|J31 is where you'd attach an external battery. Often those motherboard would have an on-board nicad rechargable battery. If you were using it you'd have the external connection jumpered as in the photo. |
The external batteries would come with a 4-wire connector with one of the holes filled in to act as a key so it could only be installed one way. If you were using the external you'd remove the jumper which I guess would disable the on-board battery and then attach the external battery there.
Your motherboard has a lithium which is not a rechargable battery but I guess their logic is the same--with J31 jumpered you're using the on-board and with it off you attach the external one there. It's kind of odd the batteries have different voltages. I assume that's due to their different locations on the motherboard so the voltage measured at the cmos chip is the same regardless of which one you use.
I'm almost sure you can still buy those external batteries at Batteries Plus, a computer supply store or online. Or you could make one with a 3 cell holder. You could probably solder the holder leads to one of the 4-wire audio cables that come with a cdrom and attach it to the motherboard that way. You have to be careful of the polarity since the cdrom cables aren't usually keyed to fit one way. You could also cut the 4-wire speaker connection from an old case and use that.
I have some of those external batteries but they're probably dead by now but I could check the polarity if you decide to build one. But if it was me I'd just buy an external one. That way you know it's done right.
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