Regenerative Braking Question

[RossiT]

Hi

A bit of a random question I know but on a Hydrogen fuel cell car with regenerative braking, I understand that the energy created during braking is used to turn the motor which becomes a generator and charges the backup lithium battery.

What I can't understand is, how is the braking energy used to turn the electric motor. What holds the energy?

Thanks

[buggalugs]

RossiT said:

What I can't understand is, how is the braking energy used to turn the electric motor.

I understand that the wheels of the car turn the motor,

Hth,

Neil

Edited by buggalugs on Sunday 5th December 12:40

[Ozzie Osmond]

RossiT said:

.... charges the backup lithium battery.

RossiT said:

What I can't understand is, what holds the energy?

I think you've answered your own question.

[davepoth]

You know how a dynamo on a bicycle makes it really hard to pedal, and slows the bike down really fast? that.

[RossiT]

I think I could have worded the original post better, when the car brakes what gathers the braking energy that was created?

[freecar]

RossiT said:

I think I could have worded the original post better, when the car brakes what gathers the braking energy that was created?

You do understand that a motor spun by external work becomes a generator? With this in mind the turning wheels are slowed by magnets in the motors, this generates electricity to charge the battery.

Edited by freecar on Sunday 5th December 12:46

[CraigyMc]

RossiT said:

I think I could have worded the original post better, when the car brakes what gathers the braking energy that was created?

When you start braking, a mechanism attaches a generator to the wheels. The wheels turn the generator, and the electricity from that charges up the battery.

When you're not braking, this mechanism is idle - it's not connected to the wheels.

Capiche?

C

[RossiT]

Thanks very much, makes sense now.

[tyranical]

Perhaps i'm daft, but why not have the motor connected at all times so it constantly charges off the rotation of the axle?

[CraigyMc]

anonymous said:

[redacted]

That's how normal car alternators work (well, they are actually driven off a belt on the engine rather than connected to the wheels). Without an alternator, your car would stop working quite quickly...

C

[PhillipM]

CraigyMc said:

That's how normal car alternators work (well, they are actually driven off a belt on the engine rather than connected to the wheels). Without an alternator, your car would stop working quite quickly...

C

It's still not 100% effiecent, and you're dealing with a lot more power through a regen system.

[skid-mark]

its usually controled through a second motor in the gear box which becomes activated when the brake pedal is pressed and helps charge up the battery, then the 1st motor is used to deliver the power

Edited by skid-mark on Wednesday 8th December 22:35

[CraigyMc]

PhillipM said:

CraigyMc said:

That's how normal car alternators work (well, they are actually driven off a belt on the engine rather than connected to the wheels). Without an alternator, your car would stop working quite quickly...

C

It's still not 100% effiecent, and you're dealing with a lot more power through a regen system.

Nothing is 100% efficient. As for your other point, well, I don't see what you're getting at?

BMW's version of this doesn't actually ever drive the wheels, it *only* charges the battery up (they don't even market it as hybrid) - but it does recoup energy while braking...

C

[thinfourth2]

anonymous said:

[redacted]

No

There is no permeant magnets in an alternator. They are electro magnets otherwise known as the field coil. These are controlled by the regulator. So by varying the voltage to the field coil you can vary the output and therefore the amount of power consumed by the alternator.

A demo of this is when you jump start another car. If you leave the running car at idle and connect the jump leads to the flat battery the engine note will change slightly as the alternator loads up.

So this is exactly the same mechanism as used by the electric motor in a hybrid just it can also become a motor as well as a generator.

[The Wookie]

skid-mark said:

its usually controled through a second motor in the gear box which becomes activated when the brake pedal is pressed and helps charge up the battery, then the 1st motor is used to deliver the power.

The motor and generator are almost universally the same device, there is no physical mechanism to swap the connection to the wheels between the two, they are the same thing, it is purely the way the motor is controlled/wired that results in it turning from a motor into a generator.

You might be getting confused with the Prius setup, which uses a second electric motor to control the effective gear ratio of the engine relative to the electric motor.

[anonymous-user]

It's also worth noting the enormous short term power* capability of conventional "friction" brakes, which makes competely replacing them with a regernative system just about impossible. Hence even if you have a regenerative system the manufacture still needs to fit friction brakes, so vehicle mass goes up as does cost!

• even a rubbish car can pretty much stop from 100mph in 5 to 6(ish) secs these days, compare that to cars that can get to 100 in 5 or 6 secs and look at the power they have to produce to do that!

[Ozzie Osmond]

Max_Torque said:

It's also worth noting the enormous short term power capability of conventional "friction" brakes.

Yes indeed. Which helps explain why there's little point going on a trackday in a road car (heavy) with ordinary road brakes. In almost every case an expensive set of brakes can be destroyed in minutes. Tyres too.

[skid-mark]

this might help http://auto.howstuffworks.com/auto-parts/brakes/br...

[The Wookie]

skid-mark said:

this might help http://auto.howstuffworks.com/auto-parts/brakes/br...

"When the driver steps on the brake pedal of an electric or hybrid vehicle, these types of brakes put the vehicle's electric motor into reverse mode, causing it to run backwards, thus slowing the car's wheels."

While technically correct, the wording for this is extremely misleading, it makes it sound as if the motor is put into a reverse gear and is turned in the opposite direction for regenerative braking when in fact no such thing occurs, it is purely the power flow that is reversed, causing a torque that opposes the motion of the vehicle rather than one that assists it.

[davepoth]

skid-mark said:

its usually controled through a second motor in the gear box which becomes activated when the brake pedal is pressed and helps charge up the battery, then the 1st motor is used to deliver the power.

No it isn't. Why would you bother carrying around 2 motors when one can do both jobs perfectly well?