Fortunately, your laptop's USB ports can transfer power to other devices, including to your phone. While it not seem like the best way to top up your phone battery, charging via your laptop's USB ports is perfectly safe. Instead, both devices negotiate a safe power draw, and only that is delivered to your phone. This is the technology that ensures your phone charges efficiently and safely when you connect your phone to a laptop or to a high-powered wall adapter. The USB ports on laptops are typically designed to deliver a limited amount of power, which means your phone will charge slowly, and can take hours to juice up completely.