Re: HP21 bacteries


Thanks David, I never knew that. Next time it's time to replace the NiCds in my 29C I'll switch to NiMH. Easier too on the environment, I bet.


Yes, they do not contain the heavy metal cadmium. Also they are much less prone to "memory effect" that requires you to fully discharge the cells before recharging them.

On the HP29C, you should NEVER charge the batteries in these machine. It is probably the most fragile of the woodstock machines when it comes to potential damage from high voltages caused by a bad battery or connection. It is GREATLY recommened that you use an external charger or another (preferably dead) woodstock machine. The same warning should also be followed with any of the woodstock machines.

If you must charge the battery in a woodstock machine, turn on the power before connecting the charger. If you have enough juice in the battery enter 888888888 in the display. The load caused by the LED display helps to keep the voltage lower... but still not low enough.

I have a pile of over 25 dead ACT chips removed from woodstock machines. I have never had to replace an ACT in an HP67 or HP97... they have a real regulated power system that does not cause the Voltage of Death.



Eeek! You've made me very nervous. I've been using my HP29 off and on since 1979, with replacement NiCd's in self-rebuilt battery packs. I've always used the charger to recharge the battery in the machine.

Should I forget about using NiMHs in the calculator and stick with tried and trusted NiCds? I can use an external charger for either type, but one that accepts the individual cells only, not the pack. But I don't want to keep rebuilding the battery pack every time it's time to recharge the cells. Do you have any suggestions, short of me trying to find one of those pack-only HP rechargers.



Just a thought. Could the type of HP charger have anything to do with the "Voltage of Death" you mention?
I use the 220V HP charger that came with the machine? Could it regulate voltage differently to the US 110 charger?


The type of battery or charger does not matter. The machine uses the battery pack as a voltage regulator/power filter. If the battery is not good and/or making good contact the full unregulated voltage of the charger gets put on the circuit board. This voltage can easily be enough to zap a chip... especially a 30 year old CMOS chip.

On the 25C and 29C machines, the RAM chip is always powered up, so you can zap a machine even if it is turned off... actually especially if it IS turned off. The load produced by the LED display helps hold down the voltage somewhat.

If your battery has enough juice to light up the machine and you have something showing in the LED's, it is making good contact and it is probably OK to charge it in the machine... but just to be safe I always use an external charger for the 25C and 29C.


Could you suggest some possibilities for an external charger?

I realize there are chargers for AA cell batteries, but those require the batteries to be outside the plastic battery pack that Woodstock models use.

Is there any present-day (aftermarket) item that could recharge the batteries _inside_ the battery pack?




You could probably adapt just about any battery charger with using a pair of clip leads. Even better you could build some kind of clamp/battery contact assembly out of wood and screws. Best of all would be to get a real HP extranal charger or a cheap (ugly, even dead) HP21 or other woodstock series machine (try the classifieds here or Ebay). With a little luck you can bag one for under $30... very cheap insurance for protecting a $250+ 29C.

I have converted many a fried/carved/corroded HP21/25 carcass into chargers. I like to install an LED/resistor in series with the big charging resistor so that you can tell when juice is flowing into the battery pack.

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