Creating a battery charger indicator

[lrprath]

I have built a battery charger for a cordless drill. It works fine, however, I would like to add an LED indicator to let me know when the battery is fully charged. I'm getting tired of checking the voltage across the battery all the time to see if the thing is charged. I'm not picky on how exactly this indicator works. The simpler, the better.

 

[MATLABdude]

Welcome to PhysicsForums!

The difficulty with the charge indicator (and you neglect to mention the chemistry and number of cells here) is that the difference between a fully charged and a 75% run-down battery is usually fairly low (under a half volt) per cell. Nevertheless, it is possible to design a circuit using comparators and Zeners (or even better, with an ADC and a microcontroller) to indicate this difference in a battery of cells. You can even Google for schematics and circuit diagrams.

However, it may be better to detect the completion of charging using temperature or voltage rise (especially if you're wanting to monitor the charge for the purpose of stopping the charging).

 

[skeptic2]

You might try interrupting the charging, measuring the no-load voltage across the battery, then momentarily putting a resistor equal to full load across it and measuring how much the voltage drops from no-load. A uC should be able to do that.