Rechargeable Batteries


This might be a super dumb question.
I recently made up a battery pack for one of my HP calcs.
The replacement batteries were of a very much higher mAhr capacity.
How can I tell when they are full?
If I get a voltmeter can I measure the voltage and assume they are full when the voltage gets to 2.4v (2*1.2v in series}?
Thank you.


There's no easy way to tell if the batteries are fully charged. Output voltage on NiCD cells is fairly constant until they near exhaustion. You can trickle-charge the cells at 0.1c for about 14-16 hours. Charging them at a faster rate requires a temperature sensor that can detect increases in cell temperature that occur when a cell is nearing full charge.


Thank you. I'll leave them on for a couple of days. That should charge them up. The trouble is the new ones are 2000mAhr vs 700mAhr of the originals.


Of course you could charge them with a ndv charger outside the calc, as many ndv chargers are made to charge cell pairs.


Which the battery model that you used, and did where buy (site)?


Many conditionals apply to the following general statement: Ni-cads are considered to be fully charged when they reach approximately 1.35-1.4 vdc per cell. Not perfect for many reasons but a good place to begin discussions. 1.1 vdc is (generally) considered to be the bottom of the discharge curve, that is when it begins to fall like a rock.

Another (inaccurate) way to assume full charge is (cell capacity/charge rate) = time required. For example: an original 600 mah classic nicad pack requires 10 hours at the standard 82002 charger rate of 60ma. Change those to 1100mah cells and it *should* require ~18 hours to reach full charge. Extrapolate to suit.


Your calculations for charge time ignore the inefficiency of the charging process. For a trickle charge, it take about 1.4c to fully charge the battery.

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