# Testing Batteries With a Multimeter



## Kent10 (Feb 4, 2009)

Hi:

I recently bought a multimeter to use with my computer. I also heard that you could test batteries using the multimeter, but that it is not reliable unless the battery is under a load.

I would like to test many types of batteries including button batteries such as CR2025 and CR2032 as well as some smaller battery watches such as a 386, 395 and 377.

I suppose I could buy one of these http://www.ztsinc.com/mbt1.html but they are rather expensive and I already have a multimeter.

What would I need to buy to use the multimeter to test various batteries. I read somewhere that a resistor such as this http://www.radioshack.com/product/index.jsp?productId=2062291 
could be used. Would that work? And would I need a different resistor for different batteries. Could I use the same one for all 1.5v batteries, for example.

Thanks so much for any help. Kent


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## bobelectric (Mar 3, 2007)

When in Doubt, Change Tnem Out.


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## Kent10 (Feb 4, 2009)

Thanks bobelectric. That would work. It is just that I have a couple of kids and a wife who like to mix used batteries and new batteries and so it would be nice to quickly test them. Also I have clocks, for example, that may use up one battery but the other is still good so only one will need to be replaced. And it is kind of fun for us to test the batteries. Thanks.


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## Pudge565 (Jan 27, 2008)

Kent10 said:


> Thanks bobelectric. That would work. It is just that I have a couple of kids and a wife who like to mix used batteries and new batteries and so it would be nice to quickly test them. Also I have clocks, for example, that may use up one battery but the other is still good so only one will need to be replaced. And it is kind of fun for us to test the batteries. Thanks.


Never use new and old batteries together.


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## Kent10 (Feb 4, 2009)

Is that true even if the older battery is still mostly charged. I have a clock that uses two 386 batteries and when the clock goes weak, one battery seems to be strong while the other is depleted. If I just change the one battery, the clock works very well.


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## Pudge565 (Jan 27, 2008)

I don't know I just follow the directions on the package and that is what they tell me.


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## bobelectric (Mar 3, 2007)

At least ,change out Smoke Detector batteries, Get recharables for all the gadgets.


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## Ultrarunner2017 (Oct 1, 2008)

Here's an inexpensive battery tester sold at your local Radio Shack store.
I have been using their older model for many years, and it's still working.

http://www.radioshack.com/product/index.jsp?productId=2991084

FW


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## Gigs (Oct 26, 2008)

The problem with using one stronger and one weaker cell in series is that toward the end of the life, the weaker one can get reverse charged and burst.

As an exaggerated example lets say you have one cell at 1.5v and one at 1.0 volts. Then they discharge and one is 0.5 volts and one is 0 volts... now the stronger one will force the weaker one into negative voltage. It's not that simple in real life but that's the idea.


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## Yoyizit (Jul 11, 2008)

Load a new battery so the voltage drops 10%. Apply this same load to the BUTT (Battery Under TesT). www.hosfelt.com and allelectronics has resistors for this. 

Or get a schematic for a battery tester and use those same values and same power ratings.


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## AllanJ (Nov 24, 2007)

The act of testing under load takes power from the battery and can significantly shorten the life of a small button battery.

If you must test a battery, do it as quickly as possible to reduce the power consumed.

When a battery has questionable strength left, the kind of tester that displays the word "good" or something like that on a little screen will take so long to not display that and much of the remaining power in the battery is consumed.

The size of a resistor for loading depends on the size (in terms of power rating) of the battery. It is definitely not a case of the same value resistor for all 1-1/2 volt batteries etc. Too large a resistance will not perform a meaningful load test on a large battery such as a size D. Too small a resistance will deplete a small button battery in seconds.


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## Yoyizit (Jul 11, 2008)

The 386 has a 120 mAh capacity. This is the "C" rate. 
Usually a battery capacity is measured at the 20 hour rate, which in this case means discharging it with a constant current of 6 mA. 

For a watch battery like this to last 30,000 hrs. would take a max. drain of 4uA, but batteries behave strangely at very light loads.

At 1.55v this is a 1/8w load resistor of 258 Ω for a constant resistance discharge. This load shouldn't harm the battery but I don't know how low the voltage should drop for a used battery for a pass/fail judgement. The manufacturers may help you with this. If you e-mail 5 you might get 1 response.

Also, try
http://www.google.com/search?client...lver+oxide+discharge+curve"&ie=UTF-8&oe=UTF-8
but substitute each battery's techology for "silver oxide".

What you're really testing for is the increase in internal impedance as the battery gets lower in charge, but finding values for this for these types of batteries is like pulling teeth. 
A new AA cell has about 0.2 Ω, which means a 1A load would drop the voltage 200 mV.

Please post back with what you find. . .


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## Kent10 (Feb 4, 2009)

This is all very interesting to me. Thanks for all your replies. Gigs: I understand what you are saying about using a stronger and weaker battery together. It probably isn't a good idea. With the alarm clock I mentioned, I wonder if one battery is used for the alarm and the other for the clock because one always gets used up a lot quicker than the other. So except right when I put in 2 new batteries, I think one will always be weaker than the other until one is depleted. I have another wall clock and the same thing happens. One depletes much more quickly than the other so it seems one is always weaker except when 1st installed. Has anyone seen a weak battery burst? I have had leakage from batteries kept in an unused device too long. Is this the same thing? Thanks for your help.


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## Gigs (Oct 26, 2008)

The open circuit voltage isn't as useless as you guys make out. Yes, it can be misleading, but at the same time, it's still gives a rough approximation for most batteries.


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## PaliBob (Jun 11, 2008)

Gigs said:


> The open circuit voltage isn't as useless as you guys make out. .......


 I'm with Gigs, if You measure a bunch of batteries of the same type and they mostly measure within a few percent of each other then they are keepers. If any are significantly lower, trash em.


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## DoctorIz (Jan 16, 2011)

*According to Radio Shack (**http://www.radioshack.com/uc/index....ibrary/BuyersGuides/research/Batteries05.html)*

*How to test batteries* 

The most important thing to understand about testing batteries is that *batteries will not show their correct voltage unless they are under a load.* If you simply take a multimeter and connect the leads to each battery terminal, you will not get an accurate reading. 








Battery testers are designed to place the battery under a load in order to get an accurate reading and are best for normal consumer use; however, multimeters provide a more accurate reading of the voltage potential. If you are needing a high level of accuracy (such as for design or testing), then you should use a multimeter. If you are just wanting a device to let you plug in a battery and know whether or not it's still good, a battery tester is a better choice. 








When testing batteries with a multimeter, you must have the meter set to measure DC voltage and place the battery under load by using a resistor in parallel with the test leads of the meter. If the voltage reads the same on a multimeter with the resistor or without, then the battery is either low or dead and should be replaced. 








*A battery can be used until it is down to about 30% capacity*. After that, it typically will not supply enough voltage to power the product in which it's being used. The actual cut-off voltage will vary.


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## DoctorIz (Jan 16, 2011)

Duracell says:



> Open circuit voltage ranges from 1.5 to 1.6 volts. Nominal voltage is 1.5 volts. Operating voltage is dictated by the state-of-discharge and the actual load imposed by the equipment. The voltage profile under discharge is a sloping curve


They also provide a graph that can be used to tell how fresh your battery is and how long you can expect it to last. Worth checking out.

http://www1.duracell.com/oem/primary/alkaline/alkvoltage.asp


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## a7ecorsair (Jun 1, 2010)

Kent10 said:


> Thanks bobelectric. That would work. It is just that I have a couple of kids and a wife who like to mix used batteries and new batteries and so it would be nice to quickly test them. Also I have clocks, for example, that may use up one battery but the other is still good so only one will need to be replaced. And it is kind of fun for us to test the batteries. Thanks.


I've read through this entire thread and agree with much of it as far as testing, the series resistance of the battery, and the use of various testers. The idea of reverse charging a dry cell battery with another in series is a little far fetched. 
Kent, OP, in this clock mentioned above, how did you determine that only one battery was dead or weak? If the clock has two separate circuits then yes, only one may be bad. 
Batteries in series, like a two cell flashlight, with discharge evenly assuming there isn't some other issue with one of the batteries. Weak batteries will have a low voltage when measured with a multimeter. When a 1.5 volt standard battery, i.e. AA, C, D and the like, measure 1.2 volts, it is pretty much dead. Some equipment won't work when the open circuit voltage of a battery is less than 1.3 v. Button batteries come in several voltage levels so their application will dictate what is the cutoff level.
When a flash light battery gets weak the light just gets dimmer and dimmer, but with electronic circuits, the device just quits working.
Use your multimeter to test batteries. The more you use it to test various batteries the better feel you will get for what is good and what needs replacing.


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