Battery SOC--Brake or Coast?

Its a good question. Coasting will generate a small current for a longer period of time. Braking will generate a larger amount of current for a shorter period of time. I'm not sure which actually puts more charge back in the battery. My guess is that coasting would generally win, as the chances are better that the light will change before you get there. The amount of energy you use accelerating up to speed from a stop is much greater than what you will recapture through the regenerative brakes slowing down. Many folks would probably argue that gliding (no arrows) is even better than coasting for the same reason. Even though you are getting no regenerative braking, you spend even longer using no energy and have an even better chance of the light turning.

The "spare the brakes" thing applies less to the Prius, as it is mostly using the electric motor to brake anyway. This doesn't apply to "panic" stops, where the Prius aggressively asserts the mechanical brakes, but in general use the wear on the brakes is very little. Many folks are going 100k+ miles on the original pads. I think even the taxi drivers are getting 30-60k which they consider phenomenal.

Rob

 

He is partly right. A long coast is better. Hard braking will charge the battery quicker, but it is still more efficient in the long run to coast..

 

The most efficient way to brake is to not brake at all. If you can coast to the light with just enough pedal pressure to not get any arrows, then you really have it. Anything else is less efficient. Braking gently is second best, since you don't do as much braking.

Tom

 

Think of it this way: momentum is a form of energy storage, and
requires no efficiency-robbing conversion process to use.
.
_H*

 

Only been doing this for a couple of weeks but it seems like if I take my foot off the accelerator and just touch the brake peddle I can feel the regeneration kick in. I assume this is the point where the brake switch kicks in to turn on the lights. My guess is that there is none or almost no pressure on the pads at this point.

If the light doesn't change I add a little pressure as needed to keep from hitting the car in front of me. I am hoping to get the best of both worlds, benefit from the coast and maximize brake wear.

Bob

 

As soon as you lift your foot from the accelerator pedal regenerative braking starts. It does this to simulate engine drag as found on a normal car. As you add pressure to the brake pedal, the amount of regenerative braking increases. You can brake pretty darn hard without using the friction brakes. The friction brakes only come into play at low speed (a ~7 mph), in a panic stop, loss of front wheel traction, when the battery reaches a high SoC, or if you need more braking than regen can provide. Mostly they only kick in at low speed.

Tom

 

Without detailed knowledge of the charging formulas that Toyota uses between coasting and braking, it could be argued that they should be equal. Either a little power generated over a long time or a lot of power generated quickly. As long as the battery could absorb all the quickly produced energy fast enough.

You are, for the sake of discussion, stopping the same mass from the same speed. If time and effort (stopping power) are the variables they should be inverse relations to each other.

Now I'm sure some real world techno babble design issue may upset this balance. Does any body know what the actual chargin parameters are?

 

Sure, from the standpoint of simple physics you are dissipating the same amount of energy during a stop, regardless of whether you coast gently to a stop or slam into a brick wall. What makes a difference is the relative efficiency of capturing and reusing that energy. From a practical standpoint, the most efficient mode is to not have to capture and reuse any energy. It's better to directly use the energy by coasting, where the kinetic energy is dissipated overcoming friction. If you do have to capture and reuse energy, then the efficiency of generating and storing energy becomes the limiting factor. Many factors are involved, including losses in the motor/generator, the inverter/charger, and the battery. These losses are related to heat. The internal resistance of the battery is a key player in heat losses, and it varies by temperature and rate of charge. Empirical data posted on this forum suggests that 60 Amps is about ideal for charging during regeneration.

Tom

 

I want the regenerative braking program to be driver configurable... like in the ACPropulsions' eBox regen braking slider. It is said it is configurable to be from no regen to max regen (no need to step on brakes to stop normally).

Na.... too much user parameters for Toyota to do something like this. But it is a cool toy for us geek heads though. :target:

 

This has been discussed so many times previously, but I'm still not convinced it makes that much difference whether the soc is replenished directly by the ice or regen. Either way the energy comes from the ice. There are losses either way. I do my best regen on long downhill sections leading to known stops. If I must stop going uphill, I try not to brake, try to glide or very slightly brake. I try to keep my soc up to a point where it will do me some good so I can borrow some energy when needed, and don't like for it to get too low. The battery is there for a reason, so your car can borrow from it when needed... and you will always need it occasionally. Those who don't like to regen... get a go-cart and you will have the same power needs and mpg (although not as nice of a ride!)

 

I'm not sure I understand your post. Why would it make a difference if the SOC is replenished by the ICE or regen? The losses should be about the same either way. Either way you are using the battery, which is where most of the losses occur. Regenerative braking is better than friction braking since some of the energy is recovered, but not braking is more efficient than any form of braking. Obviously you can't avoid braking when driving, but you can minimize it by looking ahead. Likewise with the battery. It's there so we can drive a car with a small and efficient engine. Real world conditions demand that we use the battery, but minimizing its use will improve your efficiency. Life is full of trade-offs.

Tom

 

I am not clear on the advantages of gliding to a stop rather than recovering some of the energy used to accelerate in the first place. Once you reach your driving speed, the energy needed to get you there is a sunk cost. If you can keep the engine off and use the regen systems, you can regain at least some of that cost. By gliding, you are losing all of the initial energy input to friction losses. If you did not have the battery, gliding would make sense. But since you will have to eventually stop, it would make sense to regain whatever you can in the process, even if there are efficiency issues to be accounted for. (This assumes that the battery is not fully charged and the recovered energy has someplace to go).

Pat

 

I think you are getting tied up in details and loosing sight of the big picture. You are correct that some of the energy used to accelerate a vehicle to speed is stored in the vehicle as kinetic energy, the rest is consumed by various losses. When slowing down, some of this kinetic energy can be recovered through regenerative braking, or all of the kinetic energy can be used to overcome friction and drag. If you recover some of the energy through regeneration you still have the same losses with friction and drag, but now you have to expend other energy to overcome the friction and drag. The energy you use in this latter case is produced at less than 100% efficiency, unlike the kinetic energy which comes from your sunk cost. It costs you more to run the energy into and out of the regeneration-battery-motor cycle than to just use it while coasting.

Tom

 

Hi All,

The OT's (Old Timers) know what they are talking about here. Do not buzz up to speed just to glide for a few thousand yards and have to brake off that speed to the battery.

Recently, I have been using the Atilla Vass program. He has a braking effiiciency meter on it. And he has done allot of work to figure out the optimum braking rate. Off-pedal coasting to a stop is not very efficient recovery of energy apparently - according to the Vass program about 5 to 8 percent efficiency. Medium braking does allot better 50 to 100 Percent. This was allot harder than I was braking.

But again, try to plan to zip on through those lights when green, and do not waste gas if you know that light always goes red before you get to it 1/4 mile on down the road. But if for some reason the light goes red unexpectedly, what seems to work from a 40 mph glide, is to wait till your 300 to 500 yards out then do a medium brake down to 15 mph, and lightly brake on in the last 50 yards. These are just guestimate values based on a week of using this program. So, do not hold me too them in a few months, years.