HP-12C batteries - Page 1
Forum section: electronics / repair

author: Peabody
author_rank: Super Contributor
timestamp: November 09, 2021, 11:55:26 pm
content: I have an early HP-12C that uses three button cells. What's in it now are EPX76 cells, but I don't know what's optimum for it these days. Does anyone have recent experience with this issue? Based on what's readily available, I would probably default to using 357s, but if there's a better choice I'd like to know about it. The calculator is used very little because my 38C still works. :-) So shelf life is probably the most important consideration. Actually, something I can order a three-pack of on Ebay for little money is probably the best choice.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 10, 2021, 12:20:27 am
content: IIRC EPX76 are silver oxides, 1.5ish volt.I'm a HP50 person (my 48 died a few years ago) but my dad is in the 41's and 15'sHe just use LR44s, they're cheap, alkaline (less harmful for the environment IHMO) and thats it.He also uses 4 x LR44 instead of 4Ns on the HP41, because they're pretty expensive and very hard to hetHow's your experience with LR44s?

author: Peabody
author_rank: Super Contributor
timestamp: November 10, 2021, 05:45:23 am
content: I don't think I've used LR44 cells before. I did find this line in the Wikipedia entry for "button cell":"one manufacturer (Energizer) offers three silver oxide cells of the same size, 357–303, 357-303H and EPX76, with capacities ranging from 150 to 200 mAh, voltage characteristics ranging from gradually reducing to fairly constant, and some stated to be for continuous low drain with high pulse on demand, others for photo use."The EPX76 is specifically for photo use. I'm not sure what that means. The highest capacity of the Energizers is the 357-303H, but I don't see that on Ebay or Amazon.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 10, 2021, 09:42:36 am
content: Here their datasheets:LR44/A76 - Diameter 11.6 to 11.25 mm, height 5.4 to 4.9mm, 1.5V, 175mAh @ 6.8K drain - Cheap and easy to sourcehttps://data.energizer.com/pdfs/a76z.pdfEPX76 - Diameter 11.6 to 11.25 mm, height 5.4 to 5mm, 1.55V, 200mAh @ 6.8K drain - Has a MUCH better discharge curve, that's probably what they mean for "photo use" the plateau line up to 450ish hours. Thats what you want from a battery, the constant voltage over dischrge, so its a better batteryhttps://data.energizer.com/pdfs/epx76.pdfI'd say you should try a set of quality LR44s and report

author: Haenk
author_rank: Super Contributor
timestamp: November 10, 2021, 10:10:15 am
content: LR44 are also known as AG13 - probably the most commonly used (by far) button cellAnd yes, I used the AG13 as well for the C series calculators. However I had standard "N"-sized batteries, these are still not too hard to find; e.g. Amazon has them for less than 1 EUR each. I also had N sized NiCd cells, however those were expensive and the voltage was too low, so the my 28C/S shut down soon.

author: george.b
author_rank: Frequent Contributor
timestamp: November 10, 2021, 02:23:01 pm
content: I use LR44 on my HP-15C and HP-20S.

author: Peabody
author_rank: Super Contributor
timestamp: November 10, 2021, 06:16:04 pm
content: The LR44 is an alkaline cell. The EPX76 is silver oxide, but isn't available anywhere at a reasonable price. The 357 is silver oxide, but has less rated capacity than the LR44. The best datasheet is for the 357-303H, but there's no product associated with that, even on the Energizer site. So I'll probably just go to a local store and buy whatever appears to be the newest.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 10, 2021, 06:44:32 pm
content: LR44 is rated at 175mAh (energizer @ 6.8K ohm discharger)EPX76 is 200mAh (same discharge rate)https://data.energizer.com/pdfs/epx76.pdfI see no reason no to use LR44, cost pennies and get the job done

author: Peabody
author_rank: Super Contributor
timestamp: November 10, 2021, 10:37:30 pm
content: Well, I feel like a total dumbass. I was curious what battery voltage would cause the low battery indictor to begin flashing, so I took the old batteries out to measure them. But all three were above 1.5V. So I cleaned them, and the contacts, and no more flashing low battery indicator. It seems these may have a good bit more life in them after all. Shoulda done that in the first place. But it's strange - I did take the middle one out before to see what battery it was, and still had the flashing indicator when I put it back. I guess the crud messing things up was somewhere else.

author: WattsThat
author_rank: Frequent Contributor
timestamp: November 11, 2021, 01:08:29 am
content: The discharge rate is so low in those calculators that the silver oxide cells are reported to last more than twenty years with use. It has been said it is more about how stable the chemistry is rather than the discharge curves, not my opinion, just what I’ve read by those much smarter than me.The early Voyager units with the symmetrical battery contacts do tend to be fussy about contamination, they are just tin plated and likely more susceptible to oxidation due to battery out-gassing, as little as there is with such small cells. De-oxit does an excellent job to clean up years of exposure.HP redesigned the battery contact arrangement early on but this was really done to reduce manufacturing costs - the contact redesign was just a bonus of going to a single pc board. The internal changes are well documented over on the Museum of HP Calculators.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 11, 2021, 12:39:42 pm
content: Not if you own a HP 35S, that thing eat batteries 2xCR2032 in a few weeks just sitting on top of the desk. I've not measured the stand-by current tough, I guess I've found a task for my HP-4140B heheehe

author: Peabody
author_rank: Super Contributor
timestamp: November 11, 2021, 03:03:52 pm
content: This calculator was my brother's and I got it in 2011. The batteries have been in it ever since, and I don't know how old they already were in 2011. And I assume some current had to be drawn to maintain the ram. I've used it very little, but at least this indicates the shelf life of silver oxide is indeed quite lengthy, and powering down reduces current to essentially nothing, even while preserving the ram values. Maybe the voltage drops off a cliff at some point, but with the voltage still above 1.5V, maybe they could last another 10 years.

author: rvalente
author_rank: Frequent Contributor
timestamp: November 11, 2021, 03:44:08 pm
content: The EPX76 has a 10 year shelf-life, if you got with a little use in 2011 + some ram maintenance + some light use, I'd say these batteries are incredible within spec, they covered shelf life with honors + some use/discharge. Can't complain at all