NiCd battery replacement in vintage computers

Mark J. Blair nf6x at
Sat Feb 6 20:35:21 CST 2016

> On Feb 6, 2016, at 16:59, Chris Pye <pye at> wrote:
> This is what I normally do, preferably mounted away from the board. Generally you can get away with simply using a diode to prevent the circuit trying to charge the battery.

I think that a blocking diode added in the positive lead of a non-rechargeable replacement battery should work in this application. After further thought, lifting one end of the diode in the existing charging circuit would not be ideal because that would mean the RTC would always draw power from the battery, rather than drawing power from the main power supply when the computer is turned on.

> On Feb 6, 2016, at 15:37, Brent Hilpert <hilpert at> wrote:
> My favorite solution, for the right circumstances, is to mount a 2, 3 or 4 cell AA or AAA battery holder on the outside/rear and use common alkaline cells.

That's not a bad idea!

> Right circumstances comes down mostly to current draw. If the function is purely CMOS memory retention the current draw is equivalent to static/idle leakage and you can expect shelf life from alkaline batteries. An RTC is going to draw some current due to the active circuitry. I'm not sure what the current draw for an RTC of that era is, the tech should be CMOS and the draw small but not as small as simple memory retention.

Let's see, the RTC in this case is a Ricoh RP5C01. It has 26 x 4 bits of RAM, but I'm not aware of any parameters stored in it (neither have I investigated thoroughly; I just don't recall ever seeing the sorts of parameter settings on Amigas that PC family machines usually have). Its rated maximum operating supply current at Vcc = 5.0V is 250µA. It's probably quite a bit lower at typical conditions while operating from a backup battery at Vcc = 3.0-3.6V, but still a lot higher than the 500nA ratings I recall seeing on much newer RTC chips.

> On Feb 6, 2016, at 17:03, Chuck Guzis <cclist at> wrote:
> I see all the talk about NiCd cells--does no one use NiMH nowadays?  Why go with the toxics?
> --Chuck

NiMH is worth considering; I would just need to study the charging circuit carefully before changing secondary battery chemistry.

Mark J. Blair, NF6X <nf6x at>

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