I was having a particular perplexing problem last week. On Friday, the temperature was well above 100 (about 110 or so). Because I had Friday off, I was running errands all day. In the morning, everything was working just fine. By the afternoon, my regen braking was not working and my battery was not getting a charge. Whether I was coasting or braking softly or a bit harder, it would only go to the first blue block past zero. As the afternoon dragged on even more, my regen braking would not go below the zero line (to me, that seemed as if it was not regen at all). In addition, the battery was not getting charged - which was not a surprise if the regen brakes were not working. It would actually regen more on a slow coast than it did when I was braking.

Anyone ever encounter this or have any suggestions?

I haven't seen any similar effect, but I haven't driven in any 100+ weather either. It could be an issue with the battery temp (although the battery compartment vents into the cabin, so the temp should be cooler than 110, unless you had the AC off for some reason). Could you still hear the electric 'whine' from the brakes? What was the battery level like?

I've seen something like this. I even tried slamming on the brakes from high speed, and the regen would barely work. In my case , it was accompanied by the ICE turning on much easier than normal, and at lower speeds. I mean, that EV mode was not possible above 33mph, whereas usually I can use EV mode up to 42mph.

I'm in New England - the few times that I noticed this, it was not particularly hot, certainly nowhere near the temps you had.

-Gren

I've had a similar thing for a short period before. It was shortly after coming off the freeway with a long offramp that I got a lot of regen off of. I wonder if the regen brakes have a temp sensor that turns them off when it gets too warm?

I believe it's the battery that has the sensor that stops charging if the battery gets too hot. The problem with 110+ degree heat is that the sensor gets confused. There is a distinct MPG loss from about 97 degrees on up.

I think I could still hear the "whine", but the battery level was very low and did not increase at all.

Then my first suspicion would be the battery temp, and not the brakes themselves, although GrendelKhan's report casts doubt on that theory. I'll keep an eye out for this behavior, but I haven't seen it yet.

We just had a 5 day heat wave... temps of 105-108. It killed our MPG.

It's part of the safety feature for the HV battery. Be glad it keeps from shortening the battery life. Hot batteries get even hotter when being charged. I need to read the tech manu more and see where the energy goes when not to the battery. It there a resistance element somewhere to absorb the power from the gen during braking?

When will this terrible heat end? It's by far the worse summer I ever had here. I am really really tired of it.

Yes...battery regeneration is limited to protect the battery from overtemp.

The car's brake controller calculates how much braking will be done by regeneration and how much by the regular hydraulic brake system. If the battery's state of charge is high, or if the temperature is too high or too low, it will limit how much energy it can accept safely. If you brake softly/coast, the car will regen as much as possible back to the battery. If you brake harder, the friction brakes will do the majority of the work instead.

Of course other things effect it too--for example, the Vehicle Dynamics Control/ABS systems. If the car detects that a front wheel is about to lock up under braking, or if the car is about to slip, then it will redirect braking torque to another wheel. Imagine if you were driving on a slippery road and only the front brakes were working...this is what's happening when the generator is doing the braking. So, it has to account for regeneration's effect on stability control too.

For temperature relationships, remember that the AC is powered by the HV battery--so the hotter it is outside, the more the battery is getting used to power the AC, which leaves less available for driving the car, which means the engine must then come on sooner to assist under acceleration or to maintain the battery's state of charge. And since the battery is getting used more because of AC, its temperature increases, which also leads back to its ability to accept regeneration.

It's a delicate balance to maintain driveability, climate control comfort, fuel economy and reliability all at the same time!