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  #1  
Old 02-08-2010, 01:06 PM
blood63 blood63 is offline
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Could I use a multimeter to test a battery?

I have dozens of dead batteries laying about and I have multimeter which I used infrequently to measure if a wire is live or not.
Could I use this to test a battery?
What setting would be best?
Even dead batteries produce some voltage so voltage is not a good test.
If I measure for current, I am draining the battery as I test (and possibly damaging my meter).
Is there another setting I could use?
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  #2  
Old 02-08-2010, 01:37 PM
Patty O'Furniture Patty O'Furniture is offline
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A meter is good for testing a cell's terminal voltage and not much else. Because of a multimeter's high input impedance, it won't tell you anything about how well the cell will perform under load. If you have a 1000 ohm resistor, place it across the cell's terminals and then measure the voltage. Bonus points if the voltage stays constant for more than 10 seconds or so.

An easy way to determine how much useful work a cell can do it to put in a simple load like a flash light, where the output can be "measured" with our innate senses.
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Old 02-08-2010, 01:48 PM
Rhythmdvl Rhythmdvl is offline
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Relevant thread from November:

Can I use a multimeter to test a AA battery?
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  #4  
Old 02-08-2010, 02:38 PM
UncleFred UncleFred is offline
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Well, there's always the Red Green approach from his 'Handyman's Corner'. Mostly he uses it for car batteries but it may work for smaller ones;

"Use a wire or a wrench if you have to but connect your tongue across the battery terminals. Now look at your watch. If you've been out more than an hour the battery is good."
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  #5  
Old 02-08-2010, 03:00 PM
Sinaptics Sinaptics is online now
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A slightly related question:

Twice recently I've went out in the morning to find my car battery dead. The battery is brand new, less than 5 months old. I was planning on taking out the alternator this weekend to have it tested but the snow screwed me up.

Assuming that the alternator isn't bad and that for some reason there is a slow drain on the battery (i.e. some piece of electronics staying on when the car is off), is there a way to test this at the battery with a multimeter?

I would think that it's only going to show me the terminal voltage/amps and not tell me if any juice is leaking out.
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  #6  
Old 02-08-2010, 03:25 PM
Gary T Gary T is offline
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Quote:
Originally Posted by Sinaptics View Post
Assuming that the alternator isn't bad and that for some reason there is a slow drain on the battery (i.e. some piece of electronics staying on when the car is off), is there a way to test this at the battery with a multimeter?
If it includes an ammeter of suitable range, yes. Up to 50 mA drain is considered acceptable. There is a way to do it with a voltmeter and a resistor as a shunt, but I don't recall the details.
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Old 02-08-2010, 04:16 PM
engineer_comp_geek engineer_comp_geek is offline
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Quote:
Originally Posted by Gary T View Post
There is a way to do it with a voltmeter and a resistor as a shunt, but I don't recall the details.
Use a resistor with a very small resistance (note that if there's a chance that significant current is going to flow you'll need a fairly high wattage resistor or else your resistor might catch fire). Measure the resistor so you know its precise value (a 1 ohm resistor isn't likely to be 1.00 ohms). Disconnect the battery and put the resistor in series with the battery terminal. Then just measure the voltage across the resistor.

Then just use ohm's law. V=IR. Since we're trying to find I, it's just I=V/R. You just measured V and you measured R before you put the resistor in place.

Note that some radios and alarm systems don't like to have the battery disconnected like that. Follow your manufacturer's guidelines for stuff like that. Ammeters have to be in series with the circuit as well, so the same rules about disconnecting the battery apply if you are using a real ammeter too.

If you detect excessive current flowing, remove the fuses one by one until it goes away. Then you at least know which circuit the offending device is on. Note that some devices don't go through the fuse box under the dash, so if you go through all of the fuses and don't find the guy drawing the current you'll have to do some more digging. Everything in a car is protected somehow, so if it's not protected by going through a fuse then there will be some other protective device somewhere, like a fusible link under the hood somewhere.

By the way, a quick and dirty test of the alternator is to measure the voltage when the car is off and compare it to when the car is running. Usually you'll be somewhere around 12.6 volts or so with the car off, and it will go up a volt or two while running (more or less depending on how fast you rev the engine).
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Old 02-08-2010, 10:01 PM
Gbro Gbro is offline
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Check out this page.
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  #9  
Old 02-08-2010, 11:02 PM
Rick Rick is offline
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Can you test a battery's condition with a voltmeter?
of course you can.
Both automotive batteryy's and alkaline disposable battery's voltage has a direct relationship to its state of charge.
For example on a car battery the following numbers apply
Above 12.72 surface charge, turn on head lights for 30 seconds, then off, wait 2 minutes and retest
12.6-12.72V= fully charged.
12.4V= 75% charged
12.25V=50% charged
12.00V= 25% charged

AAA, AA, C, D cells all have similar state of charge curves, but off the top of my head, I don't know what they are.

to find a draw in a car, there are a couple of ways to do it. The stupid but it works way is to use a 12V test light. Disconnect the neg battery terminal and connect the test light between the battery post and the battery cable. If the draw is large enough to be a problem the light will light. No light = no problem.
You can also use the amp meter function of the multimeter (NOT the voltmeter function) as Gary T says anything less than 50ma is not a problem.
Word of warning, on some modern cars the electronics shut down can take anywhere up to 10 minutes or so, with varying draws during that time. Refer to proper service material to make sure you are not experiencing a normal condition.
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  #10  
Old 02-09-2010, 08:07 AM
Gary T Gary T is offline
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Quote:
Originally Posted by Rick View Post
Word of warning, on some modern cars the electronics shut down can take anywhere up to 10 minutes or so, with varying draws during that time. Refer to proper service material to make sure you are not experiencing a normal condition.
Good point, and some take quite a bit longer than that.

The situation is that after some module circuits are deactivated, which will happen whenever the battery is disconnected, then upon being reactivated the module will go into a "ready" mode which draws a significant amount of current. After a period of time, the module then goes into a "sleep" mode and the current drain returns to normal. I know of some systems where the period of time is 30 minutes or so, and I believe there are some where it's an hour or two. So if you disconnect the battery in order to hook up the ammeter, you don't get an accurate reading of normal parasitic drain right away, you have to wait until the modules go to sleep. The way to avoid this problem is to have the ammeter leads connected to the battery terminal and the battery cable end before disconnecting so there is no interruption of current flow. Easy to say, but sometimes very tricky to do.
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