NiMH pulsed charger - need opinion on charge method

[damo_shelt]

pulsed charger

Hi guys,
we are trying to develop a product which will operate at times off a 6v NiMH battery. (5 x 1.2volt cells)
It will have a microController so we are hoping to fast charge the battery using PWM.
The battery capacity is 300mAh and the voltage input is approx 13.1 volts.
We currently have a 47Ω resistor in series and the input is switched 1 second on 20mS off. This should give a charge rate of approx 1C - correct?

We are using an a/d to detect a -ve Δ V of 10mV (2mV per cell) as the termination method. We also have a timeout after 1 hour.

what i have seen when measuring the voltage across the battery (with an oscilloscope) seems to be compatible with most "voltage during charge" graphs i have seen.

**broken link removed**

The voltage goes rapidly up to 7 volts where it remains until the timeout.

When i load the battery however - with a 23.5Ω load, the charge does not seem to hold at all and the voltage is reduced very rapidly.

Does our charge method sound reasonable? Any assistance/input would be greatly appreciated.

Kind Regards,
Damian

 

[BABESH]

NiMH pulsed charger

hi Damian,
You have 13V for charging that is very good,
when you apply this voltage to battery in series with 47R resistor then the current will be (13V-Vbat)/47R.
As Vbat increase the current thru battery will change,
To charge NiMH battery You need a constant current source not just current.
So design a constatn currrent source for 300mAmp to make it 1C charging and try measure battery voltage during current is off, then you get the correct voltage of battery.

this is basic setup you need before charging Nimh battery.


Thx
Babesh.