How do I improve battery health

There is no actual way to improve your battery health, the only way that can be done is by getting a new battery. Battery health, as you might be aware, degrades overtime irrespective of what you do with your device. There are someways to slow your battery's degradation. A Battery's health decrease as you keep charging it overtime. Say for instance your battery is at 5%, once you charge it to 100% that might count as 1 battery cycle. A battery usually has a certain number of charge cycles, your iPhone might have 800 (a wild guess, I am not aware of the charge cycle count on the iPhone 11). So every time you charge from a lower percentage to a higher one, you use up 1 cycle. As you keep increasing the cycle count you are decreasing the battery's capacity to hold charge. Hence 3000mAh battery would reduce to 2900mAh in 150 cycle counts (also a wild guess, not sure by how much the capacity reduce per cycle count).

So as you can see battery degradation is inevitable. To slow down the degradation you can try these:

1. Don't let your phone overheat (one of the fastest ways to degrade battery health). Take off your phone cover and stop using the phone if it does overheat.
2. Maintain a charge level between 30% to 80%: Make sure your charge level never drops below 30% and that you never charge it beyond 80%. This may reduce cycle counts from increasing to an extend. However, you will have to drain your battery to a very low percentage and charge it to 100% occasionally, say once a month.
3. When doing intensive tasks that drains battery, it is advantageous to have your phone connected to a power source so it might offset some of that load from the battery. If using it for longer you might eventually have to charge it beyond 100% and keep it on till you are done with your task. Make sure that the phone does not overheat as it usually produces a lot of heat when doing intensive tasks or charging, doing both together is sure to double the heat generation and in such a case it is better to avoid doing both. If you live in a cold environment, you might be able to avoid the overheating.

Hope this helps!

With lithium-ion batteries, they only go down. There's no way to chemically reverse the degradation other than to recycle the materials and turn them into new batteries, and to replace the battery.

There are occasionally cases where the battery health reading goes up and down, especially with certain type of battery health readings. You can see that with coconutBattery and where the number might go up with temperature. But that's only because battery health is only an estimate based on observations. I've actually seen battery health go up about 10%, but that was in a Mac that had an unplanned shutdown where it dropped maybe 12% and then after some use it crawled back up because the system observed there was more capacity. iPhone batteries don't seem to change their readings that rapidly going back up. I've only heard maybe of an iPhone battery health reading (from Settings) going up maybe 1%.

There might be some steps that can be taken to improve overall battery longevity, but they don't reverse the loss of battery capacity. The most important is to reduce heat generated during charging, and they often recommend charging without using a case that might trap in heat. Also - possibly turn on Battery Optimization, which will try to minimize the time that a battery is near 100%.