From what I grasped on SRAM, they can retain save UP TO 30 seconds. Which means that you could have 30 seconds to solder our battery... Or if your banks and block full, everything vanishes in the half a second after you disconnect the battery.
What you can do is to solder a temporary battery (with wires)on the power feed, if the battery tabs allow for it. so you have time to fiddle around.
But, question. If you don't want to lose your saves is because your Tennokoe bank retain saves? Or does it keep them, but for a short period?
If your Tennokoe bank retain save saves for more than one day, then you don't need to change the battery.
Sadly, there is no predictable way to know when the batteries are going to run out.
If it doesn't retain saves, then you probably only have your Tennokoe 2 or CD-ROM2 attachment/Duo saves left. So why do you need to keep your Tennokoe card powered? Leave your CD-ROM2 system on and take time to swap your battery.