Batteries for older HP & Fluke instruments

[1audio]

Some older instruments from both HP and Fluke used large sets of NiCad batteries. In some of the 1960's vintage HP instruments the batteries were part of the voltage regulation. Or, for example, the Fluke 540B running on battery is essential for isolation. However most of us use these instruments very occasionally so the battery shelf life (3 years?) becomes a real issue.

I'm looking at making a battery for an HP 432A but I might just leave it out since its not necessary.

Is there a best practice for dealing with these issues? I really don't want to buy 12V or 24V worth of C cell NiCads (or NIMH) for maybe six uses a year tops and have then deteriorate. I'm sure there are other instances of this as well.

[bob91343]

I am reminded of the time, several years ago, that I purchased a used Brother label maker that was defective because it worked only on batteries but not with the usual ac adapter wall wart.

After using this for some time I decided to make it work with an ac adapter.  I assumed the problem was in the closed circuit jack but that seemed okay.  So I put a bench power supply on the unit and disccovered that it indeed worked fine via the jack, but required a narrow voltage range (too low or too high and it quit).  I put in a little regulator and it's worked fine ever since.

[aargee]

Is it worthwhile designing something akin to a NiCd simulator?
From the machine it looks like a NiCd but itself is powered by a NiMh or Lithium cell?
The energy density of a set of LiPos could take the physical space along with a small circuit to provide the correct voltage/charge current.

[james_s]

There's a cheap way to get a bunch of NiCd or NiMH cells, buy a used power tool battery, or a cheap knockoff replacement. Roomba batteries are also readily available and have 10 NiMH cells.

[1audio]

These older HP instruments I have need 20 Nicad cells. For the $32 power meter it is not a real issue since it works fine with no battery and I'm not sure if I need battery operation. However the HP 8062 sound level meter uses the batteries as part of the regulation. I may just install some big Zeners. The Fluke 540B is more of a challenge. I think replacing the rechargeable with some LI primary batteries may be the cleanest. The 10 year shelf life LI batteries make sense for a 1-2 times/year instrument.

[james_s]

You could also probably get away with using NiCd or NiMH AA cells, you can get cheap ones that don't cost much, and it's probably not too hard to drop the charge current enough to not cook them.