Trying to charge NiCads using a wall wart

[seanspotatobusiness]

I made my own dynamo-powered bike light with a very simple circuit which charges some NiCad cells to keep the light on when I stop at traffic lights. Due to a disconnected wire, I used it without input which of course drained the cells. I tried to the charge them in this manner once before - by connecting via crocodile jumper leads to a variable wall wart power supply. Now I'm trying to do the same thing but the light on the power supply turns off whenever connected across my cells. What might be happening inside that power supply? Does it short the batteries? I imagine it was never designed for this purpose but it did work once before set at 6 V and at 7.5 V (I have five cells connected to form a ~6 V battery).

I'm connecting the positive output to the positive terminal of the battery and vice versa with the negatives.

 

[Harald Kapp]

Probably your battery is very drained having a voltage substantially lower than 6V.
Measure the voltage of the battery.
Measure the current when you connect the battery to the wall wart.

NiCd batteries dislike being deep-discharged. They can even reverse polarity and become defect. You may try to "resurrrect" a dead NiCd battery with a strong current pulse. Google "revive nicad batteries" for instructions.