Re: HP 29-C

[Jerry Siegel]

A friend at work told me that rechargeable NiMH batteries
pose a threat to the Woodstock series, and that I should
use NiCDs instead.

Does anyone have any info to confirm/deny this?

[David Smith]

I have an EE degree with over 30 years experience including many projects involving rechargeable batteries (some that required over 10 years of uninterrupted operation in VERY harsh field conditions).

I also took the time to track down an applications engineer with Sanyo and confirmed that NiMH cells work fine in NiCad applications that use slow charge rates of (say less than C/10, where C is the battery capacity). Basically, if your pack is not getting overly warm when sitting on charge, it is OK. Charge rates slower than say C/40 are not recommended since it is hard to maintain charge balance between multiple cells in a string.

[Randy Sloyer]

I would be curious to hear the explanation of how a NiMH pack poses a threat to a Woodstock machine. Is there concern when a cell fails and the voltage rises? Is this any different then the failure mode of a Nicad? Just wondering.

No matter which way you go, heed the warnings of all here concerning the fragility of the 29C. Do what ever you can to charge the pack out of the calculator.

[David Smith]

At one time there was some idle speculation that NiMH cells would somehow explode in a small thermonuclear explosion, etc. if charged in an HP calculator ;). Like all bogus info on the internet, once it gets out there, there is no stopping it.

There are some minor differences in the discharge curve of the NiMH and NiCad cells. Depending upon the circuit this can cause the "low battery" light to come on too soon (usually) or later than a NiCad... no big deal. In my trusty, dusty, rusty HP67 the low battery light comes on with an hour of run time left (out of over eight hours of continuous use).

This is the only difference that I have seen between operating NiMH cells and NiCads and have used them in numerous Classic, Woodstock, and Spice machines. NiMH cells do self discharge a little quicker than NiCads, but do not have "memory effect" problems with charging a partially discharged pack.

I have one HP97 pack also (but Sub-C NiMH cells are surprisingly hard to find and ludicrously expensive for a comparitively modest increase in capacity... it cost 60 bucks last year) so do not have much long term experience with them in that configuration. So far it does seem to work quite well though.

[Randy Sloyer]

Thanks for enlightenment David. Thermonuclear explosion. Yeah. Reminds me of an old Steven King short story called "Thermonuclear War" where there is a war fought between plastic soldiers. A double-A cell would be just about the proper scaled size for such a weapon.

Another urban legend laid to rest.