My Bluetooth headset was getting about 6 hours on a single charge and sometimes even lower if it wasn't turned off after use.
So, I decided to replace the battery with a beefy 18650 Li-ion battery.
I have tested this setup over a month and had to charge the battery once. It hasn't given me any problems so far.
One of the main concerns I had was if the Lithium-ion have the same characteristics as the 200mAH Li-polymer battery in the headset. Li-ion and Lithium Polymer have slightly different battery chemistry but has very similar voltage profiles. Since I am not using the headphone to charge the Li-ion , dealing with the headset's BMS for charging is not an issue. Also, I measured the absolute cut-off voltage for the headphone for operation. Its around 3.35V, which is well above the cut-off voltage of 2.5V from Li-ion. ( INR18650-30Q datasheet).
Step 1: Find the Battery
The battery should be located in one of the headset pieces. In my case, it was located on the right side of the headset.
First thing is to remove the headphone cushions and remove the screws that hold the headphone cups together.
After removing the screws ,remove the 200mAH battery , drill a hole for positive and negative wires through the headset to exit to the outside.
Make sure you note the positive and negative on the wires.
Step 2: 18650 Holder and Cable Management
I used quick glue to hold the 18650 Li-ion battery holder to the side and made sure it stayed secure with full headphone articulation. Then did some cable management, soldered the wires to the right length and it worked!
I plan to use this headset for home use only, so this mod should do the job.