Home made battery capacity tester for primary and rechargeable cellsI have found the life of fully charged NiMh rechargeable batteries is incredibly variable within one batch. I suspected my fairly new rechargeable batteries are either self-discharging at an alarming rate, or the capacity varies.
I decided to design a tester which almost anyone can make, at minimal cost, mainly from old junk. The tester must produce repeatable results, and compare batteries to make matching sets. It should also give a good approximation of capacity in mAh.
When replacing a watch battery the other day, it hit me.. Why not make a battery tester out of an old quartz analogue watch! It has a non-volatile memory (the clock face) and reacts to the battery going flat (stops). I had a couple hanging around in a drawer, one being a watch I bought last year for 3.99 at a local market, which quickly deteriorated. The tester uses a shunt resistor to drain the battery. The watch times how long the battery takes to go flat. As NiMh batteries terminal voltage drops very quickly as they get to the end point, the actual voltage at which the watch stops isn't too critical. Mine stops at exactly 1 volt, which is fine.
I designed the circuit to draw 250mA from the battery. This rate is within the abilities of good NiMh (Nickel Metal Hydride) and NiCd (Nickel-Cadmium) cells, and represents a very high drain device for AAA and moderate drain for D size. The discharge rate is too high for meaningful results with Zinc-Chloride below C size, and Zinc-carbon below D size.
ResultsOut of a batch of 10 identical NiMh 750mAh batteries, I found the batteries to last the following times.
The discharge rate is around 250ma so 8 minutes corresponds to 33mAh and 2:32 corresponds to 633mAh. Your equipment will only function for as long as the weaker battery holds out.
MiMh batteries are said to need 5 charge/discharge cycles for them to reach final capacity. The very poor 8 minute battery improved to 35 minutes then to 1 hour 15 minutes before returning to 56 minutes on it's fourth charge/discharge cycle.
NotesThis unit also helps recondition batteries. Nickel cadmium batteries benefit from complete discharge/recharge cycles to help remove memory effect. NiMh cells benefit from complete charge/discharge cycles when new to bring capacity up to scratch. The complete discharge cycle doesn't work in most equipment; the equipment will only run until the weakest cell fails. The other cells will not get their full discharge cycle. This unit will continue discharging until the cell is really dead, without any chance of accidental reverse-charging.
The following table assumes an average battery terminal voltage of 1.175v. In practice, it may be slightly higher than that, but will usually be within 6%.