Regenerative braking speed considerations
 

Hello,

I have a highlander hybrid. There is an issue of regenerative braking that I've been thinking about and I don't know if people here can provide any insight. Here is the issue:

I'm imagining that there is a limit to the rate of current that can flow in to the hybrid battery by regenerative braking. Now, lets say you use the regenerative brakes to slow from 20mph to 10mph over 10 seconds. Compare this to to slowing from 50mph to 40mph over 10 seconds. The amount of energy that can be stored in the hybrid battery is different in these two situations because the kinetic energy of the vehicle is proportional to the square of the speed. The change in kinetic energy going from 50mph to 40mph is 3 times as much as the change in kinetic energy from going 20mph to 10mph.

So... given that the kinetic energy being recovered by regenerative braking is much higher when braking at high speeds, will I be exceeding the limit to the rate of current that can flow in to the hybrid battery? How should I adjust my driving to account for this in order to minimize throwing away kinetic energy? I don't really know.

Lipo packs have to use a charge controller for numerous safety reasons, so the max amount of charge put back in the pack is based on whatever that charge controller outputs. There's no way without a lot of measuring to simply know that, unless the factory advertises the engineering specs somewhere, which I doubt they do. Otherwise your logic is sound, but its not an easily answered question without knowing all the details of the system that you are driving.

Well, I've been thinking about it. I've got a hunch that the amount of power that can be used for regeneration is probably similar to or a little bit less than the amount of power that can be discharged by the electric motors. The car's a 2011 toyota highlander hybrid and the motors are 26kw (I think) total. The curb weight is 2,105 kg. Will think more about this later.

edit: unsure of specs on motors;

I can provide some insight, mainly how my own Insight behaves :)

As you know the regenerative energy you can harvest at higher speeds easily exceeds the capacity of the battery. But you need not worry as it gets limited to a safe level by the electronics.

My Insight has a permanent charge/discharge indicator which shows whether I'm on its limit or not. If the needle dips to the deepest part of the green zone (I need to mildly press the brake pedal to make it do that) it is on max regen, and any more pressure on the brake pedal will make it use the disk brakes.
No doubt the Highlander has a feature like that too.

So, if I need to come to a stop and I have the space to do so I just keep it slightly below max regen until the speed is low enough to coast the last bit without braking, retaining some speed for if the lights go green before I get there.
If I need to brake harder I do that in the first phase, aiming to bring the speed down to where I can slow down using only regen for the last part. Doing so, I max out the time I can have max regen as I would take longer (in time) to stop than if I brake overly hard in the last part.

That said, for max regen you need to gradually build up the brake pressure as the speed slows.
A very light touch on the brakes will send it completely in full regen at highway speeds.
At 20 mph you need to brake quite hard to get full regen; power is force times distance per second, so the shorter the distance the higher the (braking) force needs to be to produce the same power.

If I leave the highway and need to brake for the lights and they come up faster than I can manage with just regen, and there is nobody on my ass I can either:
- brake hard to shed excess speed, then gently to brake only on regen, which gradually becomes harder and harder braking as the speed drops, ideally ending in very hard braking right at the lights, or:
- brake even harder to get well below the ideal speed, then leisurely let it roll towards the lights rather slowly, hoping they turn green before I arrive there to retain some of the speed.

The latter holds the risk of the lights turning green before you expect them to, and red again before you arrive. Like it happened to me twice this afternoon; though that was more a case of letting the pack of cars chase away from me and seeing the last of the group dive through at stale orange. But I could have known I'd miss it and let go earlier for less gas usage and more regen... So hard to get it perfect every time.

In a Prius, you can set up X-Gauges on your SG2 to give you your braking Watts read out.

The simplest and most efficient method is not to brake at all :thumbup:

Can't you just add capacitors to increase the charging capabilities before the charge controller???

OP: You're mixing up "Power" and "Energy", mathematically. It doesn't work that way.

Correct me, if I'm wrong.

Had time to look it up.

Power P (watts) = M·v·a (mass · velocity · acceleration)
Mass in KG.
Velocity in meters per second. (the change in speed, be it 0 to 60mph or from 50 to 40mph)
Acceleration in meters per second squared.

Energy is the capacity to do work. Energy is power integrated over time.

Power is the rate at which work is done, or energy is transmitted.

Energy = joules = watt-seconds

Power = watt = joules/second

Point being that it takes the same amount of power to accelerate from 0 to 10 mph in X seconds as it does from 40 to 50 mph in the same X seconds. And the reverse is true for braking/regen, from 50 to 40 is the same as 10 to 0.

You get more regen braking force at higher speeds either because you have much more voltage being generated (therefore you can suck more watts out of the motor) at higher rpm, or because the inverter is programmed to brake less at lower speed and/or is programmed/limited/designed to how many amps it can suck out at any given moment (amps x volts = watts).