What you will need.
1/ Rechargeable batteries ( Tagged ) 2/ Soldering Iron & Solder 3/ Universal or the correct connector 4/ Heat shrink sleeving 5/ Heat gun
Step 1/
First find out chemistry battery your old phone battery is, usually Nickel Cadmium or Nickel hydride. The Nickel hydride was used in many of the older phone and has now been replaced with Nickel Hydride.
Step 2/
Find out the size or your phones rechargeable battery most are 2/3AA size and many now are 2/3 AAA as the phone are getting smaller. Most of the older type phones used the standard AA size.
Step3/
Once you have selected which batteries you need you must purchase them with solder tags fitted it is not a good ides to solder directly to the tops of the cells as this could damage the vents under the posatice end of the cell. There are many sites that will supply with tags and again all sizes can be found at www.espspecialbatteries.co.uk
Step 4/
Have got your batteries you now need to solder link the tags in series i.e. Positive to negative leaving one tag for your positive connection and one for your negative.
Step 5/
You now need to fit your connector most of these can be found at www.espspecialbatteries.co.uk and select components and connectors
Step 6/
Now solder the red positive lead of your connector to the positive solder tag on your newly made battery pack and then solder the black negative lead to the remaining negative end of your new battery pack.
Step 7/
Finally if you have some heat shrink sleeving the cover the new battery and using a heat gun shrink it down so it fit tightly over your new battery pack. Alternatively you could get away with using insulation tape, but this is not as neat.
Completed assembled batteries can also be found at http://www.cordlesstelephonebatteries.co.uk/