Robert Leroy Kruse has written:

'Experimental determination of the stable-trim attitudes of two proposed, general-purpose, heat-source modules' -- subject(s): Aerodynamics

In general, battery is source of electricity and not the heat source.

However, for common use of battery, battery can be a heat source though it is not intended to be. It is because the flow of electrical current through resistance would give out heat. If your battery heat up without cause then it is suspected for the short-circuit. Over charging or oversupply of electricity would also cause the battery heat up.

The main purpose of a heat sink is to expel heat from a generating source. Heat sinks work through the process of conductive and convection heat transfer. Heat sinks are a passive form of cooling, as they have no moving parts and require no power.

the sun is our source of heat

That flame is the source of heat for the hot air in the hot air balloon.