Hi there Johnnybht,
While it's difficult for us to determine exactly why your battery requires service at this time, it's normal for this message to appear if the battery is degraded to the point of needing replaced whether the maximum capacity percentage is at 80% or not.
A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions.
Keeping that in mind, it could be related to the charge cycles and not necessarily the capacity. Have a look at this link to learn more: iPhone Battery and Performance - Apple Support
To get more information about having your battery replaced, check out this link: iPhone Battery Repair & Replacement - Apple Support
Take care.