|Re: 82120A Battery Pack|
Message #5 Posted by Ellis Easley on 3 July 2003, 3:21 p.m.,
in response to message #3 by 41Jeff
It seems that NiCads are supposed to receive 1.5 times the capacity during recharging, so 80*1.5=120, 120/17.8=6.74 hours. I have no documentation for my battery pack so I don't know what the recommended charging time is (*). Generally, all NiCads can be charged at a current equal to 1/10 of the Amp-hour capacity for 15 hours, and quick-charge capable cells can be charged at 3/10 of capacity for 5 hours, to achieve a full charge. In principle (according to my 1988 Sanyo NiCad databook), quick charge-able cells (everything Sanyo made at that time) could be left charging at 3/10 of capacity indefinitely with no damage. They get warm after charging is complete because of a chemical reaction that prevents buildup of gas, and this temperature rise can be detected to signal that charging is complete, but leaving the current on is not supposed to be harmful - and I think this is what HP communicates in the Woodstock and Spice manuals, which specify 4-6 hours for full charge but say the calculators can be left charging indefinitely without harm.
I have tried NiCad "N" cells in the standard HP41C battery holder. According to some data in the 2000 DigiKey catalog for Panasonic NiCads, the recommended quick charge rate for their 150 mA-h "N" cells is 38 mA, instead of 45 mA which would be 3/10 capacity, for 6 hours instead of 5, which comes out to 228 mA-h which is close enough to 45*5=225. This suggests that smaller cells shouldn't be charged in 5 hours at 3/10 capacity, now that they think about it. "N" cells are the smallest ones in that catalog, I don't know how much more Panasonic might derate the quick charge current for 80 mA-h cells. 17.8 mA is 0.2225, or 2.225/10 of 80 mA-h. 38 mA is 2.533/10 of 150 mA-h.
They say what ultimately kills NiCads is reverse charging that happens to the weakest cells during "deep discharge". The weakest cell stops pulling its weight (pushing electrons) and the current from the other cells flows through it in the opposite direction from charging current. This is why it is important to turn off NiCad powered equipment as soon as the battery indicator comes on - the system has detected a sudden drop in voltage of about 1 volt from the battery pack, which means the weakest cell has lost its charge and is going into reverse charge. Aggravating this situation (as I have just learned through making measurements on my HP34C) is that the voltage regulation in the DC-DC converter causes the battery current drain to rise as the battery voltage falls, so just as the weakest cell gives up, the (now reverse charging) current jumps up. The reverse charging current causes little needles of metal to be deposited is such a way that they short circuit the cell. Next time you connect the recharger, the shorted cell results in too high charging current for the remaining cells, since HP didn't use constant current charging circuits and depended on some small resistances (~6 or 7 ohms from the transformer winding plus an 8.2 ohm resistor in the case of Woodstock and Spice) and the normal operating voltage of the NiCads to establish the charging current. With the regular charging current being 3/10 capacity, the new excess charging current is more than the cells can withstand without overheating, producing too much gas, and venting the corrosive electrolyte through the pressure relief valve.
(*)I should mention that I got my rechargable pack in corroded form and have never used it! The cells are no good but the electronics cleaned up. I got a set of 110 mA-h (1/3 AA?) cells, which will require trimming the plastic case to fit, and I haven't done it yet. I also have a set of unmarked cells, very close in size to the actual cells, which a local battery store found for me, but I haven't installed those either - I'm waiting till I perfect my own spot-welding technique!