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Full Version: HP-25: charging of battery pack
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Dear HP Enthusiasts,

Maybe I miss that point, which has already been answered in other HP clasic threads ...:

Is there any problem / hazard to charge a HP-25 / HP-25C (Woodstock Model) by wall net power supply, when the calculator switch has been set in OFF position ?! Thanx for your feedback in advance ! I know about the problem, but I think it is only related to an Woodstock Model, which is in ON calculator position !

Kind regards from Germany,

Thomas alias Dr. Mabuse

Yes, correct. Just always remember to disconnect the charger before turning the calculator on.

Simple answer is no it is not a problem as long as the calculator is OFF.

The problem arises when the fallibility of the mind turns on the calculator while it is plugged in. The design risk that kills the ACT, ROM or RAM or a combination of one, two or three of those occurs when one of the three faults occurs:

  1. Oooops, I forgot I took the battery pack out: infinite resistance.
  2. Ooops, the battery pack is in but the batteries are chemically dead: infinite resistance.
  3. Faulty battery contacts causing, you guessed it: infinite resistance.

All three place the 9 volts (?) of the wall wart through the ACT and further into the circuitry.

Therefore, to remove the possibility of destroying one or more ICs in the calculator, charge the batteries outside the calculator.

Correct! Nothing to add except...

... If you like calculating while charging or even without battery pack inserted you need to add a voltage regulator component inside the "Woodstock" calculator. Then all components will be save. I can offer you to visit me for doing the modification.

Dear all,

Many thanks for the clarification. Due to the fact that a external charging device is still missing, this question comes up.

Best regards from Germany,

Thomas alias Dr. Mabuse
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