Use a compatible power supply with the correct voltage and current limit
Use a USB port for small rechargeable devices that support USB charging
Use a solar panel with a proper charge controller
Use a hand-crank generator designed for batteries
Use a car battery charger adapter if the battery type matches
Use a bench power supply set to the battery’s required charging voltage
Use another battery of the same voltage with a suitable resistor or charging circuit
Use a DC adapter with the correct output and polarity
Use a battery pack or power bank only if the device supports it
Use a dedicated charging circuit for the specific battery chemistry
Use an emergency jump-start method only for lead-acid batteries and only when appropriate
Replace the battery if it is damaged, swollen, leaking, or not accepting charge