Battery discharge meter

[Ron Marcus]

I have been asked to provide a battery meter for NiMH rechargable batteries. Since they have a flat discharge curve, I was thinking of measuring current use vs. capacity. The device would need to keep track of charging time also, in case it was not charged up fully. Anybody done something similar? Is there another way to measure rechargable capacity prior to them dying without placing a heavy load on the cells?

Thanks,
Ron

[Acetronics2]

Hi, Ron

This problem is often solved through current measurement, as voltage itself do not tell anything ( look at voltage vs load curves ...).
So, TC 9400 or 9402 or AD 654 V/F converters just drive a LCD counter ...

Now with Pics ... a current to voltage translator ( to use full 0-5v Pic's ADC range ) and a "small" 12F675 can do the job, say, via time interrupts.
you add at every interrupt ( say 1 or 2 per sec. ) the mean ADC result during the interrupt period and show the result on a LCD.

But, now, look : it's a false problem, as the capacity given by the battery depends of the drawn current ... ( look at the datas ...)
It will be impossible to you to know when your battery is empty !!! Or very, very difficult. so, digital capacity monitors are just here to sell electronics.

The old method of measuring voltage ( even corrected with load ...) certainly is the most accurate.
Remember ... battery discharge limits are given in volts ...not in Coulombs

So, what do we do ??? ...we measure Voltage AND Current and , from the data curves, calculate a " corrected voltage vs load " ...

If corrected voltage is under the 1.0 or 1.1 Volt/ cell ( for a safe stop ) ...it's time to feed the batt again.

Have a look to : www.gpbatteries.com.hk ... everything in their datas

Alain