Ev battery goes from 0 to 35% after long trip?

Might you have enabled Charge Mode?
Pressed and held EV/HV button?

 

+1

Sure if you were on a long decline over a mountain, at the very bottom of the hill, you can gain traction battery charge from regen alone. But on a flat road, it is very unlikely, unless you really try hard to manipulate the HV mode.

 

When did you check that your car was in EV mode? Did you check when you parked two days ago, or when you got back in the car yesterday and realized that you have 35% traction battery SOC left? If the latter, then the theory of CHG mode was used inadvertently while you were driving 4 hours on a relatively flat highway two days ago on Dec. 27 is still a likely possibility. CHG mode is canceled after you shut down the car. Equally possible would be that you had more than 35% SOC before the 4 hours drive on Dec. 27, and you have manually switched to HV mode and never depleted the EV range.

 

/\ /\ /\ /\ This

One of these two scenarios happened

Since you thought you put it in EV Auto, it was probably put in HV or CHG mode.
EV MODE is always default when starting the car with charge.

 

I think the occam razor principle applies here. In this case it's a result of an action of the OP and not a defect/feature that no one else has ever noticed in the Prime.

 

4 hours on "mostly flat" is 240 miles at freeway speeds. I do that kind of driving monthly. After the first 25 miles or so it should have been in HV mode for about 200 miles. That's plenty of time for putting some charge back in the traction battery.

Besides a downgrade, there are many other things that give the car a boost. Drafting a truck (at a safe distance) or picking up a tail wind will both allow both allow the car to generate more power than is needed to maintain a steady speed.

 

Are you saying for ~200 miles of relatively flat highway drive after you deplete EV range to --%, you are able to GAIN 35% of transaction battery charge? While it is possible to do that if you carefully manipulate HV/EV switches to put back charge in traction battery during regen and store it for later EV mode use, from the reading, OP was not actively engaged in such activity. It is very unlikely that normal driving on HV mode to gain 35% of charge without a long decline and active management of regen by switching HV/EV back and force.

 

From mid-August to mid-December I did that kind of mileage daily (work days 5x/week). Under no circumstances did I ever capture more than 7% (once, on a detour that sent me to a long, steep decline). Best I got otherwise has been 3-4%. Without engaging CHG mode, on flat terrain, I can’t see how a 35% charge would be physically possible.

Unless, paradoxically, in EV AUTO mode and travelling at 120 km/h or more on cruise control AND with some hilly terrain at the start of the trip. I’ve made such a journey in those conditions multiple times and the extra power demands on the hilly terrain caused the engine to fire up pretty immediately. Even so, I arrived home (160 km in this case) with 14% on the best day (usually 9-11% in those particular conditions). Same drive at 105-110 depleted the EV portion of the battery every time (but I used less gasoline). Still a far cry from 35% on a flat surface.

 

I did not say that I would get 35%. The only thing that I do special is to switch the car go HV mode any time I resume my journey after a rest stop. Then I set the cruise and cruise along. I've noted that the car recharges a little every time it has to brake for a slow moving car/truck.

I should have mentioned that that on those journeys the route seems level but there is a drop of more than 2000 feet on the last 250 mile leg of the trip. That's only 8 feet of drop per mile, but it can add up.