How to extend the life of a chargeable battery.
A battery that is not being used will lose its charge from leakage between the terminals. The temperature affects the rate of charge lost. A battery kept in the fridge or stored in the snow (inside a dry pack) will last longer because the loss of charge which be much slower. The battery will of course need to be warmed up before functioning efficiently.
Fully re-chargeable unused batteries can go flat in 14 days while a battery stored in the fridge can last twice as long.
There is no point having a phone switched on if you do have a signal. Turn it off and save its life until you have a signal.