Can Battery Performance Management be manually reapplied?

BSummers74 wrote: "It is the original battery. After an unexpected shutdown sometime in the past, I guess I manually disabled Battery Performance Management..."

The shutdown was probably due to the original battery having been weakened through age & use. You can hold on to that old iPhone 6 Plus, but do have its original battery replaced:

iPhone Battery & Power Repair

About genuine iPhone batteries

Repair Provider Verification

(Articles found by searching Apple's Knowledge Base.)

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BSummers ~ Does your iPhone 6 Plus still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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Can you elaborate? What do you mean by "manually reapplied?" There are a number of hardware and software optimizations in place that are designed to help keep your battery at peak performance. You can read more about it here:

iPhone Battery and Performance - Apple Support

It sounds like you're referring to optimized battery charging.

Optimized Battery Charging is on by default when you set up your iPhone or after updating to iOS 13 or later. To turn off the feature, go to Settings > Battery > Battery Health

Regarding having the phone last as long as possible, it might be worth considering having the battery swapped.

iPhone Battery Replacement - Official Apple Support