You can solder the wires from an old PC speaker connector or something similar to a (non rechargeable) 3.6V lithium battery. For extra tidiness you can wrap it all up in heat shrink tube.

Doornkaat wrote on 2020-10-09, 20:34:

You can solder the wires from an old PC speaker connector or something similar to a (non rechargeable) 3.6V lithium battery. For extra tidiness you can wrap it all up in heat shrink tube.

I have some speakers, but they seem to only have connections for the far pins on the header. What's that middle pin for then?

Some battery packs are keyed so you can't connect them the wrong way around. I think it is electrically a second ground pin. But I can't gurantee that so you better probe it with your multimeter.

Are you sure it has to be 3.6V? As I remember, 4.5V was used for external battery as in these battery packs: https://i.imgur.com/FG82OJo.jpg

You can get part #BH3AA-211-ND from DigiKey which holds 3xAA batteries. It comes with the keyed connector ready to go, and also has velcro so you can stick it in the case somewhere. I'm using one of these on one of my systems.

When I did something similar the motherboard I was planning to use measured 5V between the far pins when I measured using multimeter when powered so I assumed it would be ok for charging a triple AAA battery holder with rechargeable batteries, so I soldered one of those speaker connectors to the battery holder. The pc has been working fine without losing any of the settings for 7 months now even if there has been months between powering up the system some times.