Once a Battery unit or Tank exhausts its ammo supply it can no longer attack or counterattack, though it can still move.
A good way to represent the ammo supply is to wrap small rubber bands around the figure, one for each round left.
If reloading from a unit of the same type the supplying unit must have ammo to spare. The supplying unit can give the reloading unit up to as much ammo as it is carrying.
The 'close enough' rule does not apply to reloading.