HP-29C Low-battery warning, help needed please!

[jjohnson873]

I measured the "low battery warning" threshold on three (3) different HP-29C calculators using a Fluke 87V DMM and a bench power supply where I could vary the voltage to the calculator. Two of the HP-29C calculators were made in America, one in Singapore.
The POR/Low Battery IC is very consistent in measuring the battery voltage between the three HP-29C's. I entered all "8's" in the calculator display for a worst case current. At 2.28 vdc, the decimal point did not flash (low battery indicator). At 2.27 vdc, all of the HP-29C's indicated low battery. I could vary the voltage back and forth between 2.27-2.28 vdc and the low battery signal to the ACT was very consistent. The current draw on the battery pack increases as your voltage decreases and the calculator current was around 192mA at 2.27 vdc. When I entered a single "1" in the display, the calculator current dropped to 160mA at 2.27 vdc. Larger currents will deplete the battery faster and, of course, leaving the calculator on will deplete the battery's charge.
Once the low battery warning indicator began to flash, I continued to lower the voltage until I noted the display starting to flicker. With all "8's" in the display, the display began to flicker between 1.90-2.00 vdc. I haven't done a study on power use between the low voltage threshold indication and where the calculator begins to fail, but I'm guessing you still have quite a bit of time before the battery pack gets to 2.00 vdc.

I wondered how different NiCd and NiMH battery discharge curves looked but found that the initial "fully charged" voltage of between 1.2 - 1.4 vdc/cell is very similar between battery technologies. NiCd and NiMH perform quite similarly with regards to discharge curves with the voltage/cell remaining around 1.2 vdc for 85% of the discharge curve. A fully charged, 2 cell, NiMH battery pack should be around 2.4 - 2.6 vdc. If there's a load on the battery pack, the charge drops to just above 2.4 vdc and stays there for most of the discharge time. If your NiMH batteries are good, they should stay above the 2.28 vdc low battery warning threshold for quite some time before they need to be charged again. Of course the NiMH batteries don't exhibit the memory charging issues that the older NiCd batteries were famous for. (I know I'm conjuring up Katie Wasserman's wrath by mentioning batteries since she's the guru in this area).

The batteries are the main factor that should be investigated for a single bad cell or both cells not holding a charge at 1.2 vdc each for a long time. Another possible issue is battery contact resistance, either with the springs in the back of the Woodstock battery pack or the calculator contacts. Use some sandpaper or a small metal file to clean these contacts. You can use distilled white vinegar to clean off any corrosion.

If you really want long battery life for your HP-29C, get Panamatik's low power replacement IC kit and replace your ACT with a very low power HP-29C emulator micro that will give you weeks of battery time! I haven't used my converted HP-25LP much but the battery is measuring 2.54 vdc after 2 weeks of turning it on once a day. The auto shutoff feature really saves battery power by turning off the display after a period of no activity.