Constant current NiCd battery charging.......only one current level?

Hello,

We are charging four series (1V2 each) NiCd cells.

They are SAFT VNT D U cells (4250mAh)


Our charger is from an inductive coupler, and it is set for 0.1C. (425mA) .....so how do we do trickle charging?.....because we cannot reduce from the 425mA level.


.....is it OK to do pulsed charging, kind of "PWM charging" where we switch the charge current source on and off very quickly so as to get a lower charge rate?

Since its a current source, we can stop the charge current by shorting it at the coupler. ( i cannot really say more about our technology for IP reasons)

A possibly simpler way would be to use a shunt resistor across the cells to shunt excess current away. say 4 X cell = 4.8V, current to shunt away - 400 mA? R = 4.8/.4 = 12 ohms at 2 watt.
Frank

thanks but too dissipative

You can use PWM for pulsed charging to get a lower average charging current. By adding a filter at the PWM output, you can get a pure DC for charging the battery. By varying the duty cycle, you can vary the voltage and thus constant charging current.

Since it doesn't look as though this will make me a million dollars, I'll post my battery charger project here, free.



I constructed a real version of this to charge a 24V battery bank from PV panels. I found that it works.

It makes use of hysteresis using an op amp.

I adjusted this simulation so it will charge a 4-cell battery pack until it reaches 5.6V, then let it drop to 5.2V, then resume charging, etc.

In this way each cell should be maintained between 1.3 and 1.4 V. That's the range I usually find my batteries are at, if they were charged recently.

The batteries will self-discharge a bit over a period of weeks. At some point the charger will come on, and take an hour to bring them back up. This should be appropriate for long-term battery usage.

I can only get this to work under certain conditions. Example, it will not work if the voltage coming from your charger is above a certain level.

It may or may not suit your purpose.