Unfortunately I have checked every car on Spruce Falls bridge, all of the houses and cars in Milton and the farmhouse.

There is no battery in the mountain town region for me, so i decided i had to travel to broken railroad. on my way out i checked the broken car at the bottom of Milton Basin, SUCCESS - A BATTERY!!!

Only to realise that the position and unopenable broken bonnet means I can't access the one battery I have found :'(

I'm at Broken Railroad now, I just have to make the double rope climb back from the Basin.

NE from Milton is Spruce Falls Bridge and there are dozen cars there, so if you need batteries, that is the most abundant place in the whole game for those.