Charger for cordless drill (Tinkering with)

[soldar]

I have a cordless drill that came with NiCd battery which died and I had it unused. I have rebuilt the battery pack with NiMH cells. The cheaply made charger is just a transformer followed by a full diode bridge and capacitor. The resistance of the transformer itself is what limits the output current.

When charging the transformer gets HOT. Like really hot. I did some measurements.

The primary, 230 V, 475 ohm.
The secondary, 25V, 6 ohm.
After rectified, no load, 32V dc

When connected to the battery the voltage falls to about 22 V and the current is about 500 mA. The transformer gets very hot. Maybe it is designed for that but I would rather charge a bit slower and not have it get so hot.

The first thing I thought was to insert a resistor in series with the output but then I thought it is more effective to put a resistor in series with the input. I tried several values and I made a chart with the resistor value in ohms versus the charging current in mA.

Even a small decrease in current makes a very noticeable difference in temperature. Decreasing the charging current to 300mA and the transformer barely gets warm.

So I am thinking of having three settings:

- High (say 400 mA)
- Low (say 200 mA)
- Trickle (say 50 mA)

Or maybe I could have four instead of three. Maybe even add a timer or voltage sensor to switch from high to low and from low to trickle.

Maybe someone can find something useful in this.

Maybe someone has ideas I could use.