NiCD Fast Charger Questions

I have a few questions I would like to ask, which I do not get any direct answers for on the net.

1. It seems like the only thing that matters in the charging is the current and not the supply voltage. I am not talking about the delta effect now. Can you for instance put a 17VDC supply on a 9.6V battery pack and then limit the current via a resistor to C/10 or C/2 or whatever?

2.Now this current limiting resister is a bit of a dilemma to me. I have a transistor and a diode in my charging circuit so I took the supply voltage of the regulated power supply i.e. 17.2VDC and subtracted 1.4V for the transistor and the diode and used ohms law to restrict the current to 1100mA. Thus I had to use a 15.8/1.1 = 14.36 Ohm resistors, isn’t it? It seems not to be that simple because if I connect the battery and measure the current and volts across the battery I get a reading of 10.92 Volts and some current of 650mA or so? How does the battery affect the whole circuit and how do I determine the correct resistor value for the circuit? It seems like I have to use some other formula when the battery is connected into the circuit.


Your help would be greatly appreciated please.

NiMH charging

Hi there,

I am currently building a NiCD charger, using a PIC16F877 to detect the DeltaV on the charge cycle. Now detecting deltaV on NiCD battery packs is relatively simple because the Voltage drops about 20mV per battery so for a 6-Pack Battery pack you need to detect roughly 120mV fall in the voltage of the battery pack for you to know it is fully charged. The problem with NiMH batteries is that the deltaV per battery cell is around 2mV per cell is. This can easily disapear in all the noise measured!! What is the easiest way to detect the deltaV for NiMH cells?

You should use a constant current source, rather than just a crude resistor, this overcomes any problems with supply voltage and battery voltage changes.