johnr's Ni-MH discharger

Ni-MH battery discharger
drains individual AA cells to 1V

see copyright notice. Page created 26-May-2007

Soon after I started using Nickel-Metal Hydride (Ni-MH) cells in my digital camera, I noticed both their discharge and their recharge times getting shorter. Sometimes the camera would say the cells were empty after just a few minutes' use, then the charger said they were fully charged after a fraction of the "proper" time. This prompted me to look into the whole issue of Ni-MH cells and how to get the best out of them - and there's more to it than the writing on the bubble pack tells you!

Two related problems conspire against the Ni-MH user. The cells have a high self-discharge rate; and this and other parameters can differ widely amongst individuals of the species. If you start with a set of 4 fully-charged cells, chances are that in a matter of weeks, or even days in hot weather, they'll have a mixture of charges left, some may even be "flat". In an appliance where they're series-connected (the usual arrangement) the power will fail as soon as one cell is empty - regardless of the state of the others. When you put all 4 back into the charger, unless it's a very clever one it'll tell you they're ready as soon as the "fullest" one has been topped up.

So how should we use these beasts? Charging them immediately before use is a good strategy - if you're planning an important bit of photography, wait until the day before, so they don't have time to lose their oomph. When your charger says "ready" it probably drops down to a trickle, or "maintenance" rate rather than stopping altogether - so it's a good idea to leave it running for a further couple of hours to let the weaker cells catch up. But it's also important to start the charging process with all 4 cells fully discharged. My camera's designers realised this and put a "battery discharge" function on the menu; unfortunately the cells appear to remain series-connected so this function isn't really much better than using the camera in the normal way! To do the job properly you need to let each cell discharge until it reaches about 1V (no further, as that would risk permanent damage. They're relentlessly fussy little ******s). This gadget achieves that goal with minimal circuitry.

Of course, a simple resistor connected across a cell would discharge it, but you'd need to keep monitoring the voltage and disconnect when it got down to 1V. This design uses the fact that rectifier diodes drop a fairly constant voltage when conducting current; about 0.7V for conventional silicon, and 0.3V for schottky barrier ones. One of each type in series with a 1 ohm resistor has proved empirically to be just right for discharging an AA Ni-MH cell down to 1V. You still need to remove the cells after a few hours (I usually do a quick voltage check first) but it's nowhere near as critical as with a simple resistor drain.

The 4 single-cell holders are fixed to a strip of wood with double-sided tape, with the components soldered directly across the terminals. The holders I used had a well-intentioned safety feature, in the form of a plastic "pip" next to the positive terminal, to prevent the cell's negative end making contact if inserted the wrong way round. Unfortunately my Ni-MH cells have shorter positive caps than most alkaline cells, and the safety feature prevented them making contact even the right way round - so I made the necessary "adjustments" with sidecutters and a file! I also cut off the retaining clips so that cells can be inserted and removed without undue wear.

Ni-MH battery discharger drains individual AA cells to 1V

Ni-MH battery discharger
drains individual AA cells to 1V