Charging Batteries

An old, non-HP (gasp!) RPN calculator has recently come into my possession. It is powered by 3 "AA" sized rechargeable batteries wired in series. The output voltage for each battery is specified as 1.25 volts (They are brand new and I believe they are NimH?). There is about 3.8 volts present at the charging port when the calculator is turned off and the batteries are fully charged, which they were when I received the calculator.

The problem is, the original recharger unit was not with the calculator and the manual gives no information about it other than that it takes "about five hours" to reach full charge starting with dead batteries.

I have a bench-top variable voltage DC power supply which I intend to use to recharge the batteries. However, I do not have a lot of experience with rechargeable batteries (at least not beyond charging them with the original factory charger). Can someone who is more electrically/electronically astute than I recommend a charging voltage? The highest I have dared turn the power supply while connected to the charging port is 4.00 volts, but for all I know it will take a year to fully charge the batteries at that voltage. Is there a "rule of thumb" ratio of charging voltage to design output voltage? I just want to be able to fully charge the batteries without damaging them or the calculator.

Thanks in advance to any/all who can assist me with this.

Take care.


PS - The calculator is a National Semiconductor 4640 in truly outstanding condition (other than the missing battery charger).


The proper charging rate is in terms of current, not voltage. If your power supply has a current limit set it to 50 mA. Any AA sized Ni-Cd or Ni-MH cell will take at least 50 mA during charge. Do not set the voltage higher than 4 V. If your power supply does not have a current limit I would put a resistor in series with it to limit the current to 50 mA. In this case the voltage can be set somewhat higher to allow for the drop across the resistor.

Possibly Related Threads...
Thread Author Replies Views Last Post
  Safe charging LED HP calcs Anoop Sahal 6 797 03-29-2013, 09:08 AM
Last Post: Anoop Sahal
  Source for N-Cell NiMh Batteries and Charging adapters TnC_Products 14 1,420 12-16-2012, 10:26 AM
Last Post: Dieter
  Charging Current in HP97, 82143A, 82162A Les Wright 5 733 06-17-2012, 02:11 PM
Last Post: Les Wright
  HP 9114 disk drive battery charging David Ramsey 8 822 05-25-2011, 12:15 AM
Last Post: David Ramsey
  Charging times for HP-41 rebuilt battery pack Timo 2 460 01-07-2011, 11:31 AM
Last Post: Timo
  Charging 'classics' from a USB port Katie Wasserman 16 1,351 10-06-2010, 02:54 PM
Last Post: Juergen Keller
  Problem with HP 110+ / Portable Plus battery charging (+workaround) John AS (UK) 2 421 04-21-2010, 01:32 PM
Last Post: John AS (UK)
  Charging NiMH cells on calculators Luis 4 585 02-11-2008, 12:26 AM
Last Post: db (martinez, ca.)
  Charging time for repaired HP 82120 Matthias Wehrli 0 234 03-03-2007, 05:31 PM
Last Post: Matthias Wehrli
  Woodstock - Battery Charging Indecation Mike T. 4 498 09-11-2006, 12:40 AM
Last Post: Hal

Forum Jump: