Hi @kdkf, thanks for your message.
First of all, for most batteries, even when they are brand new, you will never get 100% of the design capacity. The design capacity is based at the factory with no load.
The remaining full charge capacity, the measure you will read on Accubattery, can be affected by many factors like using fast charging against normal charging, etc… You have taken good care of your OPT to still have 85% of its design capacity 
As you know we are not selling new smartphones but pre-owned devices. Now they have to go through a long set of tests and can be repaired if some components fail to meet the expected quality.
All our refurbished smartphones come with batteries that have been checked to be fully operational and with a full charge capacity at least 80% of the design capacity. This is actually not always the case across the industry.
Don’t forget your device also comes with a one year warranty should there be any hardware failure during that time.
Any other questions, please let us know.
Alexis