Safe charging LED HP calcs


Apologies if I have failed to find posts about this , I have tried various phrases with poor return.

I would most welcome advice about safe charging of LED HPs. I note the battery pack is a sort of voltage buffer and requires contact to work properly. So, I dare not move my LED HPs during charging.

Another recommendation is out of body recharging. I would welcome advice about how best to do this. Croc-clipped wires from an HP charger to the battery pack perhaps or are there better ways?

I bought a USB cable which charges at lower currents than a mains re charger. Any experiences with these available?

I live in the UK where we have a juicy 240 volts out of a mains socket

Thank you all for your help.


It depends on which model you have. If it is a classic series (HP35, 45...) you can safely charged the batteries inside the calculator. This does not apply to the HP65 though. For the woodstocks I personally use an old broken HP21 as a charger. But it is also easy to take the cells out of the battery pack and charge them in a standard charger.


Thanks Harald

I have a 34c, 35,45 and 67

I am looking for a 27

Do the cells prise out of these packs?


If you cut out the center bar between the cells (shown in the picture below), you can pry the cells out. The pack seems to hold the cells securely enough without the center bar.

The above applies to the battery pack for the 27, not sure if you were asking about the others. The cells do not pry out of the classic (35, 45, etc.) pack, the pack has to be sliced along the seam on each end and then separated into two halves to remove the cells. For the 34C, if I recall corrctly you can just insert the cells in the battery compartment as long as you figure a way to connect the batteries in series on the side away from the contacts. I have seen aluminum foil used for that purpose.

Edited: 28 Mar 2013, 12:39 p.m.


Thanks Jeff

Out with the Acto knife!


In general it is safe to charge the battery pack in the calculator if the calculator electronic is disconnected from the pack/charging circuit.

For the classics, it's safe to charge the battery in the 35,45,55,70 and 80. Not the 65, the card reader sense amplifier chip is connected to the battery and can be damaged if the battery pack is open-circuit (or makes bad contact).

The 67 is similar, but I am told the sense amplifier was redesigned so it could stand the overvoltage. I don't want to risk it.

For the Woodstocks (20-series) and Spice (30-series) machines, it is safe to charge the battery in _non_ continuous memory models (21, 22, 25, 27, 31E, 32E, 33E, 37E and 38E IIRC) provided the machine is not turned on. It is NOT safe to charge the battery in continuous-memory models (25C, 29C, 33C, 34C, 38C).

I think the Sting series are similar. It's OK to charge the battery in a 10A, not in a 19C.

For the Topcats, the 'normal' ones (91, 92, 97, 97S) are OK if they're not turned on. Actally, there is a circuit to protect the machine even if it is turned on, but I wouldn't want to rely on it. If you are lucky enough to have a 95C, do not charge the battery in the machine. There is a risk of damaging the RAM chip (at least) if the battery is open-circuit.


Thanks Tony ,

I presume disconnected calculator electronics means the calc is switched off?

By the way what has happened to modern HP's they seem so far removed from the design ethos of the classics.

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