Cost of a new phone set with three headsets (receivers): $50
Cost of a new 500mah 2.4V battery (branded): $34
Cost of rechargeable (low drain) battery (2000mah 2.4V) (set of 8): $13.16
The cheapest and best solution was to make my own rechargeable cordless phone battery.
Soldering Iron + solder wire
Glue gun + glue sticks
1" of copper wire or small strip of metal
2 AA rechargeable batteries (NiMh or NiCd)
Step 1: Connect Batteries in Series
Solder a small wire to the positive and negative terminals of the rechargeable batteries to connect them in series (1.2V + 1.2V = 2.4V).
Check the voltage output using a Voltmeter. Once the desired voltage is achieved, glue the wire and batteries in place using a hot glue gun.
Step 2: Connection Plug
Strip the open ends of the wires.
Solder them correctly onto the open ends of the batteries (red wire - positive terminal, black wire - negative terminal).
Check the voltage again using the voltmeter. Once the desired voltage is achieved, glue the terminals and wires to the battery using a hot glue gun.
Place electrical tape on all exposed wires.
Step 3: Summary and Final Product
Finally connect the connection plug to the receiver and check that it works. Install the battery cap and place the phone receiver in the phone cradle for charging.
From past experience NiMh and NiCd batteries can be charged using the same charger. (please verify this for your self). Do not mix new and old batteries. Do not mix NiMh and NiCd batteries in the same circuit.
With the original phone battery: talk time ~ 200 minutes
With new DIY battery: talk time ~ +700 minutes
I still have 6 low drain batteries left over for other use.
Total cost of new replacement battery $3.29 (assuming I have use for the other 6 batteries).