Braking to a red light

I've read some conflicting info regarding braking into a red light. One post said above 7 MPH, provided the brakes are not applied "hard", the regen system is used to slow the car and the pads are not engaged. Below 7 MPH or when applied hard the pads are used. If this is correct, if I know I have to stop at a red light and nobody is behind me and I can lightly ride the brakes wouldn't that be best for recharging the battery and if this is true it seems would lead to very long lasting brake pads?

I think you have it nailed. I try to start coasting so early that I don't have to brake, but of course for many reasons, it's not always possible, then your way comes into effect. I theorize it's better to slowly charge and discharge the battery as opposed to big loads, which if I'm correct leads further importance to your method.

With the Gen III Prius, the cutoff speed for regenerative braking has been lowered, so it works even below 7 mph, although obviously the amount of available regen drops at low speeds.

For best efficiency, you don't want to brake at all. Try to anticipate the red light and coast. Failing that, brake at a moderate level, as that seems to produce fairly efficient battery recharging without invoking friction brakes.

Tom

Hey - I also use the shifter 'b' brake mode to assist in recharging - down shift to the dynamic braking mode also provides a wonderful way to recharge the battery without pressing the brake pads - try it.

Cool feature

Mr.Bill

'Recharging the battery' is never necessary. The computer's target is 60% State of Charge, and it will burn extra fuel to get back there. It's rather like your toilet cistern, with a ball valve controlling the flow of water - if the tank is empty, the valve opens to refill it from the water supply, and the valve opens wider the further down the tank goes. If overfull, you can drain some without opening the valve and consuming water from the supply.

The car will leave the engine off if power demand is low, below a certain speed limit where it has to run the engine to prevent overspeed on the motors. This does allow the 'tank' to drain a bit, but the further it drains, the lower power demand you can get away with before it restarts the engine.

Because of the shape of the engine's efficiency curve, it can sometimes be more efficient to run the engine a little faster, to provide the road demand and charge the battery, than to just run the engine alone. Most of the battery charge comes from normal cruising, not from regen braking. Regen is a bonus, but the car doesn't account for any regen at all. You don't capture a great deal of energy from any one stop.

+1. Yeah. I keep B mode as much as I can.

why?

A previous discussion did not come to a consensus, but I have to claim that braking has to be moderate or even light, depending on speed, to prevent pad engagement. Regen braking power is limited by the battery current limit, and common street braking rates that people are accustomed to on their non-hybrids very easily exceed this limit.

I try to make sure the the 2010's HSI display does not peg the recharge bar, whenever possible. It takes significant advance planning and change of habits to achieve this. And the accuracy of this display has not been confirmed.

Sorry, but B mode wastes energy.

The OP has the right idea.
Drive like your brakes are stuffed. Don't brake if you can coast to a stop, brake as long and light as you can if you have to brake. Best of all, time traffic lights so you don't have to stop at all.

A spreadsheet I've been fiddling with suggests that the harder you brake, as long as you keep it within regen limits, the more energy will be recovered (as less is lost to wind resistance which is proportional to the cube of speed), but I don't know how recharge efficiency fares with different amounts of recharge current. Battery University says:

"The overall charge efficiency of nickel-cadmium is about 90% if fast charged at 1C. On a 0.1C overnight charge, the efficiency drops to 70% and the charge time is 14 hours or longer."

"Nickel-metal-hydride should be rapid charged rather than slow charged. Because of poor overcharge absorption, the trickle charge must be lower than that of nickel-cadmium and is usually around 0.05C. This explains why the original nickel-cadmium charger cannot be used nickel-metal-hydride.

"It is difficult, if not impossible, to slow-charge a nickel-metal-hydride."

C is the rated capacity of the battery, i.e. for 6.5Ah a charge rate of 6.5A which is only about 1.5kW. Wind resistance at 70mph is about 12-15kW, so you can see we're not getting a lot of braking at that rate. You can actually dump up to 120A into the pack which is about 20C - I don't know how efficient it is at that rate.

Still, coasting - losing a small amount of speed - is better than burning fuel to maintain speed then braking hard, and avoiding the stop - with the consequent need to accelerate - is even better.

Please explain how by using the 'b' mode wastes energy.

All the things you mensioned in your privious paragraph should always be done first ..... but given the option between the 'b' mode to brake or pressing on the 'brakes' to slow the vehicle - I was lead to believe that the 'b' mode actually created greated breaking efficiencies and greated voltage being generated for the battery. This statement also is predicated on a less than full battery on the receiving end.

If you can please add some light to this please do ... I don't have any scientific data to back this claim ... just my Toyota Service Technician talking in 'bla-bla'bla' language as conformation.

Kind regards for now - Mr.Bill

Maximum regeneration is limited by the capacity of MG2 and its controller, and the ability of the HV battery to absorb charge. Normal braking (non B mode) is easily capable of producing maximum regeneration, so except in a few esoteric cases B mode offers no regenerative advantage. What B mode does do is spin the ICE as an air pump to throw away energy. This is B mode's principal purpose. Think of it as downshifting a manual transmission or using a jake brake on a truck. This is the reason B mode wastes energy. Because of this, you generally don't want to use B mode except on steep downhills where the HV battery will rapidly charge to its high limit.

Tom

I worry about the people behind me seeing me if I slow down using the"B" .
Does anyone else?
Karen - a one week Prius owner

It's no different than downshifting a manual transmission or using a jake brake on a truck. All you are doing is slowing down a bit, not braking, so there shouldn't be any trouble with the people behind you. That said, you can't fix stupid, and there are plenty of stupid clueless drivers on the road.

Tom

Except that the 'recharge meter' goes up noticably when engaging B as opposed to coasting down the hill.

Not as much as hitting the brake does, though -- probably about the same for the amount of slowing being applied.

Hi, try to read this:

http://www.techno-fandom.org/~hobbit/cars/b-mode.html

To put it short, in B-mode you convert part of your kinetic energy into useless heat, produced by compressing the air in the cylinders of the ICE (although regen does not cease unless your battery is full). On the other hand, in the D-mode (if you brake lightly enough) you can convert greater portion of your kinetic energy into the electricity, stored in your traction battery for later use. B-mode makes sense only when a) you have reached full SOC or b) going long downhill section where regen is not sufficient to keep the car from accelerating - you don't want to stress and possibly overheat your friction brakes, so it is better to switch into B even when your SOC is not at maximum. In all other cases, avoiding the B-mode produces better results.

Speaking of the JP Gen III, the threshold speed is now 24 mph.

Ken@Japan

I haven't been able to puzzle out all the requirements, but my GenIII did a lot of B-mode braking with 0 RPM on the ICE, at speeds around 25 mph and a few times pushing as high as 35. Most of it happened one evening three weeks ago in the elk viewing areas of Rocky Mountain National Park.

One of the conditions is 6 or fewer bars on the battery, but there are more requirements that I failed to identify. I was able to replicate this a few times during the remainder of the trip, but most tries failed, with the ICE spinning up immediately.