One of the joys of buying old mobile phones is that they tend to come with free biohazards, courtesy of their rapidly-decaying NiCad batteries. This one was no exception.
Once I prised open the battery casing (wearing gloves!) I quickly removed the gunky old NiCads and wrapped them in kitchen roll, ready for disposal. Actually, they weren't too badly decayed, given that they probably haven't been charged since the Millennium.
There was more good news. Before buying this phone, I knew that the CM-H333 battery pack is rated at 4.8V. This sounded like code for 'probably just 4 AA rechargeable batteries in a box' and it turns out I was right. All I needed to do was insert a battery contact at one end of the prised-apart battery box, tidy up the main terminals to ensure they made proper contact with the phone's +/- pins (with a bit of very carefully positioned tin-foil) and put the two halves of the battery pack back together.
Huzzah! I now have a 2,450mAh Ni-MH battery pack for the old Sony, containing nearly-new batteries.