I assume you mean the High Voltage Battery. Don't worry about the HVB health. It should last the life of the car in the Hybrid C-Max. You don't have to do anything special as the HVB is operated normally between about 40% State of Charge and 54% SOC. The displayed battery symbol on the dash is not an accurate measurement of the actual SOC of the HVB. Filling the battery symbol to the tip (appears to be 100% full) is acutally only about a 70% SOC. I've attached a conversion chart showing the actual SOC of the HVB with the displayed battery symbol.
One only gets to the 70% SOC when going down long grades or braking from higher speeds. The traction motor in the transmission simulates engine braking when coasting down hill by acting as a generator and using the kinetic energy to make electricity that is then sent to the HVB for storage. Same thing happens when braking (non friction braking). This is generally referred to as "regeneration" or "regenerative braking". So, you should be able to get the symbol to approach the tip if slowing to a stop going down a longer hill.
As you can see from the chart, your 15% is actually about a 32% SOC and your 80% is actually about a 57% SOC -- pretty normal. More than likely, the 15% is after running in EV+ mode and the 80% is when cruising at higher speed for a period of time on fairly flat roads or after regenerative braking.
There are many Diagnostic Trouble Codes for the HVB and hybrid operations. Since, you have no DTCs, everything should be fine. Has your fuel economy changed? If your SOC seems to be declining more rapidly than before, perhaps you are making better use of the EV capability or you have more electrical load to supply like: running the A/C, seat heaters, and so forth.
Edited by Plus 3 Golfer, 04 July 2018 - 06:41 PM.