CMOS Battery for RiscPC

I have had a look at the CMOS battery in my 21 year old Acorn Risc PC and notice it is starting to breakdown so have obtained rechargeable batteries and two holders. What is the highest and lowest voltage which should be applied using NiMH rechargeable batteries. I have two triple A battery holders and wonder if I should connect them in parallel or series to give either 1.2v or 2.4 volts. The batteries are rated at 650mAH each. On the other hand will just one battery be sufficient?

I can’t answer this directly. However, I can refer you to this guide that I found and which I followed for an A7000+.

This thread on stardot might also be informative.

The original battery is a Varta V280H, rated 1.2V.

So, basically, any AAA cell will do the job. You can use a normal alcaline if in series with a diode to prevent charging. Alternatively, a little NiCad cell?

Note that NiCad (original) is not the same as NiMH.

That said, those little triple-A cells that you can find in cheap solar powered garden lights (a quid a pop, or less!) seem to be pretty tolerant, so fitting one of them may do the job.

That reminds me, I really ought to see if I can get at my RiscPC and see if the thing still works and the battery hasn’t killed it. Last used it… oh my… 2010? 2012? Some time that is increasingly far away…

That reminds me, I really ought to see if I can get at my RiscPC and see if the thing still works and the battery hasn’t killed it. Last used it… oh my… 2010? 2012? Some time that is increasingly far away…

A gap that long suggests that you may open the case and see a little salty encrustation by the battery and you can probably kiss that part of the PCB goodbye.
However if you check on Chris’s Acorns then down the bottom of the page are the circuit diagrams to show which bits you need to patch to what.

Note that NiCad (original) is not the same as NiMH.

Are your saying that the Triple A NiMH will not do the necessary as a replacement for the original round cell?
I cannot see there is a difference, except in the materials used to manufacture them. Having said that I do not know. They are both rechargeable and deliver 1.2 v. I could have used a double A NiMH but do not at present have a holder.

Will it be ok to use the Triple A NiMH battery?

Yes, a single AAA NiMH should work fine – that’s what CJE sell as one of their standard battery replacement kits.

http://cjemicros.co.uk/micros/individual/newprodpages/prodinfo.php?prodcode=4D-REMBATTAAA

I used that too on the two RiscPCs I kept long-term. No problems in the six years they ran like that. (Scrapped a couple of years ago when I got my first Pi.)