A step-by-step tutorial showing how I repaired my $60 Dirt Dog battery with just a AA battery and some tape. We start off with a Dirt Dog battery that has a voltage of 15.87V which is indicative of a bad cell. When we complete the repair we are at 17.12V and that is without even charging the pack.
Step 1: Remove Triangle Screws
The iRobot battery has a bunch of annoying triangle shapped screws. I've read that some people can easily unscrew them with a appropriately sized flat had screwdriver. I used another technique which works for most "secure" screws types.
Step 2: Dremmel the Screws
I used my dremmel to cut flat lines across the screws. Later I cut a second set (not shown) so that a phillips head can be used to unscrew and rescrew these. This is a cute trick to use for all sorts of stuff you are not supposed to service yourself.
Step 3: Get That Case Open
There are claims that you can just pop the battery lid after removing clips and doing some prying on the irobot batteries. I think there is a little more to this. The Dirt Dog in particular seemed to have a well glued on battery lid. It was no match for a dremmel. Be careful to not cut deep. Good ventilation is useful here as the plastic smells terrible when you are cutting/burning it off with the dremmel.
Step 4: The Battery Cells
Here are the cells. These are 12 sub-c sized / 1.2V / 2600mAh / NiMH batteries. These vary between different models so yours might be a little different. The dirt dog seems to have a cheap ass NiMH (low amp hour) versus the higher end models.
Step 5: Make a Mess
Yeah, now this is a good time. Go ahead and rip off that glue on paper stuff. It's cool. We need access on both sides to the batteries so we can find our dead cell.