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 Good Ideas done Badly

  Previous Issues

Bad Alkaline Battery Charger  

A while back I bought a battery charger, which claimed to charge alkaline 1.5v cells.  I seem to use a lot of 1.5v batteries.  I have many flashlights, computer mice and MP3 players which only work with standard 1.5v alkaline cells.  The markings on almost any alkaline battery say that you should not attempt to charge their devices.  They warn that doing so could cause the battery to leak or even explode.  Still, many people claim to have some success in putting some energy back into these otherwise disposable batteries.  The purchased charging device claimed to have the “intelligence” to properly charge batteries.  So, I thought I would test the thing.
Alkaline/NiCd/NiMH Battery Charger Battery Warning
I chose some fairly freshly used AA cells.  I usually dispose of standard alkaline batteries when their open voltage measured 1.35v or less.  I picked 4 units which measured 1.30v. 
The charger manual said that the unit’s microprocessor will automatically detect the battery type and charge the battery accordingly.  I popped the 4 AA cells into the charger.  The manual said that the charge indicator LED for each battery would turn red when charging and would later turn green when the cell was done charging.  I noted that in 2 of the 4 cells, there was a LED color change from green to red, but after only a few seconds, those two LEDs changed back to green.  The other two indicator lights for the batteries being charged showed no color change at all and stayed green. After a few seconds, all 4 cells showed a solid green LED indication. So, what does that mean?  Surely the cells were not fully charged in only a few seconds. I decided to leave the batteries connected to the charger overnight.
After about 8 hours, I went back to the charger.  All four charge bays still showed a sold green light.  Two of the 4 cells showed signs of leaking electrolyte.  The other two looked OK.   I guess the battery manufacturers were right.  You can’t charge these batteries, at least not at the charging current rate this charger was using.  All four cells measured an open voltage of 1.6v when taken off the charger but gradually dropped to about 1.5v in a few hours.  A quick test with a 15 ohm 100ma load resistor showed that the cells did indeed take a charge, since they were able to source about 100ma of current.  Although the two leaking cells were able to source 100ma of current, I tossed them into the trash.  I don’t want to put a leaking battery into any device.  Some research on the Internet indicated that other people had similar problems with alkaline chargers.  In some cases, the batteries did not start leaking until days after they were taken off the charger.  I think this charger was putting too much current into the batteries. The charging current caused gas pressure to build inside, pushing out the electrolyte fluid. I think the current needs to be much lower.  Perhaps some kind of selector switch would be needed to tell the machine the size of the battery being charged.  As an example, a big D cell should be charged at a higher current than a much smaller AAA cell.
I measured the charging current at each bay using a multimeter.  I measured 55 milliamps into a 1.5v battery.  The open voltage at each charge bay measured 8.5 volts.  Could the charger be using nothing but a series resistor to limit the current?  Perhaps.  It sure does not seem to be an “intelligent” charger.  I might take the thing apart later and see what more I can find out about the charger.  For now, I think I will not be using the thing unless I can carefully monitor the charging process.
A good alkaline AA cell should have an Amp-hour capacity of about 2 Amp-hours or 2000 milliamp-hours.  If you were to charge a cell at say 83ma for 24 hours, you would pump 2 Amp-hours into the cell.  In theory, that should be enough to bring the battery back to life.  But, the charging process is not 100% efficient.  There are losses, so if the goal was to put 100% of the original energy back into the cell in an 8 hour period, either the current would have to be increased or the charge time would have to be increased. But, charging these cells at this kind of current is way too high.  I think a slower charge rate would be better for the cell, giving it time to reverse the chemical reaction, which produces the electricity. Even a three day 72 hour charge time would be OK by me.  Perhaps the charger circuit could monitor the cell voltage increase and tapper the charge current as the voltage increased.  Maybe after three days of charging, the charger would lower the charge current to perhaps C/200 or just 10ma for an AA cell.  A big D cell might be able to handle 50ma of final charge current while a small AAA cell might terminate the charge cycle with just 5ma of current.  I think the ideal charger would charge perhaps 4 cells at time over a 4 day period.  Once completed, the cells would then go into a storage rack, like the one shown below.  Alkaline cells have an excellent long term storage characteristic.  They should last many years once charged.  Maybe each battery would be thrown away after two or three recharge cycles.
Leaking AA Battery Cell  Leaking AA Battery Cell
Battery Storage Rack with Voltage Tester

First Quarter,  2012    


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