New batteries in old HPs


Hello all:

I just wanted to share my recent battery adventures with the faithful. I made a few battery packs for the HP classics using new NiMH 1600 mAh batteries (Sanyo industrial and Kodak retail). Unfortunately, a standard classic charger will take 36-48 hours to fully charge these batteries (and they might not fully charge since the current is relatively low). So, I modified a charger to output about 180mA on the battery-charger circuit rather than the normal 50-60mA. This should yield about a 12 hour charge time. It was a very easy modification. In my wall transformer, there is a 13 Ohm resistor being fed into a transistor. This resistor determines the current available from the "current source" circuit on the battery-charging side. By putting two 10 Ohm resistors in parallel with the 13 Ohm, I was able to get the net resistance to about 3.7 Ohms. This resistance theoretically should yield 190mA, but I measured less--about 175mA.

Now, the really good news: these 1600mAh NiMH batteries last a LONG time. I decided a good test would be to start my HP-55 timer (with hundredths-of-a-second displayed---meaing eight digits and two colons), and see how long it lasted. I started to get the all-decimals low-battery indicator after 9 HOURS and 45 MINUTES. I finally gave up (and went to bed) after 10 HOURS and 20 MINUTES (it was still running fine at that point).

A test in my HP-67 yielded similar results, in that a running program lasted about 8 HOURS. Unfortunately, the 67's low battery LED comes on after only an hour or two---even though the calculator runs many more hours. I haven't quite figured this one out. I'm not sure why the low-battery threshhold on the 67 would be different than the 55.

Anyway, I'm a happy camper! 8 to 10 HOURS on a charge. Woo-Hoo!



thanks. i was wondering about that just last weekend.


Hello all:

I just wanted to amend the previous post I had written about modifying a charger to charge 1600 mAh NiMH batteries. It appears that I recommended too long a charge time. It seems that about 4 hours is more appropriate for the charge current of 175 mA that I quoted.

I figured this out by looking at the charging current of two "smart" NiMH chargers I have. The energizer (I think) charger used 135 mA to charge AAs in about 6 to 12 hours, then dropping to 25 mA as a maintenance charge. The quick-charge charger I have used 225 mA to charge in 90 minutes, then dropping to 17 mA for a maintenance charge.

Please base your charging times on theses numbers. My recommendation would have resulted in charging the batteries for too long.


Possibly Related Threads...
Thread Author Replies Views Last Post
  Availability of scientific HPs Matt Agajanian 4 1,200 08-28-2013, 07:40 AM
Last Post: Eddie W. Shore
  HIGH END HPs.. Status Symbols?? John W Kercheval 13 2,751 05-09-2012, 12:55 AM
Last Post: Matt Agajanian
  Speed comparison WP34S and other HPs Harald 18 4,034 03-19-2012, 05:58 AM
Last Post: Tom Grydeland
  Static and HPs Fouad M. Kaadou 2 846 11-13-2011, 03:13 PM
Last Post: Fouad M. Kaadou
  Old geeks miss their HPs Juergen Keller 10 2,068 11-05-2010, 03:02 AM
Last Post: Juergen Keller
  Units Localisation request for future HPs Mark Edmonds 24 4,168 08-05-2008, 03:15 AM
Last Post: ncrook(UK)
  Re: Who buys HPs these days? Ren 0 539 11-13-2007, 11:17 AM
Last Post: Ren
  Who buys HPs these days? Frank Rottgardt 23 4,228 11-02-2007, 12:35 PM
Last Post: Norris
  Recent Apple Gadgets Reminiscent of HPs of Old Les Wright 7 1,612 09-26-2007, 04:16 PM
Last Post: Gene Wright
  Programs Shrinkers for ours beloved HPs Patrice 6 1,370 09-22-2007, 11:01 AM
Last Post: Patrice

Forum Jump: