Battery for CMT-71-64R RAM module, HP-71

[smatthys]

Dear HP fans

Some time ago I purchased a CMT-71-64R RAM module for the HP-71. Now my question is: What type of battery is in there & where could I possibly get a new one?

Thanks in advance for your support.
Stephan

[rprosperi]

There are probably a half-dozen or more threads here already about this, easily found with a quick search.

But the upshot is no one has ever found (and/or at least never replied about) a source for a compatible battery for the CMT 64K RAM modules.

As in all prior threads, I'll ask that if anyone does find a source, to please post the information here (right after after ordering yours).

[PeterP]

Bob, I tried 6 different ways of searching for these threads and failed. Clearly my search mojo is weak to say the least.

Any way you can point to a couple maybe?

[J-F Garnier]

Searching on this forum with "CMT and Battery" lead to several interesting threads:

HP-71B 4Kb (CMT32k FrontPort) Memory Module "Battery" - FRAM

Lithium Battery CMT 64Kb

about 71B's CMT 64K RAM modules

and several others...

J-F

[PeterP]

Thank you! I was just wondering if there is something in the aerospace industry, hence the interest.

[rprosperi]

Often, the best way to search the Forum history is to use google.com, just like searching for almost everything else.

"CMT (71,71B) battery site:hpmuseum.org"

The built-in search tool is finicky, but generally more helpful than googles's external search if looking for posts from specific people.

p.s. Thanks J-F !

[J-F Garnier]

It's strange that no reference of spare batteries can be found in CMT flyers or EduCalc catalogues.
It may be that CMT estimated that the lifetime of the battery would be long enough for no replacement needed, since the battery is only used to retain memory when the module is removed from the 71B. They obviously didn't think that some people would still use the modules 20-40 years later.

As many, I'm using my (only) 64K module without battery, and that's fine.

J-F

[Etienne Victoria]

I'm looking for a source to order the following battery :

thin film battery

If I can order one and get it to work in my modules, I'll report back here.

Etienne

[PeterP]

Could something like This work

[Dave Frederickson]

(02-11-2024 10:01 PM)PeterP Wrote:  Could something like This work

Maybe. There are a couple of concerns.
1. The original battery was, I believe, 3V. Will 3.7V have an adverse affect on the system?
2. Will using a rechargeable battery have an adverse affect?
3. I measure the thickness at 0.65mm. Will a thicker battery fit without damaging the battery contacts?
4. If the battery tabs are cut off, are the battery contacts the top and bottom of the battery?
5. Why did cutting the battery not short it out?

[PeterP]

Dave, I am no expert here, just maybe have a few supply chain access points which could be useful.

If one scrolls down on that page, one can see that thickness as low as 0.3 is available. And while I dont know how rechargeable vs not could have an impact, they also list non-rechargable ones.

Lastly, they do offer a customization service which I dont know what minimum order would have to be, but they have gotten pretty nimble tbh.

The one thing that stuck out for me and I do not know if it is a problem or not is the 3.7V vs 3v.

I believe the battery was not shorting as they cut away from the back without combining the plus and minus tabs metal directly, but I could be wrong.

Lastly, I am no expert but as we do use some rare components in making our satellites I thought I could maybe be of help. I apologize if I overstepped.

[rprosperi]

Thanks for this info Peter; although questions remain, this is by far the closest match anyone has ever found IIRC. While not speaking for Dave, I'm sure his critical-sounding comments come from frustration at never having found ones that we've been looking for, for well over 10 years, and not being judgey about this info.

The challenge is especially difficult as there is no published technical information about the original batteries, and AFAIK, no one has ever dissected a 64K RAM module since they are both super reliable and quite expensive. For example, we don't know if the battery is trickle charged by power from the mainframe when the module is inserted, or if it was intended to me more like a CR2032 backup battery, that is a single original charge intended to cover the full "lifetime" of 71B RAM module, probably guestimated to be 10 years max. No one at CMT was thinking about how fanatical calculator fanboys would be using them nearly 40 years later.

Further, as you know, Lithium batteries may not react well when improperly charged and perhaps that could happen using this battery in the CMT module.

I think we've all assumed that the original was a primary cell, and not charged, but we just don't know. I do know that I won't dissect one of mine to find out, so the mystery may simply continue.