How to select a correct battery for your devices?

How to select a correct battery for your devices?

 1.You need to know your device

      a.What's your device's input voltage (V)

      b.What is its power consumption? (Wattage)

      c.What is maximum current drain? (A)

      d.What is your expected running time by a battery pack?

  2.Device battery pack you voltage you must be equal or a little higher than your device's need

  3.Decide battery pack's capacity is depended on how long you need to run your device (hours)

caculated the capacity as the follow:

(Ah)=Device's Wattage (W) X Time to run (Hours) / Battery Voltage (V)