Highway speed brake regeneration question

The state of charge bar graph is very coarse, it's not going to jump a bar right away when you're braking. It takes a fair amount of charging for each uptick.

Are you using the Hybrid Synergy Display, the one with the horizontal bar graph showing your acceleration zone (not the one with the car pic)? It'll show when you're charging, and I'd speculated as long as the charge zone is not fully filled you're doing (more-or-less) just charging, little or no physical brakes.

 

It seems to make more sense to me that the harder you brake the more the friction brakes will engage and the less regenerative power you will be making.

 

I'd only use B for its design purpose: to slow charging, with the aim of avoiding a fully charged battery on a LONG downhill.

 

If you're trying to slow the car from 70 mph to 20 mph and using the friction brakes, you're not beginning your speed reduction soon enough. If the bar in the display is pegged to the left in the CHG section, you're probably using the friction brakes.

 

One very simple way to get better mpg is driving to avoid breaking, as much as is practical. Yeah I know, preaching...

And over in the recall thread they're discussing 0-60 times, lol

 

In general, you want to get as close to filling the CHG bar as you can, without filling it up. Once you go over, I think it tends to keep the friction brakes on (at least partially) until you stop. (Or let off the brakes, I suppose.) You can press the pedal harder as you slow down to maintain the level on the CHG bar; because kinetic energy is proportional to the square of your velocity, you can brake harder as your velocity drops and maintain the same amount of energy transfer to the battery (which is the bottleneck). In any case, the CHG bar will keep up with this, so just keep it full-ish.

As far as the meter's behavior, it's probably a combination of very coarse numbers (and not all bars are equal measurements; the 6th and 7th bar are close to twice as "wide" as the other bars when you look at the actual SoC), slow updates, and things like that.

 

I suspect that the maximum braking efficiency might be around 70% or so of the CHG bar, I've read an app note about BLDC motor regenerative braking which found that the max efficiency point was about 70% of the max braking force point (source: search Google for microchip regenerative braking, 1st link, slide 18).

 

Actually the optimum regenerative braking rate is 60 Amp or about 12.5 kW if you have a Scanguage or Android Torque app connected. This is with the battery temps in the 50 to 80 degree F range. On the HSI display in the regen region 60 Amp braking can correspond to 15% into the regen region at 70 mph, 50% into the regen region at 40 mph and 100% into the regen region by the time you get to below 10 mph. No regen occurs below about 6 mph, and it is 100% friction. So actually at 70 mph, just taking your foot off the accelerator without touching the brake gives you maybe about 30 Amp regen, because you are going so fast braking from 80 mph generates a lot more braking energy (Amps, kWs) than braking from 40 mph about 4 times as much braking energy. As Toyota want drivers to feel a similar level of regen drag at high speed as at low speed, thus a higher amount of regen happens at high speed. You will need a OBDII guage displaying regen current or power to really know how much to push the brake pedal for most efficient regen but in general vary the braking from 30% at 60 mph gradually down to 90% by the time you get 10 mph and you will be pretty near the sweet spot. Of course not using regen at all and gliding (zero regen) is most efficient of all but try do that with a large pickup with an angry looking grille on the front driving behind you with an even angrier driver as you exit the highway ramp.

 

Let's say you're going downhill a long way, there comes a point where the battery is all charged up. At that point regen will stop and the car will start speeding up unless you apply friction brakes. It's pretty obvious when it happens, and there is this initial panic that maybe there is something wrong with the vehicle.

 

Actually I don't live or drive near any long downhills, but I would have thought the Prius would have some logic to start using a less dramatic version of the engine compression braking used in the "B" gear, once no further regen can be captured. At least the Prius brakes will typically not fade as quickly as a conventional car as it's only after regen has been exhausted that the friction braking only starts to come into play. I'm not sure what elevation drop and grade (potential energy) that the regen system can absorb, although it may be a good idea to exhaust the battery before the top of the previous incline, one of the few situation s where lowering the traction battery charge can be good.

Turning on A/C, headlights and rear defroster will also help lengthen regen braking potential.

 

It absolutely does do that. It roughly mimics the same resistance that the usual regen "engine drag" would produce. Similarly, in B mode, it uses fairly heavy regen (in addition to some engine drag) if the battery is not full, and ramps up the engine drag once the battery fills. It also sometimes does this before the battery is totally full, if the battery is warm (e.g. at 78% SoC instead of 80% SoC); for me, this tends to happen when I'm about to fill the battery for the second time on the same trip.