Why battery power remaining time on Asus Transformer Book T100HA always change in Windows system?

Actually,the remaining Asus Transformer Book T100HA battery level that shows in the system is an estimated value, instead of the actual value. The system will calculate the remaining time based on power usage and the amount of software running.

When you select High Performance mode or are running a lot of software at the same time, the remaining Asus Transformer Book T100HA battery time will decrease. 

On the other hand, when you select Power Saver mode or run few software at the same time, the remaining ASUS battery time for available power usage will increase.

This is a normal condition for Windows 10.

Asus Transformer Book T100HA battery

Asus Transformer Book T100HA battery

Upgrade the Transformer Book T100HA battery, ASUS Transformer Book T100HA 3.8V 7850mAh laptop batteries
Voltage: 3.8V
Capacity: 7850mAh (30Wh)
Battery cells: 2-cell
Chemical: Li-Polymer
Battery Color: white
Condition: new original battery

Warranty: 9 months