Regenerative Braking Randomly Shuts Off

[rgibson]

My ’06 RX400h started shutting off the regenerative braking after minor bumps at about 3,000 miles, resulting in a reduction in braking power as the car switched to 100% friction brakes (and having to then press harder on the brake pedal to stop in the same distance). This was easy to adjust to since I can see the dips and seams in the road… Now at 21,000 miles (8 months later), after the transmission update for the 40mph surging issue, the regen is randomly shutting off when braking. Is this normal for the later built 2006’s with the same transmission code?

[C6 Chris]

My ’06 RX400h started shutting off the regenerative braking after minor bumps at about 3,000 miles, resulting in a reduction in braking power as the car switched to 100% friction brakes (and having to then press harder on the brake pedal to stop in the same distance). This was easy to adjust to since I can see the dips and seams in the road… Now at 21,000 miles (8 months later), after the transmission update for the 40mph surging issue, the regen is randomly shutting off when braking. Is this normal for the later built 2006’s with the same transmission code?

Sorry, but I really can't shed any light on your question - I'm too new to my RX400h.

But I am curious about better understanding the issue of regenerative braking and "friction" braking; are these two different braking systems in our car? Also, how do you know when one system or the other is on?

[rgibson]

Sorry, but I really can't shed any light on your question - I'm too new to my RX400h.

But I am curious about better understanding the issue of regenerative braking and "friction" braking; are these two different braking systems in our car? Also, how do you know when one system or the other is on?

The regenerative braking is the electric motors acting as generators which will recharge the battery while slowing the vehicle. You will both hear a whine from the motors and see the green arrows on the energy screen (with the nav system) pointing to the battery from both the front and rear motors along with the energy "tach" residing in the blue area.

The friction brakes are the usual pad/rotor brakes that the ABS will control like on any modern vehicle.

If you are light on the brakes, you can nearly stop the vehicle on just the electric motors. If you press hard, you will get both the electric motors and traditional brakes working together. Press really hard, below 35 mph, and you can get the tires locked up and the car stopped before the ABS kicks in. :whistles: (The passenger was asking for it!)

The problem I'm having with my vehicle is that the electric motors will stop braking (for no clear reason - excluding a dip/bump) leaving just the pads/rotors to stop the vehicle. The main problem with this is the stopping power is reduced and the stopping distance is increased unless I compensate by pressing harding on the brake petal. I'm curious if others are having the same issue with the updated transmission code.

[C6 Chris]

Sorry, but I really can't shed any light on your question - I'm too new to my RX400h.

But I am curious about better understanding the issue of regenerative braking and "friction" braking; are these two different braking systems in our car? Also, how do you know when one system or the other is on?

The regenerative braking is the electric motors acting as generators which will recharge the battery while slowing the vehicle. You will both hear a whine from the motors and see the green arrows on the energy screen (with the nav system) pointing to the battery from both the front and rear motors along with the energy "tach" residing in the blue area.

The friction brakes are the usual pad/rotor brakes that the ABS will control like on any modern vehicle.

If you are light on the brakes, you can nearly stop the vehicle on just the electric motors. If you press hard, you will get both the electric motors and traditional brakes working together. Press really hard, below 35 mph, and you can get the tires locked up and the car stopped before the ABS kicks in. :whistles: (The passenger was asking for it!)

The problem I'm having with my vehicle is that the electric motors will stop braking (for no clear reason - excluding a dip/bump) leaving just the pads/rotors to stop the vehicle. The main problem with this is the stopping power is reduced and the stopping distance is increased unless I compensate by pressing harding on the brake petal. I'm curious if others are having the same issue with the updated transmission code.

Thanks for the explanation, but the electric motor will act as a generator irregardless of whether the car is braking or not - I think this is fundamental to all electric motors. I have heard the term Regenerative Braking, but thought it was more of a generic term to describe this basic electrical characteristic of electric motors and the reduction in speed associated with the resistance provided due to having to turn the electric motors.

Anyways, perhaps there is more to it; does the car actually use the magnets in the motor to apply braking power to the wheels? If so, then 1) I don't know why they would call this "regenerative braking" since it probably reduces battery power and 2) now I can see the delicate balance the computer must use to balance between braking provided by the motors and braking provided by the pads.

BTW, how are the brake pads powered in this car? I suspect there is no hydraulic fluid or maybe there is and pressure is maintain using an electrical pump?

[retired00]

I have heard the term Regenerative Braking, but thought it was more of a generic term to describe this basic electrical characteristic of electric motors and the reduction in speed associated with the resistance provided due to having to turn the electric motors.

I agree with you on this, rather than what is often characterized here as first we use regenerative braking, then the discs are engaged when needed. The brake pedal or simply lifting off the gas signals the planetary gear to engage the motor as a generator, providing resistance as it charges the battery, which helps slow the vehicle, but the brake pedal also engages the disc brakes using plain old hydraulic pressure from your reservoir. The more pedal pressure, the more braking is applied. The meters on the dash do display regenerative charge, and if none is occurring then either the meter or the engagement of the planetary gear is failing, and you'll likely feel less coast-down drag. The Lexus site has a good animated graphic of how the entire system functions.

[wwest]

The electric motors have PM, Permanent Magnet, rotors, so absent multi-phase AC driving power application they WILL generate electricity if turning.

Absent being in the "B" mode the electric motors are used in regenerative mode to simulate engine compression braking during "coastdowns" to charge the hybrid batteries. When the brakes are applied a stronger level of regenerative braking is used depending on how hard you press the brake pedal up to a point wherein the friction braking system MUST be used to provide the level of braking you require.

Ford has just been granted a patent concerning this issue that may be of interest here.

The first patented technique involves significantly reducing the level of regenerative braking to be used if the OAT is near or below freezing wherein the potential for loss of control from "braking" the front wheels on a slippery roadbed increases dramatically.

The second technique involves disabling the regenerative braking system altogether the instant ABS activates while braking.

Obviously, assuming Toyota has cross-licensed the patent, hitting a "minor bump" while braking might result in a brief, "one-shot", ABS activation and therefore instant disabling of the regenerative braking system.

[skyfish400h]

The electric motors have PM, Permanent Magnet, rotors, so absent multi-phase AC driving power application they WILL generate electricity if turning.

Absent being in the "B" mode the electric motors are used in regenerative mode to simulate engine compression braking during "coastdowns" to charge the hybrid batteries. When the brakes are applied a stronger level of regenerative braking is used depending on how hard you press the brake pedal up to a point wherein the friction braking system MUST be used to provide the level of braking you require.

Ford has just been granted a patent concerning this issue that may be of interest here.

The first patented technique involves significantly reducing the level of regenerative braking to be used if the OAT is near or below freezing wherein the potential for loss of control from "braking" the front wheels on a slippery roadbed increases dramatically.

The second technique involves disabling the regenerative braking system altogether the instant ABS activates while braking.

Obviously, assuming Toyota has cross-licensed the patent, hitting a "minor bump" while braking might result in a brief, "one-shot", ABS activation and therefore instant disabling of the regenerative braking system.

I think this describes the situation accurately and should address many of the questions being posed here lately on the subject.

The only thing I would add or take issue with is the notion that the "B" mode in anyway changes how any of this works. Everything functions just as described whether you are in "B" or "D". The only diference is the ADDITION of compression braking to the mix. This has the effect of displacing a certiain amount of the regenerative braking for a given deceleration and therefore the system will not capture the engergy waseted as heat in the engine.

As for the transistion between regenerative and friction braking... it can be unsetteling at times. I believe Lexus could have done a better job at integrating these two systems so that it was less apparent to the driver. I've experienced the "letting go" feeling during hard stops on less than perfect road condtions. ANY activation of the ABS (and perhaps even the Active Stabilty Control) will kick out the regenerative and leave only the friction. With that is required addional pedal pressure from the driver to compensate for the change. It is more difficult to come to completely smooth stop with this car under normal conditions, throw in a bit of skidding or performance type stopping and it gets worse.

[wwest]

The electric motors have PM, Permanent Magnet, rotors, so absent multi-phase AC driving power application they WILL generate electricity if turning.

Absent being in the "B" mode the electric motors are used in regenerative mode to simulate engine compression braking during "coastdowns" to charge the hybrid batteries. When the brakes are applied a stronger level of regenerative braking is used depending on how hard you press the brake pedal up to a point wherein the friction braking system MUST be used to provide the level of braking you require.

Ford has just been granted a patent concerning this issue that may be of interest here.

The first patented technique involves significantly reducing the level of regenerative braking to be used if the OAT is near or below freezing wherein the potential for loss of control from "braking" the front wheels on a slippery roadbed increases dramatically.

The second technique involves disabling the regenerative braking system altogether the instant ABS activates while braking.

Obviously, assuming Toyota has cross-licensed the patent, hitting a "minor bump" while braking might result in a brief, "one-shot", ABS activation and therefore instant disabling of the regenerative braking system.

I think this describes the situation accurately and should address many of the questions being posed here lately on the subject.

The only thing I would add or take issue with is the notion that the "B" mode in anyway changes how any of this works. Everything functions just as described whether you are in "B" or "D". The only diference is the ADDITION of compression braking to the mix. This has the effect of displacing a certiain amount of the regenerative braking for a given deceleration and therefore the system will not capture the engergy waseted as heat in the engine.

As for the transistion between regenerative and friction braking... it can be unsetteling at times. I believe Lexus could have done a better job at integrating these two systems so that it was less apparent to the driver. I've experienced the "letting go" feeling during hard stops on less than perfect road condtions. ANY activation of the ABS (and perhaps even the Active Stabilty Control) will kick out the regenerative and leave only the friction. With that is required addional pedal pressure from the driver to compensate for the change. It is more difficult to come to completely smooth stop with this car under normal conditions, throw in a bit of skidding or performance type stopping and it gets worse.

"...Lexus could have done a better job..."

Absent being able to accurately predict just when ABS will be needed, will activate, it is a practical impossibility to provide a "smooth" transition from combined frictional and regenerative braking to full/only frictional braking. Other than disabling regenerative braking virtaully completely as Ford does below about 35F anyway.

[skyfish400h]

"...Lexus could have done a better job..."

Absent being able to accurately predict just when ABS will be needed, will activate, it is a practical impossibility to provide a "smooth" transition from combined frictional and regenerative braking to full/only frictional braking. Other than disabling regenerative braking virtaully completely as Ford does below about 35F anyway.

I believe the weakness is in the system does not actively manage the hydraulics the way it does the ABS, Stability Control, and Regen braking. If it could apply the friction brakes as well then is could fair it all in smoothly and you would never be able to tell when one came in and another left off (except for the tell tail stutter of the ABS)

This would be true brake by wire technology which the 400h does not have, yet. In this world the driver "requests" a certain amount of breaking effectiveness and the system "provides" it form a mix of sources. This system could also enable a laser cruise control system that would work all the way down to STOP instead of dropping out at 20 mph.

[rgibson]

Reading through the replies, I have a fairly good idea of what's happening with my car and how to explain what's happening to the service rep.

Prior to the code update to my transmission (http://www.lexushelp.com/Ownership/Bulletins/EG/EG020-06.htm), the regen was active all the way to a dead stop. After the update, the transmission is stopping the regen after the travel speed is under 1 mph. I suspect that a sensor must be failing since the regen is stopping anywhere from 5-80 mph under ideal hybrid driving and stopping conditions. That is dry pavement, temps between 40-80, and gentle stopping. Battery charge state doesn't appear to be a factor either since it's been both in the green and mid blue when this occurs.

I'll be harrassing the service guys tomorrow and will reply back if they find something wrong.

[C6 Chris]

I'm still a little confused with all this. Let me get this straight. The battery is being charged all the time when coasting and is evident by see the display indicating power going from the wheels to the battery.

Regenerative Braking assists the stopping power by applying power to the electric motors to slow the car down Right? So how do you know when this is occuring? Can you tell by reading the displays or is it happening all the time when braking? If there is no visible indication, then how do you know when it is not working?

Confused, but trying to understand... :-)

[retired00]

I'm still a little confused with all this. Let me get this straight. The battery is being charged all the time when coasting and is evident by see the display indicating power going from the wheels to the battery.

Regenerative Braking assists the stopping power by applying power to the electric motors to slow the car down Right? So how do you know when this is occuring? Can you tell by reading the displays or is it happening all the time when braking? If there is no visible indication, then how do you know when it is not working?

Confused, but trying to understand... :-)

Basically, on your meter, you'll see the power arrow flowing to the battery and the dash gauge will dip below zero into blue. That indicates the drag caused by the motors and powering the generator. That's the regeneration of power. When you're stopping in a panic, there's never going to be much regeneration because the wheels are stopped or braking at threshold, so there's less wheel rotation/regeneration. If you did not have ABS and had all 4 locked in a skid, there'd be none. On the Lexus site, there's a link for How Hybrids Work mid page in a box, I think it will give some insight. Braking simply says when coasting or braking it stores electrical energy. The focus is on regeneration of power, not on braking, so when the term "regenerative braking" is used I think in terms of "regeration of power while braking." (I'm probably making this far simpler than it is, given the whole process is run by the no-doubt big $$$ Power Control Box, but the concept is what's important to me.)

[skyfish400h]

I'm still a little confused with all this. Let me get this straight. The battery is being charged all the time when coasting and is evident by see the display indicating power going from the wheels to the battery.

Regenerative Braking assists the stopping power by applying power to the electric motors to slow the car down Right? So how do you know when this is occuring? Can you tell by reading the displays or is it happening all the time when braking? If there is no visible indication, then how do you know when it is not working?

Confused, but trying to understand... :-)

I think were you are going off track is you're assuming that it takes power applied to the electic motors to decel the car. That is not the case. The drag comes from the generation of electricity from the torque being applied to motor shaft by the inertia of the car. You can run an electric motor either way...

electricity in --> torque out

torque in --> electricty out

What the RX does with regenertitive braking is take the electricity produced when the troque is applied and use that to charge the batteries.

hope this helps

[wwest]

I'm still a little confused with all this. Let me get this straight. The battery is being charged all the time when coasting and is evident by see the display indicating power going from the wheels to the battery.

Regenerative Braking assists the stopping power by applying power to the electric motors to slow the car down Right? So how do you know when this is occuring? Can you tell by reading the displays or is it happening all the time when braking? If there is no visible indication, then how do you know when it is not working?

Confused, but trying to understand... :-)

"...by applying power to the electric motors to slow the car down...."

These particular electric motors are of the synchronous multi-phase variety except they have a permanent magnet rotor. In regenerative mode you can think of them as an absolute duplicate of the industry standard alternator with the sole exception being your alternator's rotor is an electro magnet which is energized by the voltage regulator as a function of the need for generating electrical power.