How to measure the capacity of a NIMH Cell

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Alphi
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How to measure the capacity of a NIMH Cell

One thing i'm not quite sure on is how to measure the real Capacity of a NIMH cell.. instead of the one printed on the side.

I bought some cheap 1.2 Volt NIMH 2600MAH cells..

when I charge them up.. the voltage goes up to about 1.34 and when I measure the current it says about 2.6 - 2.9 Amps.

I was just wondering if anyone knew a little more about battery chemistry than I do.

If it says the current from the battery is 2.6-2.9 Amps is this directly related to its capacity (ie the 2600MAH rating that it should be able to deliver 2.6 amps for one hour) ?

Or does it simply mean that the battery can deliver 2.6-2.9 amps with no relation to the duration at all.

I know that most batteries are rated using a current draw of 1 AMP so a 2600MAH battery in theory should deliver 1 AMP constantly for 2.6 hours (Rather than 2.6 AMPS for 1 hour).. but does it correspond direclty to the Amp measurement i'm getting from the battery OR is it just coincidental that the numbers are so similar)

cheers.

Brock
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Joined: Thursday, August 16, 2007 - 19:27
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Re: How to measure the capacity of a NIMH Cell

Check out this thread over on Candle Power Forums

http://www.candlepowerforums.com/vb/showthread.php?t=79302

If you want to test them yourself I would suggest either
http://www.thomas-distributing.com/maha-mh-c9000-battery-charger.php
or
http://www.thomas-distributing.com/la_crosse_bc-900_battery_charger.php
Both can test them under load. It is a good idea to use relatively matched cells in devices to get the best performance in the long run.

Alphi
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Re: How to measure the capacity of a NIMH Cell

thanks man, I really should test them under load with the right gear before I buy more..

I bought 420 cheap green noname batteries and surprisingly they worked quite well together.

all of them are from the same manufacturer and the same MAH rating.. and thats about as far as I can go with matching their capacities up. sitting down and measuring each one individually would be quite painful.

but there is an easier option....

run the bike for a few weeks.. and then run it flat..

then open up the pack and then simply take out the dead cells... its easy to tell the duds..

they either expand due to over heating leaving traces of oxidation.. or they reverse in polarity.

the my very first pack.. lasted a few months.. then I left it in the garage for a few months.. and the pack refused to hold a charge after that.. so I opened it up and pulled all the batteries out...

to my surprise only about 20 batteries had failed.. and that was after all the harsh punishment i put them through..

since i bought them for $1.10 US each thats only $22 that I lost..

which is still far cheaper than the cost of the petrol and parking that the ebike saved me.

HCT
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Re: How to measure the capacity of a NIMH Cell

Amp/Hour capacity is only a indication of how energy you can store ( how big the tank is ) internal impedance is what size pipe or wire you have connected to the tank .
Both discharge and charge max rating are directly related to internal impedance of the cell, low impedance permit higher charge and discharge current Dissipated heat =Watts = I*( square) X R example 10 amps charge with 10 milliohms impedance would generate 1 Watt ( 10 X 10 X .01 ohm )
So when trying to decide on which is the better cell , just measure the internal impedance .
MOST RELIABLE MANUFACTURER HAVE NO PROBLEME DIVULGING THE INTERNAL IMPEDANCE OF THEIR CELL ,
The ones that do not have it or won,t specifite it , I would stay away from .
The impedance of battery or cell can be measured using a HP4328A milliohmmeter , this instrument is the standart one used by most battery manufacturer ( check their spec sheet) have recently seem several units sold on E-Bay from $100.00 to $ 800.00.
Have been using one for over 25 years and would not be without it .
It is the first measurement that I make to determine the cell state of health .Regardless of cell chemistries ,volt, amp/hour rating , the impedance is the determining factor how fast you can take the energy out .

HEAT & LOSS & INEFFICIENCY RELATED DIRECTLY to IMPEDANCE

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