I have a 2013 Lexus RX450h and there is a dial on the dashboard shown below at left that goes negative when you are braking:



When at a high speed, very little brake pressure causes the dial to max out at the bottom of the charging section. As you slow down, you may apply more brake pressure without maxing it out. Does this dial reflect the efficiency of the regenerative braking? To optimize regeneration efficiency, should I strive to not let the needle bottom out? What style of braking will maximize regenerative efficiency and do the dials on the car assist with trying to maximize efficiency?

 

The charge area of the gauge reflects how much is being captured, not necessarily how efficient your braking is. I know a lot of hybrids now, usually downmarket ones like the Prius or Honda Insight or Ford, grade your braking. This is not the case with Lexus hybrids. I think they wanted to keep it as much like a normal car as possible.

Anyways, it is okay for the needle to bottom out. As you are going at high speed and you release the accelerator, the car is naturally being slowed down by drag and so it is capturing more energy. The way to get the most energy is to break gently. You will notice if you are braking suddenly, it will not capture much and will even use the disc brakes to slow down instead of regenerative braking.

If you want, you can look at the navigation screen if your car is equipped


If you are slowing to a stop from highway speeds and brake gently, you will see more of those little blue "E"s than if you braking suddenly. I guess you can use this as a gauge

 

I've always gone on the assumption that the bottom of the gauge represents the maximum regeneration, so increased braking beyond that point decreases efficiency of potential energy recovery. No idea if this is actually true, but why would they scale that gauge that way it is if it isn't.

 

RXSF,

Please explain your graph. 30 minutes of 60 MPG and 125 Wh regeneration. How were you driving and if representative of the whole tank of gas, what would have been your MPG on that tank? Thanks,

 

^^ haha, that is not my graph. I found it on google images to make my point.

 

Well ya got me....I was thinking you managed to drive only downhill in SF. Should have been more Wh's made.

 

Quick question, as I haven't looked it up in the owner's manual - what is the "E" character in the chart meant to represent, and why are some of the characters truncated? I noticed this on mine too, but had thought it was a display glitch.

 

The "E" character represents regenerated or captured energy with a full block = 50 watt hours
A truncated block is some smaller percentage of the 50 value.

(That graph is unbelievable, I am on my third Hybrid and drive 20 miles none of which
is faster than 45 mph and I can not hyper-mile that far with that high an average!)

The Power Meter is supposed to guide your control inputs, don't give it too much of
either pedal to maximize MPG.

 

1. Keep eyes on the road and traffic, not on darn meter, please. Don't get obsessive.
2. E means you drove in full Electric mode.
3. Don't want to waste time looking it up, I had full display of complete bars posted once at toyotanation. Traffic. Hybrids shine in traffic. My post was "How many mpg to a full bar" or something like that.
4. Those graphs are child toy. They do not really represent what happens inside the power train and, as my son said, "That was best they could come up with"? Older hybrids had left hand instrument cluster gauge, akin to tachometer, that will actually show MPG. So it was clearly understood that you get best MPG when in E mode(lowermost gauge corner) and then it slowly gets worse as gauge climbed up. Then they decided to change it to Kw and that made it basically useless, as it means squat.
5. optimizing regenerative braking efficiency: coast as much as you can, plan your braking, use B mode as much as you can, plan braking with B mode, not with brakes. In B mode, charging amperage is 3 times higher than in regenerative braking. 30 vs 10amp.
6. final word: don't turn your driving into misery chasing tenths of mile to a gallon. Been there for 8 years. Done. Enjoy your car, after all, it is a device to move you forward, not to creep down the road becoming a road block and aggravation to other drivers.

 

I was under the belief that B meant engine breaking and thus should be avoided as you were wasting energy. I thought B was for big downhill segments where breaks might overheat.

Can someone cite a source to definitively say what it is?

 

ukrkoz is on track, "B" for brake increases the regenerative action of the MG unit(s) and creates more
electricity and greater deceleration. Roughly analogous to "engine braking" by downshifting a conventional
transmission to save the friction brakes from wear (and energy waste!!!). Just as you must return a regular
transmission to "D" or shift a manual trans to a higher gear, you must shift back into "D" with a hybrid when
you are done playing slowing.

 

That is the official stated purpose, to engage engine braking on downhill to limit mechanical brake overheating, especially when normal regen braking stops after battery reaches maximum allowed charge. There is no other official recommended purpose, but unofficially if you Google the world I think you'll find that 85% of comments and articles think using B-mode all the time is not a good idea, and is generally less efficient because, as it will in some cases increase "instantaneous" regen braking, it also causes loses through simultaneous engine braking. Zipping up behind someone at a red light just to use B-mode to rapidly decelerate with a short burst of higher regen is going to be more wasteful than simply coasting or lightly regen braking over a longer period when no mechanical brakes or engine braking is required. If its not in your nature to coast and / or your driving conditions often require brisk (ie mechanical) braking frequently then maybe there's room for B-mode in there as an individual solution only.

Personally, I've only heard two scenarios where B-mode sounds like a good idea outside of downhill. On the plug-in Prius, as long as the car is in full EV mode, using B-mode does translate to maximum regen because the engine remains "offline". The other scenario is braking on bumpy roads. Bumpy roads will often cause normal regen braking to go offline because of the variation in traction (those with good hearing can hear when this happens, the faint regen whine stops mid braking), the car will fall back to mechanical braking only. Using B-mode in this case will ensure some regen still occurs (apparently).

 

My experience is that B-mode does very little regen braking and instead spins up the ICE (without fuel, so it's basically just a vacuum pump) as a quicker way to bleed off kinetic energy. I can demonstrate this by coming down a steep hill in my neighborhood--in "1st" gear the energy dial on the left of the instrument cluster shows hardly any regen activity at all, while in normal mode with my foot on the brake I can easily peg the dial at the bottom of the range.