I guess we had to find out the "why" the hard way.
@Alec_t can you run a simulation using Regulated 5VDC feeding the battery via one diode like you did in post #49

 

I don't think simulating a NiCd battery as a constant voltage source reveals the critical information.

http://www.powerstream.com/NiCd.htm

Unlike some other chemistries, NiCd batteries can't be floated with a voltage, but rather the current must be controlled. The float current is quite low, about C/20.

 

If that is the case what will happen with a steady 5VDC across 3 Nicads in series ?

 

@Alec_t can you run a simulation using Regulated 5VDC feeding the battery via one diode like you did in post #49

Sure. Here you go.

I assumed the 5V source had a 0.1Ω resistance.

 

@Alec_t
What is your opinion ?

 

What is your opinion ?

As I said in post #49,

a 5VDC wall-wart and a suitably-sized series resistor could give the same average charge current as the B&D 4.5V transformer.

, and the third circuit in the post #64 attachment shows how a series diode might affect charging of a 3-cell NiCd battery by a 5V source.

 

Hi All

I have a rechargeable hand held Black & Decker screwdriver that I can't locate the power supply for. On the item itself it says it is a 3.3V device, and it charges by sitting within a charging unit via connectors on its side. Clever little thing it is!

So, I don't have any 3.3V power supplies, but plenty of 5V ones. What would experts advise I do to reduce the 5V to 3.3V from one of these PSU's please?

Tomorrows project

Thanks

Some times in my circuits i need to take 3.3 voltage from 5 voltage.in order to do the same what i do is just add two zener in series with the positive side of the 5 voltage supply .actuly any zener in forward bias mode drop .7 volts.2*.7=1.4,5-1.4=3.6,
If need be add one more .
Remember this is for small current and for tech savy guys,
You can experiment the risk involves are your zener go burnt,
3.6 could be bit over
In case of 3 zener 5-3*.7=2.9 is little low
You can try
All d best