History!
Flaperon777 Has most of the historic answers but a little thin on historic technical detail.
Historicly the #2 engine had the generator and the #1 had the hydrulic pump so the battery was mounted nearest to the #2 engine to enable the shortest cable run to reduce the electrical losses due to resistance.
So the #2 was the best engine to start in order to save the small amount of electrical power that these aircraft had in the batterys and once it was running to have a generator on line to help start the #1 engine.
The Beech King Air has the battery mounted in the wing just inboard of the engine, in these modern times I suspect Beech mounted the battery in this location because thats the way they had done it in the past.