HP Forum Archive 14

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Rechargeable Batteries Message #1 Posted by Howard on 10 Apr 2005, 7:40 a.m. 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.

Re: Rechargeable Batteries Message #2 Posted by John Limpert on 10 Apr 2005, 8:04 a.m., in response to message #1 by Howard 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.

Re: Rechargeable Batteries Message #3 Posted by Howard on 10 Apr 2005, 8:09 a.m., in response to message #2 by John Limpert 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.

Re: Rechargeable Batteries Message #4 Posted by Steve A on 10 Apr 2005, 10:21 a.m., in response to message #3 by Howard Of course you could charge them with a ndv charger outside the calc, as many ndv chargers are made to charge cell pairs.

Re: Rechargeable Batteries Message #5 Posted by Gileno on 10 Apr 2005, 1:53 p.m., in response to message #1 by Howard Which the battery model that you used, and did where buy (site)?

Re: NiCad voltages Message #6 Posted by Randy on 10 Apr 2005, 11:37 p.m., in response to message #1 by Howard 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.

Re: NiCad voltages Message #7 Posted by John Limpert on 11 Apr 2005, 3:49 a.m., in response to message #6 by Randy 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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