The Core Rule book was written with a lot of gaps/assumptions on the author's part that weren't communicated well. Looking online, it seems the consensus is, if a time isn't listed, then it is per activation. Which I am going to rule means per fight. I know per activation means theoretically you can just walk around with it on all the time, but I don't think that was the intention either. You basically get to use that shield 4 times per battery and it used the super high capacity batteries. This also makes sense with the bit in the description about taking move actions to reposition.