What Happens When the Traction Battery Fills Up?

if you don't need the brakes, b mode won't do anything. your car uses engine friction to burn off the excess charge the battery cannoth accept. b mode does the same thing, and is useful on a hill large enough to require friction braking.

 

The Prius does seem pretty impervious to full battery. Even when it's filled all bars ours (3rd gen) seems to continue charging. At least it feels like it is, no noticeable change in brake feel. I do use B, coming down a local ski hill.

Our previous Honda Civic hybrid was much more prone to "peak charge", and it was VERY noticeable: halfway done a moderate hill, charging bars would drop off, and breaking effort increased precipitously. It was almost dangerous.

 

My gen 2. Going down long mountain there would be a noticeably change the sound as the battery's became full. To me it sounded like engine braking. It didn't slow the car any better. I haven't gone down that hill with the new gen 4, so it will be interesting to see if it's the same. There are a lot of differences from the gen 2 to the gen 4. As far as I can tell so far the Gen 4 changes are nice improvements to what a very good proven platform.

 

On a Gen2 anyways when the batt is full that's when the regen braking stops generating power and now you have to use your real brakes to slow down. B mode can help reduce brake shoe wear in that case.

For us, Uniontown hill on Rt 40 in PA is about the only we hear the subtle change by the time we get to the bottom of the hill.

Now if it was me, I'd give Prius photon torpedoes and charge em up when the batt is full. Next Volt you see...let em have it. Bottom line you need a place to put the electrons, or else they need to stop making electrons with the motor.

 

Ahh, photon torpedos.

 

the best kind.

 

I've noticed in my Gen 3 that after the battery gets full, that although the display still indicates charging when braking, the car is using engine braking (combined with the disc brakes when needed). Like Gen 2 Tom, I hear the sound of the engine braking when it switches over on a long downhill. I don't use B mode even on long downhills because the car is smart enough to automatically switch to engine braking to avoid overheating the disc brakes after battery regen is no longer possible.

 

I mostly agree with Lightning Racer. However, I do use "B" on those big descents, to reduce stress on the battery.

 

OK!
It is useful to think of electricity like pumping water. Instead of pipes you have wires for electrons. So if you are pumping water into a tank, you gotta stop pumping the water when the tank is full or else the tank will split open. So Prius shuts off the electric motor once the battery gets full on a steep, long downhill. The electric motor serves as brakes, too so when the elec motor goes off, you gotta use the real brakes (how low tech can you get?) or B mode, or a combo.

 

When the traction battery gets full, electrons spill out and flood the rear seat area floor. There's a special drain to let them all out the bottom.

 

Here are two scenarios that I have experience of:

One:

You have the cruise set and the grade would cause the car to slowly increase speed. What I've seen happen is that the ICE "starts" periodically to slow the car even while regen is occurring. Once the batter is full regen stops and the ICE is solely used for slowing the car.​

Two:

You have a really steep grade that will cause the car more quickly increase speed. Leave it in D until regen has filled the battery then switch to B for increased ICE slowing. I'm not sure if you get much of any regen in B.​

I would be very surprised if Toyota didn't account for a full charge scenario, I don't think the car will let you over charge the battery. The full bar graph may not even represent 100%.

 

I'll tell you what happened in our Gen 2 (2005) when the battery went full green: 1) all regen braking ceased, 2) if you pressed the brake pedal in this condition, all you would get was friction braking, 3) if you happened to have your foot on the brake pedal when it went full green, you would feel the change in mode from regen to friction, 4) the battery would "spin up" the engine to use up excess battery energy, it literally rotated the engine to a noticeable RPM, 5) those of you who commented that you thought you were hearing engine braking were probably only hearing this engine spinning under battery power, 6) you only get "engine braking" if you select B mode on the gearshift.

We have not had our Gen 3 cars reach full charge, so can't say what the Gen 3 or newer do, but I suspect it's very similar, if not identical, to what the Gen 2 cars did...

 

For 3rd generation, and I suspect the 2nd and 4th as well, those points are:
1) Correct
2) Possibly true sometimes (see #6 below)
3) Probably true
4) Confused; power to spin the engine in that circumstance comes essentially 100% from the wheels, while the battery is just holding at its maximum allowed charge
5) Confused; "hearing engine braking" and hearing the engine spinning (under wheel power) amount to the same thing
6) Uncertain; some PriusChat members report observing automatic engine braking even without using "B" mode, but I haven't, so far.

 

To your point 4), in our Gen 2, under fully charged condition and come to a stop, wheels stopped with foot on the brake, the engine would spin at significant RPM under battery power. The motor generator (MG1, MG2 whichever is tied to the engine) is energized to spin the engine to burn off excess energy stored in the traction battery. I'll swear by it. To your point 5), I believe that unless you are in B mode, you are not getting "engine braking". Instead, if you are hearing the engine "rev-ing" then what you are really hearing is the excess energy being used to spin the ICE as described in 4). This was occurring at the "end of life" of our Gen 2 traction battery when it would swing from full empty to full charged very rapidly, within a mile or two. Notice we are in Colorado, hence lots of hills. Battery charges to full within a mile of a downhill grade during our daily driving, so I had a lot of experience observing this behavior. So, unless you actually experience this, it is completely foreign behavior for an automobile, but I assure you this is what our Gen 2 did.

 

OK, I believe you about your Gen 2 using a motor (MG1, the one tied to the sun gear) to spin the engine while the car is stationary. That's an interesting behavior I've never experienced. Whether that's because the 3rd generation doesn't do it, or because I try to avoid stopping with a "full" battery, I don't know. However, I was referring to the more usual situation of descending a mountain after the battery is too "full" to accept additional charge Then the power to spin the engine must come entirely from the wheels, assuming the battery state-of-charge is constant. Some owners have reported automated "engine braking" even when not in "B" mode, which would be logical behavior by the car in certain circumstances. I haven't observed that phenomenon, because I use "B" when it's needed.

 

On top of Bisco's first reply, I must add two comments, based upon my Gen3 experience that might not fully apply to your gen4:

(1) The battery SOC gauge shows 8 bars, i.e. 'full', a little before the battery is actually 'full' (to the upper limit allowed by the computers, which is not the same as the battery maker's data sheet spec limit). Think of this as the SOC gauge rounding a 7.5 reading up to 8 bars, as it cannot show fractional bars. But the difference is visible on other gauges;

(2) The green arrow display is not useful on my Gen3, as it doesn't distinguish between large and minuscule energy flows. The HSI display is much for useful, as its bar length does distinguish between large and small. While the Gen2's arrow display did make the arrow disappear on small-enough amounts that are close to zero, the Gen3 does not.

 

I agree with part of Lightning Racer's reply, not included here.

But I must very strongly disagree with the portion quoted above. The automatic switchover is only to a low level of engine braking weaker than in traditional cars in highest gear. It is far short of the amount of engine braking provided by 'B' mode, and is not sufficient to protect the disc brakes on extended steep downhills. Another newbie discovered this the hard way, after overheating not just one, but two sets of brakes:

3,500 foot decent = crazy HOT brakes and Burning Smells | PriusChat

One of my replies there describes the difference in RPM and speed control between full battery 'D' and 'B' mode engine braking on one particular hill I occasionally travel. The difference is quite large.