Please share your experience with us, if you know the correct answer. I do appreciate for all the answers.
The Original Battery 15.6v 3Ah
Normal voltage is 15.6v = 13 cells x 1.2v
Fully charged voltage is 19.5v = 13 cells x 1.5v
Capacity 3Ah or 3000mAh
13 Nimh cells 1.2v 3Ah or 3000mAh

Lithiumion battery 18v
Normal voltage is 18.5v = 5 cells x 3.7v
Fully charged voltage is 21v = 5 cells x 4.2v
Capacity is 3Ah or 3000mAh
5 liion 18650 LG 3000mAh 20A

Lithiumion battery 14.4v
Normal voltage is 14.8v = 4 cells x 3.7v
Fully charged voltage is 16.8v = 4 cells x 4.2v
Capacity is 6Ah or 6000mAh
8 liion 18650 LG 3000mAh 20A

The motor can accept 19.5 volts. I think 18v or 5s is the best choice for this project, but the battery case can only load 5 cells so the capacity is only 3Ah. If the motor works well with a 14.4v battery, it will be good because the 14.4v capacity is 6Ah. I decided to build both types for testing. As a result, the drill with 14.4v battery performed well with the test. I think 14.4v is my best choice for this tool.

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