battery tester / recharger question

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.

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.

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?

Thank you.


I have always been suspicious of the low-end chargers that only work with 2 cells at a time.

I have used THIS CHARGER for the last 2 or 3 years. I charge AA NiMH cells that we use in our Robot Kit. I don’t have your ability to test individual cells, but my charger does give a visual indication of the charging process. An already-charged cell goes up quickly and stops charging soon. A well-discharged cell takes a hour or more to charge. And a SET of 6 cells charge up on the bar-graph at about the same rate.

The interesting thing to this discussion is that when I recharge a set of 6 NiMH cells that have run in the robot for 2 or 3 continuous hours, and check the charging progress, all 6 cells seem to charge at the same rate and get fully charged at about the same time.

So I have 6 AAs that are discharged in series (therefore the same mA/HR) but always recharged separately.

Please share your tester circuit and code… I and others would probably like to make one!

Sorry for the delay in posting. The code is simple, but with comments it doesn’t fit in limits of the forum posting, so I attached it as a pdf, along with the schematic.

batteryTester.pdf (31.5 KB)

btCode.pdf (28.7 KB)

"The code is simple, but with comments it doesn't fit in limits of the forum posting, so I attached it as a pdf, along with the schematic." No no no no. Click Reply, select Attachment and other options, then browse to your .ino file and Attach it.

Have you tried your tester with a toggle switch in place of the MOSFET? I'm thinking about the MOSFETs rds, what type is it anyway? Also, what is the resistor wattage? Is it getting very hot?