iphone blacks out while the battery is still sufficient, won't turn on until charging for at least half an hour. Happens almost every other week.

The iphone 5S blacks out even when the battery is still above 40%, and it won't turn on until plugged in charging for at least 30 min, much longer than the usual length required for restart if the battery runs out normally. This problem happens as frequently as every two weeks. Although the phone eventually gets turned on every time, I am still wondering if this means that I should give my battery a repair service?