Re: 29C Charger Message #6 Posted by Steve Simpkin on 23 Aug 2008, 1:31 p.m., in response to message #4 by Michael Golub
This is what I did to replace the batteries on my HP-25. Carefully use a sharp hobby knife (exacto type) blade to slice through one end of the thin plastic bar that runs between the two batteries. Cut the end closest to the open end of the battery pack. With a bit of effort, you can then gently pull it aside enough to slip the batteries out, one at a time. The plastic bar will set back in place after you are done. If you don't care about appearance, just cut the bar off completely.
WARNING! BE CAREFUL TO OBSERVE THE POLARITY WHEN REPLACING THE CELLS!
When looking at the back of the calculator with the silver Hewlett Packard label end down, the Negative pack battery end should contact the terminal above the H in Hewlett. The positive battery end should contact the terminal above the P in Packard (Picture of 20 Series battery polarity). The bump on the positive battery end does not always sit well on the PC board terminal since the original NiCads were flat on both ends. If possible find Nicad cells that are flat on both ends.
See this HP Museum link for more information on replacing batteries:
http://www.hpmuseum.org/batts/battery.htm
When I originally did this conversion (in 1986!) I used ordinary 1.5V alkaline batteries. I had some issues with the Low Battery warning (all decimal points lit except the true one) randomly coming on the display. If finally cut the battery holder spring and carefully soldered a diode in series to reduce the battery voltage. I also soldered a flat piece of copper to the PC board terminal to make better contact with the positive bump on the battery. Although it still works great, I don't recommend doing all this extra work! Just used 1.2V rechargeable cells.
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