At DR5, I think the battery you will rely on is the capacitor on in the data sleeve of the software cartridge. I would have to check, but I think it is like a 5.5 volt 1 farad cap (quite beefy). With a new capacitor you can go quite a while, I have 824's that I still support that can go atleast 1 week without AC. I have heard of some that have gone months with fresh caps.
As the capacitors age, their back-up life drops significantly, I replaced a pair in a datasleeve that I think were over 10 years old and the old ones would last only minutes as a backup.
If you have a light touch with a soldering iron, you can remove the two capacitors and replace them. The traces on the board are delicate, so you need solder wick or a de-soldering tool to remove the old ones, you just can't super heat them and pry them off unless you want to take the traces with them. Ground yourself, unscrew the data sleeve, and lift out the board, easy to disassemble and reassemble. About a 20 minute exercise. Lots of refurb supply companies have the capacitors in stock with the exact same pin layout/spacing.
