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More  Battery Tester Circuits

Small Battery Capacity Tester  Sept 7, 2008
designed by David A. Johnson, P.E.

I have a client who uses a lot of batteries.  His sells a device which is powered by a 3v lithium coin cell and he sells thousands of them. It seems that many of the batteries he has been importing from China have a milliamp-hour capacity which is much lower than quality batteries from US or Japan manufacturers.  In many cases the capacity was less than half of what it was suppose to be.
His measurement technique was rather crude.  He would just insert one of the batteries into his product and time how long it powered it before dying. Many fresh Chinese batteries lasted only half as long as batteries from Energizer or Panasonic.  But, his test can take a long time.  In one of his products it takes a full month to complete a test. He wanted a more accurate measuring technique and he also wanted one that would tell him the capacity of the battery in a much shorter period of time.  I proposed an automatic tester which could give him some test results overnight. With the device I proposed, he could test a few sample batteries against some standard quality batteries. If they tested OK, he could then approve them for his product.
The only truly accurate way to measure the current capacity of a battery is by connecting the battery to a constant current load and measure how long the battery can sustain that current before its voltage drops below a recommended “cutoff” point.  The circuit below is designed to do this type of test.

The circuit is powered by a +5v source. A low power 5v regulator powers the circuit from a 9v battery. The left side of the circuit forms a constant current sinking circuit.  A 1v reference voltage is produced with a voltage divider.  The FET transistor is fed the exact amount of voltage to maintain this 1 volt drop across the 200 ohm resistor. Thus, battery current is drawn at 5 milliamps for the component values chosen. By changing the 200 ohm resistor to some other value, other test currents can be produced.  The test current is: 1/R.

The middle section of the circuit is configured as a voltage comparator.  With the component values chosen, the comparator will change state when the battery voltage drops below 2 volts.
A flip/flop is used to start and automatically stop the test.  When the pushbutton switch is pressed the flip/flop starts the test.  When the battery voltage drops below the cutoff voltage, the flip/flop changes state and stops the test.
To measure the elapsed test time, I use a modified wrist watch.  The battery has been removed and two wires have been connected to the watch’s battery holder.  The watch starts operating when the test is running and stops after the test.  You set the watch at 12:00 midnight to start a test.  At the end of the test, the watch retains the elapsed time.  If a wrist watch containing a day and date display is used, the test can last as long as 31 days.
The tester can test almost any small battery, from tiny hearing aid cells to larger coin cells.  However, the maximum load test current should be kept below 100ma with parts indicated.  With some modifications and component changes, the same technique could be scaled up to test much larger batteries. 
Keep in mind that due to the internal resistance of many batteries, the elapsed test time for a shot overnight test may be much shorter than expected.  As an example, a quality lithium coin cell might have a specified capacity of 220ma-hours but will reach the 2v cutoff voltage in only about 20 hours.  This would lead one to believe that the capacity would only be 20 x 5 = 100 milliamp-hours.  However, when the test is done at 100 microamps, the battery may last three months. The concept is to compare the test result times between quality batteries to those of unknown origins.

Click on Circuit Below to view PDF of Schematic

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