Thanks for the good wishes, redpoint5!
Coincidentally, this summer I also snagged a cheap (OK, FREE!) rechargeable (cordless) lawnmower because its battery was dead (see
Reviving a free, dead 12V cordless Black & Decker mower - EcoRenovator ). I retrofitted a couple of smaller batteries in parallel in that and got it working. Seems I've got a theme going here...
There's got to be a Roomba forum where you can find a "DIY" thread like the one Robert Smalls posted on fixing the Honda hybrid pack...