Battery health

According to this User Tip, by Lawrence Finch:

"Finally, keep in mind that batteries are "consumables". Their capacity starts dropping the day they come off the assembly line. Battery life is determined primarily by "full charge cycles" - A full charge cycle is 0% to 100%, or any combination that adds up to 100%, such as 50% to 100% or 30% to 80% twice, 80% to 100% 5 times, etc. The battery will gradually lose capacity, and is rated to remain above 80% capacity (that's total capacity, not charge level) for 500 full charge cycles for an iPhone (1,000 for an iPad). For most people this will be around 2 years for the iPhone. Apple will replace the battery for a reasonable fee (currently $49 US for older phones, $69 for iPhone X and later) if it goes below 80% after the original 1 year warranty or AppleCare+ warranty period. Within the warranty the replacement is free."