Every fitness tracker and smart watch is different, from its core features to its underlying technology, so this question is a difficult one to answer. Battery sizes and charging techniques vary, so the key is finding a wearable that works for your lifestyle. Journalist and author Kayt Sukel travels frequently, so she had a unique requirement when buying a wearable device to track her daily activity levels. “If I didn’t have my computer with me for a few days I might not be able to charge it,” says Sukel, who chose the Fitbit Zip, because of its replaceable “button” watch battery that can last up to six months. Despite having an always-on display, heart rate monitor and sleep sensors, the Garmin Vivofit has a battery that lasts longer than a year. Meanwhile, smart watches, like the Samsung Gear, last 24 hours and come with a charging dock to refresh their batteries — something that can easily be done each night, as you would for a cell phone, or on a desktop every afternoon.