Don’t Drain Your Battery Completely
For the most part, it’d be best if you keep your iPhone between 20% and 80% if you want the battery to age at a slower pace. That means that you should try to avoid draining your iPhone’s battery completely.
Charge it when it reaches the 20% mark. If you can’t charge it, try to stop using it.
On the other hand, you should (technically) try to avoid charging lithium-ion batteries to 100%. That will help your battery age favorably, and your iPhone will last longer – although iOS is smart enough to handle overcharging for you.