If you're like me you wont get this in your first playthrough because either you dont bother using batteries and just wait for your flashlight to recharge, or dont use batteries often.
Well Instead of waiting for the prompt which tells you to press Y just tap Y repeatedly while you are focusing your flashlight with LT, you dont have to wait for your flashlight energy to be fully depleted. So everytime you grab a battery pack, tap Y until you use up all your batteries