Repaired Roomba wheel but have to deal with a dead battery pack

REPAIRING AN ORIGINAL MODEL ROOMBA VACUUM

I used a file to cut down the four posts on the gearbox to allow the wheel to rotate with less friction. After reassembly both wheels seemed to have similar resistance to manual movement. I turned on the Roomba and it worked a bit but the battery was too dead to continue.

Gearbox posts filed down, before cleaning and reassembly

After a long charge I ran it again, but it died pretty quickly which tells me the battery pack is clapped out. The original pack delivers 14.4V using NiMH technology and the Roomba charger is notorious for killing batteries if left hooked up too long - since the charge circuit is quite naive. 

I was going to open it up and replace the cells inside, perhaps with Lithium Ion cellas, but it is secured with triangular security screws. A set of screwdrivers are on the way from Amazon so I will continue this towards the end of the week. Once I get inside I can decide how to make a workable substitute.

Battery pack with security screwheads