New iOS update is draining iPhone battery

Are you sure you need a new battery?

Batteries are consumables and 1-2% loss per month is perfectly normal. According to https://support.apple.com/HT208387, Apple apparently only considers it unusual (and warrantable) if a battery dops below 80% performance in less than a year after you receive it. Ergo, some storage loss as you use it is expected and acceptable. Incidentally, that possible 20% drop over the first 12 months equates to 1-2% drop per month being within the range of what Apple finds acceptable.

See this excerpt from the "Your battery's maximum capacity" section of the support article: iPhone Battery and Performance - Apple Support

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." In other words, if maximum capacity is below 80%, the battery is below normal performance. You can still use a device with a lower health percentage than 80% but for optimal usage you may wish to consider having it changed when it reaches 80%.

Go to this web site for information about how to request battery service for your iPhone: iPhone Battery Repair & Replacement - Apple Support

For cost and availability, enter your model information in the "Get an estimate" box at the bottom. (I will let you do this since the answer will depend upon the country where you are located.)

For instructions on how to get your iPhone, iPad, or iPod touch ready for service click here --> Get your iPhone, iPad, or iPod touch ready for service - Apple Support

Please double check the condition of your battery in your iPhone 7. You may have a poor battery condition which cannot provide sufficient power for peak performance. What is the Battery Health of your iPhone? Open Settings ➔ Battery ➔ Battery Health and let us know which percentage you see for Maximum Capacity. Also note what is written under Peak Performance. If your battery's Maximum Capacity shows a value of 80% or less, then it has gone through its lifecycle and therefore should be replaced, according to Apple.

Axel F.