Most motherboard's from that era requires at least a 4.5V external battery in order to keep all the CMOS settings but, you can safely go up to 6V.
This is in the case where the external battery connector has a diode to prevent charging of the batteries.
Apart from ensuring that you have enough Voltage, just check whether that external connector isn't also charging (as you don't want to connect non-chargeable batteries to such a connection). In most cases, particularly if there is only one external connector, it's non-chargeable.

You can just buy an external battery pack that takes 3x 1.5V or 4x 1.5V AA batteries.. An old CD-ROM header or PC speaker header can be soldered onto the wires of the external battery connector (unless you're lucky and it already comes with the header).

By default those batteries are 3cell ni-cd batteries, that produce 3.6V in total. (1.2V/cell)

I believe those are 4.5V tolerant as well, but don't try to push it on 6V!

Most external headers have a diode connected in series.
This will cause around a 0.7V drop in the voltage (sometimes maybe more).
If you connect a 3x 1.5V (4.5V) battery pack, you effectively start off with a supply of 3.8V, which will reduce as your batteries loose their charge.
I recall reading a post on here that the circuits were tolerant up to 6V but, if you take into account the voltage drop, you start off with a 5.3V supply.

I read a number of posts (and even in some of my 286 manuals) that recommend a 6V current on the external battery connector on older 286 & 386 motherboards.

I managed to find a manual for (it seems) a very similar board and it actually says this:

J15: External Battery
Pin 1. +6V voe
Pin 2. Not Used
Pin 3. Not Used
Pin 4. Ground

minuszerodegrees.net/manuals/misc/286%2 ... Manual.pdf

Case closed I guess? 😀

Thanks guys!

That's quite interesting. I've been using 3.6v lithium external batteries on my old motherboards and some of them don't seem to keep the time so well - I guess there's not enough voltage even though it's sufficient to hold the CMOS settings...

I've got that problem too with my 386SX and a 0.22F supercapacitor. I did install a supercap on my DX4 too, however here it's 4F big and I don't seem to have any issue so far.