How do I replace an old Roomba's battery? [solved]
I've got an old iRobot Roomba "3100 Pro Elite" whose battery has died. A new Roomba is EUR 300 and upward, so I would prefer to revive the one I have.
It's difficult (impossible?) to find replacement batteries in Austria (Europe), and they are so expensive that I haven't yet convinced myself the cost is worth it. But it would be nice to revive the Roomba as it is in decent working condition, mechanically, except for the cheap rubber treads that I have glued back on.
Do you have suggestions for where to get a replacement battery for this specific Roomba?
Would it make sense to try this DIY approach, considering I've never tried soldering on batteries? I'd hate to have this thing explode in my living room. Where would I get batteries from? The page's discussion thread refers to a U.S. website.

Ebay - that's where I got my replacement. It's NOT an iRobot manufactured battery pack (nor does it claim to be), but it works fine. I think it came from someone in Hong Kong, and the shipping wasn't bad at all - I don't know how shipping in your area would be.
I looked at opening the pack up and replacing the cells, but I couldn't figure out how to get the thing open without damaging it.
My first recommendation would be to contact the maker of iRobot and see what they can do for you, I know that in the US we can get batteries from them easily.
Now, if you wanted to go the route of building your own battery as it noted in the link you posted, it isn't "that" complicated. You can get yourself some much better detailed instructions on how to build the battery pack using instructions for "Radio Controlled Car Battery Pack" creation as they us the same Sub-C cells for the battery packs.
Upgrade Your Old Roomba to a Smart BotVac for $5