Measuring internal resistance with chargers Opus BT-C2000 and BT-C100

Cemoi

Enlightened
Joined
Jan 23, 2008
Messages
527
Location
France
Hi all,
I did a series of 10 succesive internal resistance measurements on three AAA NiMH batteries (two unknown brand and one Maha Imedion 800 mAh), all aged several years.
I first used my Opus BT-C2000 charger using the "Quick test" function. Batteries were left untouched in the charger, which I simply disconnected from mains between two measurements.
Here are the values I got (mOhm) :
Batt1: 849 / 895 / 906 / 939 / 904 / 88 (!) / 1044 / 1183 / 646
Batt2: 288 / 312 / 375 / 469 / 451 / 414 / 404 / 373 / 376 / 334
Batt3: 804 / 1016 / 1072 / 924 / 914 / 916 / 898 / 995 / 952 / 870

Even though I'm aware that IR measurements cannot be very accurate this way (from HKJ's remarks in his tests, and from a warning in the Opus manual), I'm suprised to see such a variation whereas I have not even removed the batteries from the charger between successive measurements.

Then I measured the same batteries with my Opus BT-C100. This time, the battery was put in the bottom of the battery compartment, and the negative contact was slid away from the battery between two successive measurements, in order to interrupt the contact. Resuts were:
Batt1: 777 / 1166 / 2166 / 973 / 1528 / 912
Batt2: 963 / 708 / 483 / 440 / 729 / 569
Batt3: 957 / 970 / 838 / 895 / 967 / 1224

Hence even worse discrepancies than with the BT-C2000.

I know such high values mean my batteries are no longer in good condition (except maybe Batt2 for low current uses), but I wonder how come I get so different results for the same battery.
Measuring a brand new Sony VTC6 18650 with my BT-C100, I get much more consistent values, like HKJ did in his test of this charger.

On the other hand, I have a Fluke DMM, can I use its ohmmeter function to test the internal resistance of batteries?
 

sbj

Newly Enlightened
Joined
Feb 19, 2017
Messages
172
The most important thing when measuring internal resistance is that you ensure good contact.

I do it like this:
Press the minus sliding contact against the battery in the direction of the plus pole with one hand. With the second hand twist the battery back and forth a little to rub off any oxide layer. Then initiate the internal resistance measurement while maintaining the pressure.
I usually make three measurements in a row and take the lowest value.

You must never connect the resistance measuring function of the DMM to a live component such as a battery!:candle:
 

Cemoi

Enlightened
Joined
Jan 23, 2008
Messages
527
Location
France
Press the minus sliding contact against the battery in the direction of the plus pole with one hand.
Thanks for your suggestion, which results in a much lower variation of the resulting values, less than 10%.

You must never connect the resistance measuring function of the DMM to a live component such as a battery!
OK, thanks for the confirmation.
 
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