nimh charger

Erik Friesen

Does anyone have a thought on the best method for detecting level of charge with·nimh batterys at .2c (500ma with 2500mah).· Temperature is not a good option for me.· I am using a 10bit adc to read both sides of a 1 ohm so I am fairly confident in ma readings and volt level.· However I only have 5mv resolution.·

I also need some fresh ideas on·how to power up the prop when its plugged in to the·usb.· Right now I am feeding·the five volt signal to my·3.3v regulator enable pin.· However under extremely low battery conditions the battery·will not start·up the dcdc converter sufficiently to start up the·prop.· The prop is controlling a npn pnp combo that controls the charge current.··As soon as it gets the charge started I am in the clear.· Anyone else tried something like this?··

BradC

Can you use a couple of mA from the USB 5v supply to kick start the converter and use a dodgy blocking diode or something to prevent squirting anything back down the port?

I've never seen a *reliable* way to detect the charge level of a NiMH or NiCd battery, and all the good chargers seem to rely on the "knee" in the charge curve to tell when the battery is full, with overtemp as a backup/failsafe.

Christof Eb.

Hello Erik,

I have built a battery charger using the method with the knee, mentioned above. While charging the voltage climbs up. When the battery gets warm at the end of charging, the voltage drops. There are too key features of the charger:
* The voltage has to be measured while the current is switched off, so that the resistances for example of leads and contacts are not important.
* A lot of averaging of the measured voltage makes it possible to detect a small drop.

The picture shows in pink the voltage while charging two NimH cells.

Good luck!
Christof

Erik Friesen

Thanks for the ideas and graph. Can you translate for me?