PMOS battery solution

[rdmark]

I was recently given a 386 for free that had been used as a phone and fax switch at a local theater for decades, but had been decommissioned at some point and was gathering dust in a corner.

It's a late-era 'industrial' 386 with IC date codes from 1995, and I was able to extract the PMOS battery just as it had started to leak, so no nasty nicad gunk had made it to the PCB luckily.

Now, I have read up on the simple circuitry that you have to build with a diode to be able to use a regular lithium barrel or CR2032 batteries in place of another little nicad bomb. However, before building one myself, I wanted to check with the community if someone knows of an off-the-shelf product that you can just drop in on any PC motherboard that expects 3.6V?

 

[YMK]

You could get a leaded Tadiran 1/2AA battery, like the ones used in early SEs.

Solder wires to it and keep it away from the board in case it blows some thirty years from now.

https://www.mouser.com/ProductDetail/Tadiran-Batteries/TL-2150-PBP?qs=afMqcEmPaWq65qzNkxflHg%3D%3D

 

[rdmark]

I suppose this would work, although I believe a diode needs to be connected in serial to stop the motherboard from feeding 5V into it.

Personally, I'd prefer a solution that doesn't involve soldering a battery directly onto the board, however.

 

[YMK]

So the board charges the battery?

If that's the case, and it expects NiCd cells, I'd substitute a battery holder with Eneloop NiMH cells.

They're also 1.2v per cell and don't leak.

 

[rdmark]

So the board charges the battery?

If that's the case, and it expects NiCd cells, I'd substitute a battery holder with Eneloop NiMH cells.

They're also 1.2v per cell and don't leak.

Actually, I may have been incorrect in my assumption. When I tested both the internal and external battery headers with my multimeter, neither had any noticeable voltage supplied to them when the computer was in operation. Would you say it's safe to assume no voltage means that the motherboard expects a non-rechargeable battery? It's the first time I work on a PC motherboard, so want to make sure I don't make rookie mistakes.

 

[YMK]

How sure are you that the original cells were NiCd?

Non-rechargeable batteries were the norm in PCs of that era.

No voltage present on the terminals doesn't rule out the possibility of an active charge controller, which may look for battery voltage before charging.

In either case, Eneloops are a good choice, also because they have a very low self-discharge rate.

 

[rdmark]

Good question! The battery is KTS brand and looks like this. There is scant information online.

 

[Volvo242GT]

If we can verify that the board doesn't charge the battery, another option would be a double AA holder with two Energizer Ultimate Lithium batteries. Provides 1.8 volts per battery until death.

 

[rdmark]

An update on this: I found some technical documentation for my 386 motherboard in question and realized that there's an external battery header, J1, sitting just below the internal battery compartment. I could not detect any charging currency going into this header either, and some discussion threads elsewhere suggests that external battery headers on IBM PC compatibles are supposedly always non-charging.

So I put together a very simple external battery pack using a 3xAAA battery holder, which provides 3.9V in serial. Crimped a four pin edge connector onto the leads, which fits the motherboard header perfectly. Now the little canisters of corrosive material are attached on the outside of the case, at a safe distance from the PCB.