How can I improve iPhone 14 Pro Max's battery health?

Your iPhone battery is doing better than normal! Your iPhone’s battery health is expected to drop about 1% a month or 25 charge cycles, and the decline isn’t always linear. Meaning some months you will lose more than expected, and other months none at all.

So after 20 months, you should be at about 80% battery health, so you are ahead of your battery!

Remember, your iPhone’s battery is a rechargeable lithium-ion battery, and they chemically age as time goes on. When your battery health reaches 80% or below, you should have the battery replaced.

⇢ iPhone Battery Replacement - Apple Support

The battery was designed to go about 2 years before it needs replacement.

Replace the battery when the Maximum Capacity drops under 80%