I have a client who makes use of lots of batteries. His sells a tool which is powered by a 3v lithium coin cell & they sells thousands of them. It seems that plenty of of the batteries they has been importing from China have a milliamp-hour capacity which is much lower than quality batteries from US or Japan manufacturers. In plenty of cases the capacity was less than half of what it was suppose to be.
His measurement method was crude. They would insert of the batteries in to his product & time how long it powered it before dying. Plenty of fresh Chinese batteries lasted only half as long as batteries from Energizer or Panasonic. But, his check can take a long time. In of his products it takes a full month to complete a check. They wanted a more correct measuring method & they also wanted that would tell him the capacity of the battery in a much shorter time period. I proposed an automatic tester which could give him some check results overnight. With the tool I proposed, they could check a few sample batteries against some standard quality batteries. In the event that they tested OK, they could then approve them for his product.
The only truly correct way to measure the current capacity of a battery is by connecting the battery to a constant current load & measure how long the battery can maintain that current before its voltage drops below a recommended âcutoffâ point. The circuit below is designed to do this type of check.
designed by David A. Johnson, P.E.
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 one volt drop across the 200 ohm resistor. Thus, battery current is drawn at five milliamps for the part values chosen. By changing the 200 ohm resistor to some other value, other check currents can be produced. The check current is: 1/R.
The middle section of the circuit is configured as a voltage comparator. With the part values chosen, the comparator will change state when the battery voltage drops below two volts.
A flip/flop is used to start & automatically cease the check. When the pushbutton switch is pressed the flip/flop starts the check. When the battery voltage drops below the cutoff voltage, the flip/flop changes state & stops the check.
To measure the elapsed check time, I use a modified wrist watch. The battery has been removed & wires have been connected to the watchâs battery holder. The watch starts operating when the check is jogging & stops after the check. You set the watch at 12:00 midnight to start a check. At the finish of the check, the watch retains the elapsed time. If a wrist watch containing a day & date display is used, the check can last as long as 31 days.
The tester can check any little battery, from little hearing aid cells to larger coin cells. However, the maximum load check current ought to be kept below 100ma with parts indicated. With some modifications & part changes, the same method could be scaled up to check much larger batteries.
Keep in mind that due to the internal resistance of plenty of batteries, the elapsed check time for a shot overnight check 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 twenty hours. This would lead to think that the capacity would only be twenty x five = 100 milliamp-hours. However, when the check is done at 100 microamps, the battery may last months. The idea is to compare the check result times between quality batteries to those of unknown origins.
