Ironically I found a computer today with this exact same motherboard in it. The Varta had leaked all over U8 and ate thru several traces. Got the battery out and have the board drying after washing the battery corrosion off.

Did you have to put any jumpers on the board somewhere before you put the external battery pack on?

Don't think so. Do bad things happen if you connect an external battery without disconnecting or disabling the onboard one?

Currently waiting for some parts to arrive before resuming work on this.

Hello again. I've got hold of a TL866II for reading the ROM. What IC setting should I use? The chip has a National Semiconductor logo on it, but I presumably have to remove the AMI label to see which model it is, which I would rather not do.

A 386 ROM would be 32 KB?

Guys, external battery was one standard 4.5V for all generations ? Or was there higher/lower exceptions ?

Original IBM AT used 6V. Most designs will tolerate either 4.5V or 6V

Thank you. I have to order more 3xAAA battery holders.

@Errius Did you dumped the BIOS of your mainboard ?
@Deksor Were you able to find the BIOS for your project?

I have a motherboard like that here, too, and I want to fix it

Greets Gassi

Those "barrel" batteries are usually a few ni-cad cells in series, count the bumps and multiply by 1.2 to get the voltage.

Ni-cads being rechargable will have a reverse "charging" voltage/current - You can replace them with AA(a) "dry" cells, but you need to add a diode to block this reverse current. Keep in mind that a diode will drop about 0.7v - I usually find that 3 AA cells = 4.5v = 0.7v works out to about 3.8v which is usually close enough and within spec for the devices in the circuit.

And do remove the old cells - they are notorious for corroding and leaking when they get old!