Working of AT+CBC command

Can someone please explain how does the command AT+CBC give the percentage of battery voltage left??

When I was powering my SIM800L module using a Lab Bench Power Supply at 3.7V, the command gave me 37%, but how is this possible as it should give 100% as result right?

If I am doing something wrong, then can anyone also tell me if is there is any specific way of connecting a battery( say 3.7V LiPo) to the module so that I can get correct value of left power in the battery.

I am trying to make low battery indicator for my battery powered SIM module.

Thanks in Advance!

For LiPo batteries 4.2V is 100% and 3.6V is like 0%.