No it's not that. I understood his blast attack and the shield. I misunderstood his Overload though, I thought it was just an attack with x10 power wherever I was, not a buff.
Maybe it was because I didn't pay attention that the shield consume a battery and used it too much, or used it while my energy depleted at the very same time with only one battery, or maybe that the shield ended at the exact same time it attacks... An instant combination with no way to trace back.
Batteries are important and currently the unique consumable, maybe their amount in the inventory should be more highligthed in the inventory than the permanent upgrades?