Author Topic: How to measure the mAh capacity of AA/AAA NiMH batteries using a multimeter?  (Read 55869 times)

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Offline gwet

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I am trying to measure two AAA batteries of 600mAh each. Plugging the probes into the COM and 10A input jacks gives me a reading of 1 and 1.4 amps respectively.

I know I am doing something wrong...... and I would probably have blown a fuse if I had plugged one probe into the mA input jack...
 

Online Simon

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You can't measure the mAH like that, you are short circuiting the battery, you always measure current by putting the meter in series with the load,to calculate the mAH of the battery you should discharge it and measure the current until it is empty and then work out how much current came out of the discharge period
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Online Zero999

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Lol your measuring the short circuit current of the batteries not their capacity and are lucky it wasn't high enough to damage the meter.
 

Online Simon

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I did the same with my first meter, one of the reasons in the other thread I recommended a 10A meter as a first meter
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Offline Psi

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If they are 600mAh batteries then that means they can theoretically supply 600mA for 1 hour.  You can't test this by just touching a meter (or any other test gear) on the battery for a sec and taking a reading. There is no way to know how much energy is in the battery other than to discharge it and see how long it takes.  ie, connect the battery up to some load, like a light bulb, and record the current readings at regular times until it's flat. Then you can work out what the capacity of the battery was using the current readings and the length of time it took.  If the battery was fully charged when you started then you have worked out the maximum energy the battery can hold which is what the 600mAh written on the battery means.

If you just wanted to see how 'good' the battery is then set the meter to volts and take a reading. 1V=flat  1.5V=full  or 1.25V=good (if it's a rechargable battery).


Sometimes people do connect *small* batteries up to a meter on the 10A scale for a fraction of a second to see how charged a battery is. However it's a very bad habit as it's not all that good for the battery and if you ever did it to a big battery your meter would probably explode, or at the very least blow the fuse. (even a little AA can supply over 10A if it's a good quality one and fully charged when shorted out)
The readings you get when you do do it are a combination of how much current the battery can supply under extreme and ridiculously high load and the batteries internal resistance. Because the battery is being pretty much shorted out the voltage is so low that the readings are pretty meaningless except to compare with other batteries tested the same way on the same meter. Different meters will give different readings as they're not really designed to measure current at such a low voltage.
  So, long story short.... Don't do it, its bad :P
« Last Edit: January 02, 2011, 12:27:21 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline tyblu

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Note that a "milliamp hour" number is assigned for a particular discharge curve. A battery's capacity will change based on its output current. Take a peek:

http://www.powerstream.com/AA-tests.htm
Tyler Lucas, electronics hobbyist
 

Online Simon

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usually it's based on a 10 hours discharge rate so basically using 1/10 the batteries capacity per hour, the faster you discharge the battery the less total power you will get from it, the slower you discharge it the more total power you will get
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Online Zero999

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I did the same with my first meter, one of the reasons in the other thread I recommended a 10A meter as a first meter

I've done it too but it was an accident. I didn't do it deliberately to measure the current capacity of a power supply or the Ah rating of some batteries.

I've never really understood why many inexperienced people do this kind of thing nor why they ask if they can safely power a low power device from a power supply with a much higher power rating. When I learned about ammeters I found out they have a low resistance so deduced that connecting one across a battery would be a bad idea. When I learned about Ohm's law, voltage, power supplies and their maximum ratings I automatically knew I could safely power a walkman which uses only 100mA from a 1A power supply. I knew all of this before I received any formal education on electronics so it can't be because I've had an excellant education and I was never top of the class in electronics at college, though not far off.
 

Online Simon

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Well I did it in the VERY early days long before any formal education at right at the beginning of my informal education. I think it is something to do with the way we treat voltage and you assume that amperage is treated similarly
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Offline gwet

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Thanks for your replies. I have tested the multimeter fuses (self test function) and they are ok.

Yeah, I have been a bit daring and even stupid because days ago I read that current must be measured in series!
 

Offline sonicj

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the cba from west mountain radio is a handy tool for this sort of task.

there is a lengthy discussion on rcgroups about building your own constant current discharger with logging.
 

Offline Longhair

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The real problem lies with Lithium Polymer (LiPo) batteries when one does not know the capacity.

Over charging them has some very bad effects when used with an insufficient charger :(

 

Online Simon

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I'd guess the one in the video was seriously overcharged intentionally to make it blow up like that, I expect many battery types could be a bit volatile under extreme overcharging conditions
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Offline saturation

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Batteries have different mAH for discharge currents, so alas, what you'd want to do is get the spec sheet of the brand of battery, or a model battery, and see if your battery meets or exceeds the mAH for each discharge current.

Today, you can buy such a charger/reconditioner, and set the discharge current to easily estimate mAH.



Best Wishes,

 Saturation
 

Offline Longhair

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I have some LiPo batteries for RC Vehicles (cars, trucks, small helicopters) and other than the invoice, there is no data sheet. Sometimes they have the mAH on the outside label and other times they don't - that is where the permanent marker comes in handy.

There are a lot of cheap (poor quality) battery chargers out there and people will buy them up like there is no tomorrow because of the low price. Unfortunately if they are not careful in what they are doing, there will be no tomorrow for them.
 

Offline tyblu

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I've never really understood why many inexperienced people do this kind of thing nor why they ask if they can safely power a low power device from a power supply with a much higher power rating. When I learned about ammeters I found out they have a low resistance so deduced that connecting one across a battery would be a bad idea. When I learned about Ohm's law, voltage, power supplies and their maximum ratings I automatically knew I could safely power a walkman which uses only 100mA from a 1A power supply. I knew all of this before I received any formal education on electronics so it can't be because I've had an excellant education and I was never top of the class in electronics at college, though not far off.
It is very easy to make mistakes in a new field. We've all done stupid mistakes, even you, despite your proficiency with Ohm's Law. We will never stop making them, either, though hopefully not the same one twice! E.g.: last night I set my alarm for PM instead of AM for the umpteenth time.
Tyler Lucas, electronics hobbyist
 

Offline Kiriakos-GR

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About battery tester , for any type of battery , I have not find yet better than that..

https://www.eevblog.com/forum/index.php?topic=448.msg6650#msg6650

It test every battery with load !!
 

Online Zero999

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It is very easy to make mistakes in a new field. We've all done stupid mistakes, even you, despite your proficiency with Ohm's Law. We will never stop making them, either, though hopefully not the same one twice! E.g.: last night I set my alarm for PM instead of AM for the umpteenth time.
Yes I make stupid mistakes all the time i.e. I made the mistake of blowing a DVM's fuse by trying to measure voltage whilst it's set to current but that's not what I meant. I was saying I don't see why some inexperienced people think it's fine to measure a battery's capacity by short circuiting it with a DVM, re-read my previous posts. This is not an absent minded mistake but a failure to understand something basic, which no one needed to explain to me, even when I when I first started. When I blew the DVM's fuse I knew damn well I had done a stupid thing by trying to measure voltage when it was set to current.
 


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