Is 88% battery health normal for iPhone 15 after 11 months?

Your indicated battery health is exactly as might be anticipated...

You might reasonably anticipate that the indicated battery health will fall by 1% for every 50 charge cycles. So, simple math shows that 335 charge cycles will equate a little more than 12% (100-12 = 88%).

"Battery health" is influenced by many factors.

An iPad battery is nominally rated to retain 80% of its original capacity (when new) after 1000 full recharge cycles. Any combination of charging that together sums to 100% equates to one full charge cycle (e.g., 50% + 20% + 20% + 10% = 100% = 1 full charge cycle). Reducing the number of times that the iPad is recharged will slow the rate that the iPad's battery health will decline due to recharging.

Other factors can influence overall battery health. High ambient temperatures can permanently reduce the battery's ability to retain full charge - as can maintaining the battery at 100% charge for very long periods of time that leads to chemical aging of the battery. Limiting the battery charge to ~80% of maximum is intended to reduce the chemical aging effect.

Information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

The measure of battery health deterioration/drainage is subjective and can vary depending on individual usage patterns. There is no universal standard to determine whether the battery health deterioration/drainage is normal or abnormal because it heavily relies on how the battery is used and maintained.

Protect & Prolong the Life of your iPhone's Battery:

1. About the battery usage on your iPhone, iPad, and iPod touch ...
2. Batteries - Maximising Performance - Apple (IN)
3. iPhone Battery and Performance
4. About Charge Limit and Optimised Battery Charging on iPhone - Apple Support (IN)

• Find out what to do if Optimised Battery Charging doesn’t activate.
• Find out what to do if your device stops charging unexpectedly.
• Find out what to do if you think your battery is draining too quickly.
• Find out how to charge and maintain your iPhone battery.

Try to monitor Battery usage by iPhone Activity as shown below

What is shown under battery usage? Checking that would give you an indication of the usage behavior.

1. Background Activity
2. Notifications
3. Connected to Charger
4. Or any Other

Try some of these options too.

To prolong the life of your iPhone battery, you can follow these tips and best practices:

To prolong the life of your iPhone battery, you can follow these tips and best practices:

1. Optimize Battery Charging: Enable the "Optimized Battery Charging" feature on your iPhone. It helps reduce the wear on the battery by slowing down the battery aging process. You can find this option in Settings > Battery > Battery Health. About Optimized Battery Charging on your iPhone - Apple Support (IN)
2. Avoid Extreme Temperatures: Extreme temperatures, both hot and cold, can negatively impact your battery life. Avoid exposing your iPhone to excessive heat or cold, such as leaving it in direct sunlight or in a freezing car.If your iPhone or iPad gets too hot or too cold – Apple Support (UK)
3. Use Original Chargers and Cables: Always use Apple's original chargers and cables or certified third-party accessories. Low-quality chargers or cables can damage the battery and reduce its lifespan. Identify counterfeit or uncertified Lightning connector accessories - Apple Support (IN)
4. Avoid Draining the Battery to Zero: Charge it whenever the charge becomes lower than 20%. The rest is up to you when to charge. how long to charge, how often to charge, which adapter to use starting from 5W to 140W USB-C Power Adapter etc.
5. Optimize Display Settings: Adjust your display settings to optimize battery life. Lower the screen brightness, enable auto-brightness, and use shorter screen timeout durations. Adjust the display and text size on iPhone - Apple Support (IN)
6. Minimize Background App Refresh: Limit the number of apps allowed to refresh in the background. Go to Settings > General > Background App Refresh and disable it for apps that don't require real-time updates. Switch apps on your iPhone, iPad, or iPod touch - Apple Support (IN)

You might reasonably anticipate that the indicated battery health will fall by 1% for every 50 charge cycles. So, simple math shows that 335 charge cycles will equate a little more than 12% (100-12 = 88%).

Please check your math.

If you take 335 Charge Cycles and divide that by 50 (you told us that battery health will fall by 1% for every 50 charge cycles) the answer is 6.7%. Call it 7% to make things simple.

100% minus 7% equals 93%. So, the Battery Health should normally be in the 93% range at this time.

You said that the battery at 88% would be considered normal. Not really.

Yes it's normal.

Nothing to worry about.

I have 338 cycle and capacity 82% for my iPhone 15 Pro Max.

Your battery is performing very poorly.

338 Cycles divided by 50 equals 6.76%. Call it 7% to make things simple.

100% minus 7% = 93%, which is where the Battery Health would be to be considered "normal".

Your battery is not performing normally at all. It is declining almost twice as fast as "normal."