This subject has always spurred a considerable amount of controversy
over the years; so, I am re-posting a message I sent about 1.5 years
ago that has been updated with more information from tests,
experience & some photos. lets see if we cannot more fully
comprehend what happens in these cars when using `D,' using `B' &
using our brakes.

FIRST, AN EXCLUSION
In both the first generation Prius (2001-2003) & the second
generation Prius (2004-200?) the brake regeneration process
completely stops all of its regenerative activities once the Prius
slows to 7-8 MPH. Any time the Prius brakes are applied when the
vehicle is moving under 7-8 MPH, all braking activity is 100%
mechanical with no `brake' regenerative benefits whatsoever. The
rest of this article will address Regenerative Braking in the Prius
without bringing this fact up again.

THE 2001-2003 – `THS' REGENERATIVE PROCESS
In the first version of the US Prius (2001-2003), the THS (Toyota
Hybrid System) always used `at least' 10% mechanical means (brake
pad against rotor or shoes against drums) along with about 90%
regenerative (MGs being used as generators) forces to slow the Prius
down. We say `at least' here because the first 65% of the applied
brake-pedal-pressure caused the vehicles to decelerate using a 10%
mechanical & 90% regenerative process. The final 35% of the brake-
pedal-pressures caused a 100% mechanical, heat producing, brake pad
& shoe reducing activity.

To say this in another fashion; the 2001-2003 US Prius always used
at least 10% mechanical braking until it reached 65% pedal-pressure
at which point it used 100% mechanical braking.

THE 2004-200? – `HSD' REGENERATIVE PROCESS
In 2004, Toyota introduced the HSD (Hybrid Synergy Drive) in its
Prius automobiles. The `Brake Regeneration System' was clearly
upgraded and significantly improved in these cars over the 2001-2003
model years.

HSD MAXIMUM PROXIMITY SETTING
In the HSD cars, when you first apply the brakes, the brake pads and
brake shoes momentarily (maybe 2 milliseconds) & very lightly touch
their rotors & drums to `set' them in a maximally close position to
their rotors & drums so that there is no delay in the event that the
Prius ECB (Electrically Controlled Brakes) determines that
mechanical braking is necessary.

BUMPS & SLIPPERY ROADS EXCLUSION
You can sometimes feel the above described transition from Full
Regenerative braking to Full Mechanical braking when you hit a bump
in the road & during slippery conditions such as icy or snowy
roads. From here on, it is assumed that the reader will remember
that bumps & slippery roads can & do cancel Brake Regeneration
Processes & the HSD braking activity will convert to 100% mechanical
braking as per instructions from several different `safety'
components that all fall within the ECB family. This type of
situation will not be again addressed in the remaining discussion.

VERY EFFICIENT HSD BRAKING
During a braking event & after `setting' the pads and-or shoes in
their maximally close proximity, these pads & shoes ride just off
the surface of their rotors and or drums and do not touch them at
all. There is essentially ZERO mechanical activity happening in
this kind of very advanced HSD braking. 100% of the deceleration is
being provided by the very efficient HSD `Regenerative' process
which is storing the captured energy in the HV Battery. A very big
part of the efficiency increase of the HSD Prius over the THS Prius
is found right here in the much improved Brake Regeneration System.
Take a look at my brake pads @ 8,000 miles & then again after
100,520 miles; I cannot measure any difference with a metal mm
ruler: http://www.privatenrg.com/#100kBrakePads

HSD 100% REGENERATION UP TO 90% PEDAL-PRESSURE
The HSD Brake Regeneration System maintains 100% of the deceleration
until one applies 90% or more of the total pedal-pressures that the
system responds to. Very few are the cases where one will exceed
90% brake pedal-pressures & often when one has `stomped' or pushed
on the brakes hard enough to exceed 90% pedal-pressure, the ECB is
kicking in several of the safety systems that begin taking measures
to reduce skids, sliding & other events that shall not be discussed
here & now. To put this paragraph more succinctly; almost all
braking in the HSD equipped Prius is applying Regenerative processes
to capture braking energies; why, because pedal-pressures seldom get
to 90% or above.

SOC & REGENERATION
In a nutshell, `D,' `B' & `Brake' Regeneration processes are all
able to capture deceleration energies and admit them to the HV
Battery as long as the SOC (State of Charge) of the HV Battery falls
within a range that the HV Battery ECU allows. Once the SOC has
reached a maximum, the HV Battery ECU communicates to the HV ECU to
stop all energy transfers or charges to the HV Battery
including `D,' `B' & Brake Regeneration processes.

SOC RANG E NOT ALWAYS THE SAME
As we consider the probability of the SOC `topping off' and
canceling any more charge or regeneration energy entering the HV
Battery we should be aware that there is a rather complex algorithm
involved. The HV Battery ECU considers the values of the following
variables & more to determine where the SOC should be set & how many
Amps or how much energy I/O is allowed:

1 – Vehicle Speed
2 – Cabin temperature
3 – Module temperatures
4 – Ambient temperatures (outside)
5 – Last few minutes of driving style
6 – Inclinometer setting & recent inclinometer activity
7 – Battery maintenance-regimen status
8 – Any battery conditioning status
9 – Individual module health
10 – Internal Resistance, both overall & module levels

`D' DECELERATION vs. `B' DECLERATION
Generally, if allowed by the HV Battery ECU via SOC level, `D'
deceleration will provide about 20 Amps worth of regenerative energy
& generally, `B' will provide close to 30 Amps. Most people seeing
these two values instantly rationalize that it is a no-brainer
that `B' is always the better choice. Ironically, `B' is almost
never the better choice. Now let's see why.

`B' INVOLVES UNECCESSARY ICE FRICTION & WASTES ENERGY
Most people know that `B' causes the ICE RPM to spin up and then
uses the rotational friction provided by the spinning ICE to assist
in decelerating the Prius. Many people feel that the extra 10 amps
generally provided by `B' offsets the frictional losses that the ICE
is wasting as heat. This simply is not correct. Many people figure
that using `B' will minimize brake wear. In the majority of
deceleration events, this too is not the case.

`D' or `D' + BRAKING IS ALMOST ALWAYS BEST
In almost all cases, Prius operators, especially those with HSD
equipped Prius, should shun the old idea that their brakes need to
be protected by choosing `B' over `D.' In almost all cases,
choosing to use `D' + brakes until the SOC is full, will not
increase brake wear whatsoever. In almost all cases, using `B' will
waste a significant amount of ICE spun energy that could have been
captured if the operator had chosen to use `D' + braking instead.
In almost all cases, using `B' causes unnecessary wear to the ICE.

USING `D' + BRAKING TO MATCH DECELERATION OF `B'
Using `D'+ Braking to match the deceleration that `B' alone provides
captures significantly more energy. Using D+ Braking in HSD
equipped Prius to match the deceleration of `B' alone will not cause
any wear to the brakes whatsoever.

`D' + BRAKING IS EVEN BEST IN THS EQUIPPED PRIUS
In THS equipped cars, hundreds of very intelligent users have tested
for thousands of miles & determined, very clearly, that the energy
captured using D+ Braking far offsets the minimal brake wear that
they get in their THS equipped cars.

WHEN, IF EVER, DO WE USE `B'
In almost all cases, choosing to use `B' after the SOC `tops off,'
is pretty much the only time that using `B' makes any sense at all.

Regards – Wayne – http://privatenrg.com