When your iPhone shut off by it self, if the battery drain faster, or if your iPhone refuse to charge than it a time to change the battery. Cell N Comp Repair use brand new 0 meter premium battery, it comes with 30 days warranty. You can extend the life of your iPhone battery by not charging the phone for long time, after you charge 90% or 100% use the battery and once it drops to 20% or less then that, you can charge again. Most of the stores they used refurbished battery which is with poor health.
Giving priority to the battery health is very important, when the battery health is good than the health of your iPhone will be good. If the battery have poor health, than it may occur unexpected shut of, which may effect to the main board.