My iPhone is requiring more frequent charging.

linus ~ If your iPhone is an iPhone 6 or later, what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

If your iPhone has its original battery then perhaps it needs replacing. See the Battery Health section in this first article:

iPhone Battery and Performance

iPhone Battery & Power Repair

You can find Apple Support Articles such as the above by searching Apple's Knowledge Base.

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linus15 wrote: "Am I correct that my iPhone doesn't have a replaceable battery?"

The battery is replaceable only by Apple or an Authorized Service Provider:

iPhone Battery & Power Repair

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