It is now quite common for pilots to warm their batteries before a flight so that they have maximum power from the start. Obviously you have to avoid having it too hot to start with, or you risk the battery being damaged by being too hot (about 60C) at the end of the flight. About 30 degrees C seems a safe starting temperature.
The heating systems in use are as individual as the pilots:- heated food boxes, model car tyre heaters, commercial units, etc.
One solution (shown here) can double up as a heated cradle for the asembled model to stop the battery cooling while you are waiting to fly. This unit uses 2 self regulating heaters to provide 20 watts of heat from any 12v supply. The box is made from 3mm ply, and the heaters are mounted on 2 sheets of aluminium insulated with Kapton tape. 6mm balsa strip is used to support the battery/fuselage and allow the warm air to circulate.
The heater elements are available from Farnell in different temperature and wattage ratings. This unit uses 2 x 10 Watt 40 degree C elements. No thermostat is required the current just reduces as it heats up.
As these units are not well known, here are some data logs of the heater in action on different voltages.