NiCd battery replacement in vintage computers

[Brent Hilpert]

On 2016-Feb-06, at 1:21 PM, Mark J. Blair wrote:
> Today I discovered that I hadn't replaced the NiCd battery in time in my Amiga 3000. Pictures:
> 
> https://twitter.com/nf6x/status/696042894939979776
> https://twitter.com/nf6x/status/696050264306921472
> https://twitter.com/nf6x/status/696065578977472512
> 
> It's a fairly typical 3.6V 60mAH 3-cell NiCd pack, 16mm diameter x 18mm long, polarized with 2 pins on the positive end and 1 pin on the negative end.
> 
> In the past I have usually replaced these sorts of batteries with new ones of the same type. This time, I'm thinking of at least installing a remote holder. Not only to prevent further PCB damage in the future, but also to make the battery easier to replace. Lots of screws need to come out to extract an A3000 motherboard:
> 
> https://twitter.com/nf6x/status/696047816938946560
> 
> While I begin to figure out how I'd like to perform this repair, I'm curious about what others have decided to do in similar circumstances. Many options come to mind:
> 
> * Solder in the same kind of NiCd pack to keep things original.
> 
> * Solder in a supercap instead.
> 
> * Reconfigure the circuit to use a non-rechargeable lithium coin cell in a holder instead. I don't think I've seen one of those leak before.
> 
> * Yet some other remote battery option.
> 

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.

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.

The advantages of this solution:
	- AA/AAA alkaline cells are commonly available and relatively inexpensive.

	- depending on current draw, you can often use otherwise-worn-out alkaline cells.
	 For example, cells that are no longer usable in your remote control or flashlight can still
	 have enough potential to maintain CMOS memory for years.

	- alkaline cells tend not to leak and generally can be ignored for years.

	- if mounted on the exterior, damage in the event of leakage will be minimal.

	- if mounted on the exterior, they're accessible for replacement.

The more cells you use, the more headroom/longevity there is as the cells wear down, but too high a voltage with newer cells also has to be accounted for.

If the original design was for rechargeable batteries one will have to disable the charging function and may have to install isolation diodes.