battery tester / recharger question
I have a number of rechargable AA NiMH batteries of questionable remaining useful life. To identify "worn out" batteries I made an Arduino battery tester that drains a AA battery with a load that draws about 100 mA. To verify that the system was working correctly I ran 2 new batteries (Duracell, rated 2500 mAh and labeled by me as H3 and H4) through the tester a number of times. To my surprise I got wildly differing results. I repeated the experiment using a different battery charger and again I got differing results (shown below). I then reviewed my Arduino hardware and software again and found no identifiable problems. The circuit is almost trivial - a MOSFET to turn the load on and off, and a button to begin the test. The load is applied when an unloaded voltage of more than 1.1 volts is detected (i.e. a battery has been inserted into the AA battery holder) and the button is pressed. The load remains applied until the voltage drops below 0.9 volts. The load is then turned off and the results are reported on a thermal printer.
I then modified the test procedure to charge the same 2 batteries, but only perform the load test on H3. After the load test, I inserted both batteries into the charger. When the recharge was complete, I re-tested H3 (only), and repeated the cycle. This time I got very consistent results.
All of the AA battery chargers that I have ever seen require charging 2 batteries at the same time. Using new batteries and considering a little manufacturers overstatement of capacity I considered anything over 2000 mAh to be a reasonable value.
Charger #1 - Energizer model CHM4fC. A red LED is on when 2 batteries are inserted. The LED goes off when the charging is complete - usually about 6 hours. http://data.energizer.com/pdfs/chm4fc.pdf
Charger #2 - Duracell model CEF14N. A red LED is on when 2 batteries are inserted. The LED turns to green when the charging is complete - usually about 6 hours. https://www.scribd.com/document/353787783/Duracell-CEF14N-NiMH-Battery-Charger-pdf
Here are the milliamp hour results
Test #1 ------------------------ Recharged with charger #1 Battery H3 Battery H4 Conclusion 561 2346 H3 failed 2523 185 H4 failed 2213 3318
Recharged with charger #2 Battery H3 Battery H4 2222 650 H4 failed 1326 18 H3 and H4 failed 172 2920 H3 failed
Test #2 ------------ Recharged with charger #1 Battery H3 2872 2725 2700
Recharged with charger #2 2698 2700 2871
Before this experiment I had no idea that batteries were sometimes not getting charged.
Can someone offer an explanation of why the charge of 2 batteries often doesn't work correctly? Is this the way battery chargers typically work? Based on the results I think this is a charger problem, but is there some other explanation? Should batteries routinely be run through a charger more than once before considering them to be charged? Should a pair of batteries always be charged using 1 fully charged battery?