Hi.
I have only worked on some TECs, but the older ones just used a 2.4 Volt Nicad battery. Generally any cordless phone style nicad button cells do the trick (as long as you can find a 2 cell pack).
If it's a plug in battery, remember to keep the power on while you have the battery disconnected, or all programming will be lost.
Hope this helps,
Mark
Mark, July 2008