Controlling charge amperage

[ignesandros]

I have built a 19.2V / 32Ah NiCAD bank and wish to use an AC charger to charge and condition it.

I got a 30v, 500mA charger from yard sale case-off and I tried testing

the voltage and amperage. The voltage came in at 35.6v. Immediately

upon testing amperage the amps jumped and then dropped to zero. Then,

the voltage was zero. I found that the supply was one of the rare ones in the consumer field that was held in an enclosure with philips head screws, so I took it

apart and found an internal glass fuse to be blown. With care, I arced the terminals

between the sides of the fuse while testing the output voltage and it

was a steady 35.6v. I did the same with amps and got up to 10A

(probably only 10 because that's my multimeter's limit). Does anyone know

of a way I can use the junction point at which the fuse was located to

determine my charging amps? At the fuse, the voltage difference is ~28v

AC. I'm not sure what the amperage rating of the fuse was.

I've read something about using transistors, but I wish to use something variable. I would ultimately like something that was temperature based, but that's only a pipe dream right now.

-Andrew

*All voltages are open circuit if that matters.

[finnsawyer]

All you have to do is connect the leads of your ammeter (or the shunt thereof) to the fuse terminals.  This does not solve your basic problem, however, as you are trying to draw more than .5 amps from a charger designed only for .5 amps.  You need to limit the current to .5 amps.  This can be done by putting a resistor of the proper value between the charger and the battery.  You could then limit the battery voltage by putting a 600 milliamp zener diode (18 volt?) in series with a resistor across the battery.  When the battery voltage rises above a certain value the zener diode will conduct and the current will be shunted through the zener and resistor.  This is not a perfect solution, but should work in your case.  You should also install a .5 amp fuse in series with the charger output.  If you try to draw more than .5 amps on a continuous basis you will probably burn up the charger.

[Nando]

For a 19.2 volts (16 NiCad cells) the maximum voltage should be 21,92 Vdc output.

You have not indicated how the charger is, how the voltage is generated and regulated if such is the case.

If the charger has electronic circuitry to regulate the output voltage, then it may be possible to adjust the output voltage and, as well, the output current.

There are two resistors that feedback the output voltage to be regulated as you noted and another small resistor that is used as reference to limit the output current ( that is fed to the regulating circuit to reduce the output voltage to reduce the charging current).

Please indicate how the charger is designed.

Also, if it is just a rectifying circuit with a fuse, then it is possible to add a voltage regulator, like the LM317 and some additional circuitry to even limit the charging current.

Regards

Nando