Hybrid cars and regenerative charging of batteries

[Roy Q.T.]

so I don't know it all, who does, but, still the drive mechanism
employed must shift the power off the motor, and transfer the (any)
output generated, back into the batteries through a controled charge
circuit, (whatchu macall it) and the best charges should still be durng
coasting or motor free axle spins.

how they do it I haven't seen..

Anyone have a website with specs or do I need to activate my IEEE
student membership to dig around files?

®oy

 

[Don Kelly]

Motors are what ! Just like Generators ?
then lets put a voltage across a generator and see how many rpms we get
from it.


Please Mr. Kelly you are standing too close to the Rift
-----------
And you are trying to look up from the bottom.

I've run (many times) motors as generators and generators as motors-- no
problem. The only real difference is the direction of power flow. Direction
of rotation is unchanged. polarity of voltage is unchanged. (note: IF a DC
machine has a series field, then it is necessary to reverse the field to
avoid voltage reversal.- this is not necessary with a PM, shunt or
separately excited machine).
For AC machines, there is also no problem. In fact there are many places
where ordinary induction motors are used as generators. In some locations
they are connected to small water wheels and are started from the grid, run
up to speed as a motor, then the water valve is opened so they are driven
above synchronous speed and generate. AC synchronous generators can be and
are used as motors.
I can suggest some references for you, if you wish.

 

[Don Kelly]

such generating switching

Daestrom,
It seems that there are not a lot of posters here that have a good
understanding
of motor operation, specifically DC. It is good to see someone here, such as
yourself, that does.

I hope you do not mind if I add some general discussion to your comments.

There is a major difference between the Dynamic Braking and regenerative
braking.
With Dynamic Braking, when the energy is dissipated as heat, The Resistor
grids will continue to take load current as long as the motor is turning and
it has field. The slower the motor goes, the lower the voltage, and thus the
lower the current, and with it , Braking effort. As the motor slows,
braking effort can be maintained by increasing the field excitation, or
stepping out parts of the resistor, to get more current for the lower
voltage. (The machines we do use DC motors with Shunt fields)

If you wish to get Regenerative Braking into a battery, as you mentioned ,
all that has to happen is the motor field is to be increased so that the
motor counter EMF is higher then the battery voltage. Once this happens,
Current will drop to zero and then reverse. The reversal of Current will
produce a braking torque, which slows the Vehicle. The problem here is that
as the vehicle slows, the generated voltage for a given field strength will
drop linearly with speed. So as the Vehicle slows The only way to keep
regenerating current into the battery (which is basically a fixed voltage
source) is to keep increasing the motor field as the speed goes down.
There is obviously a speed where no amount of field current is going to give
you a voltage higher then your battery voltage, and braking effort will
cease.

Needless to say, the design of the Motor will have to be such that at normal
road speeds the
motor field will be quite a bit lower then it's maximum value, ( in the
field weakening region) and as the speed decreases, The field increase to
it's maximum value at base speed, After this some other way must be found to
stop the vehicle.

Because of this, Regenerating into a battery, using conventional thinking,
is not really a viable option.
I am not sure how the toyota people do this. maybe they have some sort of Dc
to DC converter to step the voltage up or perhaps they break the battery
bank into it smaller voltage cells and regenerate into these.
Either way they are adding in complexity and inefficiencies.

--------------
If, as it appears, they are using the motors as generators driven by the
gasoline engine, they probably do have a fairly good and efficient method of
controlling the charging. In addition the motors may be as I have seen in
one experimental hybrid vehicle, using PM field magnets so field control is
out. These motors were surprisingly small physically for their rating (which
I forget- it was over 20 years ago). Given electronic controls for normal
operation, addition of regeneration control should be relatively cheap and
simple.

By the way, thanks, I had called regenerative braking = dynamic braking-
theoretically dynamic braking is regenerative - but not in the standard
terminology as you have pointed out.

 

[[email protected]]

In real world driving, there is probably a 4 to 8 second time period in
slowing down and stopping for a red light where the energy of a 3000 lb
car slowing from say 60k to 30k is converted to watts and stored in that
4 to 8 second time frame. That sounds like a hell of a jolt to me.

I have made the following calculation for a panic stop: Assume that you
have a vehicle of 1000 kg (2200 lbs) and you are going to decelerate it
from 100 km/h (62 mph) to a stop in 50 m at -8 m/s^2 (about 0.82 g).
This requires 400,000 J (watt-seconds) of energy and takes about 3.5
seconds.

The Toyota Prius uses a 274 V NiMH battery with a capacity of about
7 Ah (3 hour rate). If you wanted to dump all that energy into the
battery, you'd have 115.2 kW over the 3.4 seconds, or 420 A. Compared
to the battery capacity, this is a lot. I seem to recall (but can't
cite) a number that only 10 to 20% of the braking energy is recaptured
by the regenerative system, and that figure would seem to be a more
reasonable rate to recharge that battery.

The Prius battery does have several thermistors embedded in it, a
forced-air cooling system, and its own computer to decide how it will
be charged. While I agree that charging at a too-high rate is not
good for any battery, this "battery" is not just a box of chemicals.
It can stop charging itself if the rate is too high. (Also, the target
state-of-charge in most hybrid cars is not 100%; it's usually somewhere
between 50% and 80%, so there will always be someplace to dump some of
the energy when braking.)

Another clue is that Toyota warrants the battery and hybrid components
for 8 years or 100,000 miles. The 2004 and up Priuses use the same
battery as the 2001-2003 Priuses, but use it more extensively to get
better fuel economy, so Toyota must be comfortable that the battery
will last at least that long.

Matt Roberds

 

[RQ]

Thank you for addressing the actual question that started all of this.

rq

 

[Newsey Person]

You're quite right.
Apart from a couple of constraints the only difference is efficiency.

Newsey

 

[Roy Q.T.]

I read somewhere "Chrstie Industries®" was bringing an EV Production
Line into the US but nothing there after >>>> any body know of it ?

®oy

 

[SAMMMMM]

the braking energy is not nearly as much as the downhill coasting energy
that
is recovered.
on my echo, the fuel is entirely cut off at closed throttle and 1440 plus
RPMs. it makes
a big difference as there's no idle fuel being used during this time.
i was surprised at the reduction in fuel flow when i coast to a stop, (out
of gear, idling ) and
gearing for higher rpms and closed throttle.
we have a lot of ups and downs in western PA. <G>
if i could recover the downhill energy it would be even better.
sammmm

 

[[email protected]]

I understood that NiMh batteries have a shelf life of around 3 years.
In other words, they'll have a relatively low capacity after that
period.

To me, "shelf life" means "sitting on the shelf, not doing anything."
A battery that is being used, and maintained by a relatively smart
system, ought to last longer than that. I do know that Toyota specifies
an extended charging procedure when the vehicles are first delivered to
the dealerships, so the battery should be fully charged while the
vehicle is on the lot. (Not that it's there for long; Priuses are back
to a few-month waiting period...) My Prius was built in late 2000, and
I bought it in spring 2001, and I've had no problems with the battery
so far.

Also NiMh lose their capacity from heating, so I wonder how do Toyota
expect their NiMh to last for 8 years?

The pack does have forced-air cooling. Air is drawn in from vents in
the rear parcel shelf, blown across the battery, and exhausted through
the left C-pillar. The blower doesn't run all the time... only when
needed.

Everyone who has been through the experience of having to repeatedly
replacing their laptop computer battery after hardly using it knows
about this.

I think Toyota has better rockets than the coyote...

Do they have some sort of special NiMh battery?

It's made up of 38 7.2 V modules. Each module is a skinny rectangular
thing, about the same proportions as a VHS tape, but a little smaller.
They are clamped together in a big stack with long bolts. The original
Prius, sold only in Japan, used cylindrical NiMH cells, and its pack
was of a lower capacity than the later US model. Any FUD you hear about
the "turtle light" almost certainly derives from early US road tests
of Japanese-model Priuses.

Matt Roberds