Lithium batteries can lose charge much faster when exposed to cold conditions. There are few steps you can take in order to protect your Deeper from cold, and as a result increase the battery life
a) Take the Deeper out of the water whenever you're not using it, clean it from water and keep it in your pocket or somewhere warm.
b) Water temperature in an ice hole is much warmer that the outside temperature. Furthermore, Deeper that is scanning an ice hole will heat up a bit, which also protects the battery from cold. So, if you keep your Deeper in a single ice hole and use it at the same time, battery will last longer than if you're hole-hopping.
If your Deeper runs out of charge, you can also charge it up using a regular external battery pack (power bank). Just be sure to do it in a warm place (for example - your backpack or an ice shanty).