The mobile hotspot is configured (by default) to optimize battery life. There are several methods you can use to extend the battery life:
If the devices you are connecting to your Wi-Fi network are always near the mobile hotspot device, shorten the Wi-Fi range. Home screen > Settings > Device Wi-Fi Range
Reduce the LCD's brightness level. Home screen > Settings > Screen Brightness
Reduce the screen time out. Home screen > Settings > Screen Timeout.
The more devices connected to the Wi-Fi network the more battery power is consumed.