the 10 Amp fuse inside the digital multi-meter.

2 headlights x 4.6amps = 9.2 amps by themselves.

For sure, you wouldn't want to be measuring the amperage out of your battery and ask someone to crank the car so that you could measure the cranking amperage. Cranking amperage is somewhere above 100Amps.


So you need to have a general idea of how much current ( amps ) you will be measuring, and use a meter with enough range to safely handle and measure the current.

With all the lights off , I believe that a 0-10 amp meter "should" be able to indicte just how much "dark current" is leaving the battery.

Remember, even a 1 amp flow will drain a 200amp battery in 200 hours
(eight days ). That's one dome light or one trunk light.
 

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Yes, I've spent a lot of time looking at voltmeters....LOL !!

To measure AMPS ( drain ) you have to move the leads on a multimeter
to the "amps" position and select the proper range on the selector switch.

Remove the negative lead from the frame to the battery.

Connect one lead to ground and one lead to the negative battery post.

Now, all the current out of the battery has to flow THROUGH the ammeter,
and you can read the amps (flow) on the meter .

When you are through... REMEMBER !!!! ( don't ask me how I know )

Switch the meter leads back to the volts position , 'cause if you try to read volts with the meter still in the AMPS position, all you will get is a lot of smoke and a bad smell coming out of the meter :( :( :(
If you are lucky, maybe the 10AMP fuse will blow before damage is done, but don't bet on it.
 
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