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This won't take long to unlock through natural progression. Batteries are what enable you to boost your Flashlight, and you can hold up to 20 batteries at one time. Just keep doing what you're doing. If you find yourself nearing the end of the game without having unlocked this achievement, just burn through whatever batteries you have in stock. Also, if your battery inventory is full, burn out your current battery to use one so you can pick the next one up.