http://matthewbotos.com/blog/2007/05/01/sonicare

After my previous blog entry and photos on Sonicare Battery
Replacement, I got an email from Ed Elliott, who detailed his own more
successful attempt at replacing the batteries in his Sonicare 4100:
Thanks for sharing the info on the Sonicare. Based upon your photos I
was able to take some measurements so that I could take a different
approach. Rather than splitting the case in half along the original
seams, I removed just enough of the case so the batteries could be
removed and opened a small area so that I could unsolder the battery
connections from the PCB. Your unit is probably beyond hope, but you
might want to share the following info on your site.

The cuts
c1da8ad1f39e567d6eb3bb25e178693e
The battery pack
da64b49f9f86eb4c111ec6478c3a1bb3 It works! Powers up. Runs for the
full two minute cycle. I haven’t decided how to close it back up.
Epoxy? Silicone caulk + heat shrink tubing (may not fit in charger
base)? If it lasts until the new Nicads die, I’m doubt if I’ll replace
the Nicads again so making the process reversible is not a concern.
Prior to this success, I tried this process with an even older model
(it even had a different model PCB). Failure. I used the same custom-
made battery pack that was subsequently used successfully in the newer
Sonicare. I also tested the older one unsuccessfully using 2.4V feed
from a bench power supply. I believe the problem was elsewhere on the
PCB - the unit had sat in an RV unused for over 4 years through heat/
cold cycles and it had been used for several years before that abuse.
Given that my cutting approach did not give full access to the PCB
plus the fact that I already have the re-batteried Sonicare and a new
Sonicare 9500, I decided it wasn’t worth the effort to track down the
problem with no schematic.