|Re: HP 21|
Message #20 Posted by Vieira, Luiz C. (Brazil) on 8 Dec 2004, 7:37 p.m.,
in response to message #19 by Perry Schwartz
excuse me answering (or posting an aswer to) a question direct to another contributor, but I guess I can tell you about my own experience.
I bought many commercial National NiCadīs, 850mAh, and used them mostly to replace regular NiCad battery in standard HP battery packs: Classics (HP45, HP55, HP65...), Woodstocks (vintage HP2x, HP67), Spices (not so vintage HP3xE/C) and I also rebuilt a pack for an HP75D, but in this case I used a commercial, slightly modified walkie-talkie battery (pack of three regular NiCads; I had to remove a diode used for charging purposes, but the pack fit almost exactly in place). I rebuilt two pack for the HP97 (Topcat) with a couple of four NiCad cells from two mobile (cell phone) faulty battery packs. Most of the earlier cell phones used a five-cells battery pack, and in most cases, the faulty packs have only one deffective cell.
I also used regular NiCads with higher capacity, about 1100mAh, but they need extended charging time to fully charge. Because of the awkward "memory effect" observed in most NiCad batteries when not being fully charged and/or discharged, I also successfuly tested some NiMH cells. These batteries are more "well behaved" related to the memory effect. I currently use NiMH AAA cells (650 mAh) in my HP48/49 units mostly because of the power drain when communicating. In this case they are externally charged, so I always keep some ready to use spare units. The HP49G+ is the one that needs fresh cells more often.
About voltages: the NiCad units operate in the range 1.25Vcc (fully charged) to about 1.17Vcc (low-power indication) or less, depends on the voltage sensing circuit. The NiMH are close to that, and I remember reading that they actually go a bit higher. I found out later that my personal measurement instruments have not enough accuracy to indicate such difference, so I take them as close enough to work about the same.
Hope this helps a bit.