The small battery-operated heaters for hands used by hunters, fishermen and sports enthusiasts are quite efficient. The heater is turned on and tucked into a pocket, keeping the owner's hands warm for 5 to 6 hours, depending on the model. Heated socks are also available.
Battery-operated hand warmers often use rechargeable batteries. Some models have USB ports, which allow charging from a computer or 12-volt power outlet in a vehicle.
Other types of battery-operated heaters, such as defrosters and car heaters, may be plugged into a vehicle's power outlets or a cigarette lighter. These are designed for use when the vehicle is running, so the battery is not drained.