On 27.06.15 19:59, Peabody wrote:
I have a feature phone that's several years old, but still works fine. But
the battery is on its last legs, and I'm having a problem replacing it. The
battery is an Alcatel CAB3120000C1, which is 3.7V, 850 mah.

These batteries are all over Ebay, but a seller I contacted confirmed that
the battery was discontinued several years ago, so the date code on all these
OEM batteries is several years old, and they don't do very well anymore. The
most recent one I've been able to find is the one I have, which is 2013.
Most are 2010-2012. One seller offers non-OEM replacements, but didn't
respond to my question about the age of his batteries.

I wondered if it might be possible to buy a *new* replacment cell, and put
that into the battery, preserving all of the protection circuitry in the
original. The battery is very thin - 4mm - and I don't know where to find a
replacement cell that would work. The overall size is 40 x 50 x 4 mm.

I would hate to have to buy a new phone just because of the battery issue.
Can anyone suggest a solution to this problem?

Google around in your country, for a battery rebuilt company.
They open up the battery pack, and replace the old cells with new.