'B' Gear / Regen Level

It's mainly to prevent excessive heat / wear and tear on your brakes when going steeply downhill or downhill for a prolonged period of time. It does regen the battery but my understanding is that it is less efficient to use B-mode and is not recommended to use it if your only purpose is to regen the battery. The hybrid system already does a great job with maximizing battery efficiency in normal drive mode and it's not as efficient to force top it up. Unless maybe there are some specific hypermiler tricks?

 

The car is able to slow in three different ways.

One way uses the friction brakes, installed at all four wheels.

Both other ways use the two driving wheels, producing power in the transmission, and there are two things the car can do with that power. It can be sent to the battery for charging, or it can be used to twirl the engine at a higher RPM, just as a way to get rid of the energy.

If the battery reaches full charge, then all of the recovered energy after that has to be twirled off in the engine. Otherwise, it's not an either/or choice, and the car can do some balance of both.

There is not a single thing the car does in B mode that it doesn't also do in other modes, but B mode changes the balance. In particular, it changes two things;

• It recovers power a little more aggressively overall, when you lift your foot from the go pedal. That makes it feel the same way downshifting to a lower gear feels in other cars.
• As for what it does with the recovered power, it shifts the balance a little more toward twirling off in the engine, a little less toward charging the battery. The engine twirling makes it also sound the way downshifting sounds in other cars, and the smaller portion used for charging treats the battery a little more gently.

For most brief slowing and stopping, or short downhills, etc., it is more efficient to be in D, because more of the recovered energy will be saved in the battery. Twirling it off in the engine just wastes it.

On the other hand, on a long downhill, you're probably going to reach full charge for the battery part-way down anyway, and all the energy will have to be twirled off in the engine the rest of the way down. Since you can't avoid wasting that extra energy anyway, you can select B at the top of the downhill, and start wasting it a little earlier, while charging the battery more slowly and easily.

On a long enough downhill, you still end up with a full charge in the battery by the end, but with less stress on the battery.

 

There's a "CHG" area on the speedo which it will drop down to if you're coasting and if you brake lightly. That's how you want to maximize regen--long slow stops where you keep the gauge in that space. Once you go past it, you're braking with actual physical brakes.

 

Yeah, I wish it had more room though, anything more than an extremely gentle braking sends it over the line. I've trained myself to start braking way early when approaching a stop sign or stop light because it takes quite a while to come to a full stop using only the regen braking, especially going even slightly downhill it seems nearly impossible. I can see why people would be tempted to use B-mode because in B-mode you don't have to pay attention to that gauge. But I guess it's still better just to have a light foot in regular drive mode

 

B mode reduces regeneration and diverts energy mechanically to the gas engine (forces it to spin without burning fuel) so that you can descend long hills without burning up your regular service brakes.

Regen is reduced because it might be a big hill- the idea is to allow a longer duration of regen before the battery is filled.

Energy is diverted into the engine because that's the next best way to dissipate it, saving your actual brakes to be the last chance.

Yep, that's about right.

If you think about it, pretty much any other method of charging a battery involves leaving it on the charger for a significant amount of time. One of the big engineering challenges facing a hybrid is how to recapture energy that can be generated very quickly (stomp the pedal) but can only be stored at a fairly slow rate.

Getting the driver involved to help manage that timing goes a long way towards efficiency.

 

TBH, I use B mode only while going down some long, steep, hill as I do not have to push break pedal all that hard or all that often.
Does it regen more in B mode compared to simply using break pedal? Probably not. Is it more comfortable down the hill? You bet.

I was able to put 41%->66% back in battery while going down really long hill (PHV 2021). It was in B mode, I tried it in D mode as well, but my foot started to ache in the end (engine never kicked off) .

 

Should the Prime generally not fair much better as:

1. It has a bigger emotor, thus should be able to brake (=create power) harder?
   
2. It has a bigger battery, thus should be able to store energy at a higher rate?

 

Not really sure if you got it as EV battery is the traction battery

 

There is just one battery. The displays may change when switching between EV and hybrid mode, but it is physically the same battery.

B mode is there to give the driver a downshift option for controlling speed. If you didn't feel the need to downshift on your trips in another car, you'll be fine not using B. In the Prime, it does increase the regen braking level. However, when the charge nears full, it will spin up the engine. While the braking function doesn't use gas, spinning the engine means it came on to the system. So it will keep the engine on to go through the warm up sequence, using fuel, even if you shift out of B.

 

I'm not sure how much #1 comes into play.

Spot on with #2. Bigger battery that supports higher charging rates = easier energy recovery, more able to make use of regenerative braking.

 

Pretty sure, the Prime motor is the same power rating as the hybrid; the gen4 was. The difference in the Prime from the hybrid is a clutch that allows M/G1 to contribute traction power when needed. I don't think that clutch was needed for it to be used for regen. It just isn't, because the battery is the limiting factor to regen rate.

For #2, I think it is the number of cells in the battery. Each one can only take in so much energy at a time. Go over it, and the charging will cause degradation; mostly from heat. There is a whole more cells in the PEHV battery.

I think they copy and paste much from the hybrid manual for the PHEV one. The software in the Prime is built upon the hybrid's. That EV light is likely a hold over from the hybrid. It is probably disabled while the Prime is in EV mode.

 

For what it's worth, I had a shop foreman tell me not to mess with the "B" gear unless I had to. The implication was that it causes more wear and tear on components compared to "D".

 

As silly as it seems, the EV light in active in EV mode. It's almost always on, but once I switched from HV to EV mode on the highway and it took about a minute for the motor to stop and the EV light to go on.

 

I guess that's technically true, but I think it is more useful to say that safely descending long hills puts more wear and tear on components than coasting on flat ground.

 

Yeah, I think his point was: to only use it when it's designed to be used - during extended downhill runs where your brakes are gonna get hot.

 

There's a selection in the settings on both the HEV and PHEV that allow you to turn the EV light off.