Sudden battery health drop from 99% to 98% at 135 cycles on iPhone 16 Pro Max

You are right on track. On average the Battery Health drops 1% for every 50 charge cycles. The drop is not linear meaning you will not see the 1% drop at 51 or 101 charge cycle and some months may not drop at all, while other months may drop more. It is simply an average and with 135 charge cycles, it is expected you should be showing 98% Battery Health.

All you need to do is enjoy your phone. The only time the Battery Health matters is when it drops below 80%, which means it needs to be replaced. It is expected to last 1000 charge cycles.

Very normal. At 150 Cycles, your Battery Health should be at about 97%. That is because you can expect.....on average over time......about a 1% decline for every 50 Charge Cycles.

Your battery is doing just fine.

All that you really need to remember is to replace the battery when the Battery Health dips below 80%.

At 144 cycles my iPhone SE 2020, which has a much smaller battery than your 16 PM, was at 97% health. My initial capacity was ~1860 mAh; yours would have been nearly 4700 mAh,†

Remember "health" is an inexact and non-linear value not worth losing sleep.

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† — data source: MacTracker database in the Mac App Store. https://apps.apple.com/us/app/mactracker/id430255202?mt=12