This was the 4th and hottest summer I have experienced with the C-Max. There was no problem with the HVB capacity during the first two years of operation. Both of those summers were cooler than normal for NC. The car was charged outside in my shaded driveway for the first two years.
Also of note, two large oak trees on my neighbors property were removed by the beginning of the third summer so the car spent afternoons in the sun.
The third summer was quite warm and as fall approached I noticed that the HVB capacity had dropped over the summer from about 5.4kWh to about 4.5kWh. Several times during that third summer the HVB hit the maximum 113ºf and the cars programming switched to force Hybrid operation until HVB until the battery cooled below the threshold.
My plan in this 4th summer was to prevent any overheating of the HVB on the theory that it was the high temperatures that caused the capacity drop in the HVB. The goal was to not allow the HVB to exceed 102ºf. This summer also turned out to be the hottest of the 4 summers making the challenge of not overheating the HVB more difficult.
1, The HVB temperature was monitored with an xgauge programmed into a ScanGauge II.
2, 90% of charging was done with a level 2 charger and charging was always allowed to continue until the HVB was full. From empty it takes about 1 hour and 40 minutes to charge the HVB due to its diminished 4.5 kWh capacity
3, My driving pattern changed this summer. For the previously three years 90% of the miles driven were EV with the remaining hybrid miles driven on the highway. This summer 60% or more of the miles driven were on the interstate highways. All highway driving was done in EV Later.
4, Previous years miles driven per month were between 700 to 800. This year miles driven per month were between 1000 and 1200.
5. The AC was set to 73ºf and was used on every drive over the summer.
6. The car sat outside all the time as I have no garage.
7, Total miles driven in 3 1/2 years is 44k miles.
The following guidelines were used at various times during the summer to limit overheating the HVB.
* Limit the number charge cycles per day. (Usually no more than once).
* Perform most charging late at night. (Typically used Value Charging starting at 10PM to allow the HVB to cool before the charge and allow as much time as possible for cooling before the next days driving.)
* Never charge the car when the HVB was greater than 96ºf.
* Stop using the ‘L ‘ driving mode
* Try avoiding any hard braking.
* Avoid hard acceleration.
* Drive in EV Later on the highways.(Anytime speed exceed 45MPH)
* Switch to EV Later anytime the HVB temperature got to 98ºf.
* Use AC on every drive. Cooling the cabin helps get the HVB vent fan running sooner.
* Resist temptation to squeeze every EV mile possible even if it meant getting home with some HVB power remaining.
I was mostly successful in controlling the HVB temperature. The HVB reached 105ºf on several occasions and at least once went to 107ºf. In almost all cases where the goal of 102ºf was exceeded I attempted to charge the HVB more often than I should have.
The typical overnight lows during hot periods(This includes the entire month of August) were no less than 80ºf. Almost never during the summer did the overnight lows get below 70ºf. The hottest daytime highs reached 100ºf. Only rarely did the daytimes highs remain below 85ºf. There was one entire week during August that no charging was done due to persistent high overnight temperatures.
The HVB takes quite a long time to cool down. If the overnight lows approached 80ºf the HVB would start the next day around 91ºf. This usually means much of the days driving would have to be done in EV Later.
In late July and early August I took a trip from NC to NH driving a total of about 2k miles. Even though it was quite warm there, nights were cool enough for the HVB to drop into the 70s. It was quite easy to keep the HVB cool on this trip due to cooler nights, longer intervals between trips and charging only with a level 1 charger.
I am happy to report that at summers end there was no additional lost capacity. I am still getting between 4.2 to 4.5 kWh per charge. I would prefer not to have to take extreme measures, but it does seem to validate my theory that actively limiting the HVB temperature does help preserve the HVB capacity.