How bad are 3 degrees Celsius for a lithium battery and electronics?
To prevent condensation the components in the cabinet should be above the dew point.
There are two ways to do that:  


Reduce the relative humidity, for example with silica gel.  
Increase the temperature, with a heater.




When cold the chemistry in the battery slows down. Which increases the internal resistance of the battery, reducing it's effective capacity. Refer to the battery datasheet to view how much you lose.