My laser batteries are super fertile. Upon returning from a mission they seem to increase at about 1 per laserweapon used in that mission.
Example:
I have 3 laserweapons on a mission and a total of 3 Batteries (which are naturally all inside said weapons). I fire a few shots with on of those laserweapons. When I get back from the mission I have a total of 4 batteries available.