HP97 battery pack rebuild

[Dieter]

(02-11-2016 11:02 AM)Hans Brueggemann Wrote:  The DC charge current into the battery pack is approx. Ich = 170 mA (measured on my HP-97) when using the standard HP-97 charger. for minimum overcharge of the battery pack with a given amp-hours of a single cell, you calculate the charging time as

Tch = 1.1 * Qcell / Ich (assuming a fully discharged pack!)

This is not a minimum overcharge. Charging is a process with an efficiency more or less significantly below 100%. Typical values for average charge rates are 70...90%. That's why for a slow 0,1 C charge (in your example it's even less than that: 170/2400 = 0,07 C) a factor like 1,4 is common. This would yield 1,4 x 2400 / 170 = 20 hours here.

(02-11-2016 11:02 AM)Hans Brueggemann Wrote:  Tch = 1.1 * 2400 mAh / 170mA = 14.12 hours

Hmmm... my HP41 returns 15,53 hours for this. Or 15:31:46. ;-)

All this of course is only valid for a completely empty battery pack. Which cannot be checked exactly anyway. But the relative charging current is so low (0,07 C) that the batteries should not be harmed even if a half-empty pack gets the full charge.

Finally let's not forget: you may charge the NiMHs for the same time as you did with the old NiCds – and you'll get the same capacity. Longer charging times (up to 20 hours) will yield more capacity than the old cells, and this exactly is the advantage here.

Dieter