Battery health

If the battery health goes under 80%,

make a genius bar appointment with Apple for an evaluation.

They have diagnostic tools and can tell you what options you have.

Otherwise there's nothing to worry about.

Have a read here: Batteries - Maximizing Performance - Apple

How long have you had the phone? It's not specifically a defect just because it's dropping. There are many reasons why there's a drop in battery health reading. Just because it's dropping doesn't mean anything is wrong. I've seen a drop from 95 to 88%. But it had about 330 cycles on it, so it was well above typical loss of capacity. The battery health reading just kind of drops quickly from time to time.

Apple will replace the battery under warranty if the battery health drops below 80% during the warranty or AppleCare period and the diagnostics show less than 500 charge-discharge cycles.