Regenerative braking

[Sylvester]

If your assistance system features regenerative braking (Bionx or whatever), what happens if you use it on a long downhill? If you get to the point where the battery is fully recharged, do you lose retardation, or, if not, what happens to the heat you are dissipating?

I'm just having a bit of a daydream about what would be involved in designing a seamless brake-by-wire system....

 

[flecc]

On bicycles the regeneration due to the low weight and speeds is so inefficient that there's virtually no chance of fully recharging a battery.

Even if you did, the braking would continue and as you say, the effort then would be lost as heat dissipating out of the hub into the air.

Panasonic have a large batch of electrical braking e-bikes running on trial in Japan with front hub motors which are designed to be the only front brake, also regenerating.
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[D8ve]

Most of the cycling energy is used to overcome air resistance. Normally only a small percentage is used to get up hill and that is usualy used for weeeee fun agin not inefficent recharging.
You could get power back but persons make around 150 watts. It you use that for charging you aren't moving. And it takes 3 hours to recharge and ave 400 watt battery that way.
It's just not worth it.
( more detailed explanations are avalable)

 

[Sylvester]

Most of the cycling energy is used to overcome air resistance. Normally only a small percentage is used to get up hill and that is usualy used for weeeee fun agin not inefficent recharging.
You could get power back but persons make around 150 watts. It you use that for charging you aren't moving. And it takes 3 hours to recharge and ave 400 watt battery that way.
It's just not worth it.
( more detailed explanations are avalable)

If you happened to live at the top of a hill, and left home with a full battery, it's entirely possible that the battery would get saturated. So I was curious to know whether the motor would melt, or switch itself off, or whatever.

Flecc's comment about Panasonic is interesting - they clearly think it's worth investigating. I wonder how one would control the level of retardation in proportion to the load on the brake lever?

 

[flecc]

If you happened to live at the top of a hill, and left home with a full battery, it's entirely possible that the battery would get saturated. So I was curious to know whether the motor would melt, or switch itself off, or whatever.

Don't forget that the BMS in a lithium battery cuts off the charge when all the cells are fully charged, so overcharging is impossible. It's then that the braking effort will be dispersed as heat, but at bicycle regeneration levels that would have little chance of melting anything.
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[jhruk]

If you happened to live at the top of a hill, and left home with a full battery, it's entirely possible that the battery would get saturated. So I was curious to know whether the motor would melt, or switch itself off, or whatever.

Flecc's comment about Panasonic is interesting - they clearly think it's worth investigating. I wonder how one would control the level of retardation in proportion to the load on the brake lever?

Yes, this is a problem, and not just when the battery is full. The power provided by regenerative braking tends to come in high power bursts, which doesn't suit the charging requirements of the type of lithium battery used in most ebikes.

When an electric motor is used as a generator the braking effect will be proportional to the power taken, or load. The controller can regulate the power going to the battery but this will also affect the braking effect. When the battery is full the controller should stop any power being taken, reducing the braking effect to nil. There will be no load on the motor/generator, so it will not be producing any power, and there will be no heat to lose.

One could design a system such that the braking effect would be greater, and consistent, but that would require an external load, i.e. resistor, with a suitable heatsink and cooling. This is how electric train brakes work. Although some of them are capable of feeding their regenerative braking power back into the system they all use banks of resistors under the train as well.

Most types of motor used in ebikes have freewheels and are not suitable for regenerative braking anyway. For those motors that are suitable the amount of energy that can be recovered is not really worth the extra complication. Although electric brakes work well on high mass trains braking a low mass bicycle is hardly a problem, and again not really worth the extra complication.

It is a good marketing gimmick though!

 

[Sylvester]

Yes, this is a problem, and not just when the battery is full. Most ebikes use lithium batteries which don't like a high charge rate. The power provided by regenerative braking tends to come in high power bursts, which doesn't suit the charging requirements of the type of lithium battery used in most ebikes.

When an electric motor is used as a generator the braking effect will be proportional to the power taken, or load. The controller can regulate the power going to the battery but this will also affect the braking effect. When the battery is full the controller should stop any power being taken, reducing the braking effect to nil. There will be no load on the motor/generator, so it will not be producing any power, and there will be no heat to lose.

One could design a system such that the braking effect was greater, and consistent, but that would require an external load, i.e. resistor, with a suitable heatsink and cooling. This is how electric train brakes work. Although some of them are capable of feeding their regenerative braking power back into the system they all use banks of resistors under the train as well.

Most types of motor used in ebikes have freewheels and are not suitable for regenerative braking anyway. For those motors that are suitable the amount of energy that can be recovered is not really worth the extra complication. Although electric brakes work well on high mass trains braking a low mass bicycle is hardly a problem, and again not really worth the extra complication.

It is a good marketing gimmick though!

I'm thinking of a system with an electrically-operated friction brake working in parallel with the motor/generator. A control unit would take the brake demand signal (ie, load on the brake lever) and share it between the two retardation systems, favouring the motor/generator as much as possible. A lot easier said than done, I know, but it's basically how current F1 cars work.

 

[flecc]

One could design a system such that the braking effect was greater, and consistent, but that would require an external load, i.e. resistor, with a suitable heatsink and cooling. This is how electric train brakes work. Although some of them are capable of feeding their regenerative braking power back into the system they all use banks of resistors under the train as well.

Panasonic's front hub electrical brake/motor combined doesn't appear to use any external resistors and is a fully functional brake, covering everything from gentle braking to emergency stops since the bikes have no other front brake. It's probably the battery effect concerns that has resulted in the large scale long term trial in Japan.

Typically for Panasonic, they haven't released any technical details.
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[danielrlee]

I couldn't disagree more with what is being said here. Having recently had my eyes opened to the world of direct drive hub motors and their regenerative ability, I am all but done with geared hub motors. Only on lightweight folders would I now consider one.

 

[flecc]

I couldn't disagree more with what is being said here. Having recently had my eyes opened to the world of direct drive hub motors and their regenerative ability, I am all but done with geared hub motors. Only on lightweight folders would I now consider one.

But internally geared motors totally outclass DDs on hill climbing, if both are equally power rated legally. Also the many attempts at regenerative e-bikes have almost all failed, even when being from some of the most respected makers. There must be a reason for that.
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[jhruk]

Panasonic's front hub electrical brake/motor combined doesn't appear to use any external resistors and is a fully functional brake, covering everything from gentle braking to emergency stops since the bikes have no other front brake. It's probably the battery effect concerns that has resulted in the large scale long term trial in Japan.

Typically for Panasonic, they haven't released any technical details.
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That's interesting, I haven't seen that one. I suppose one could get strong electric braking by reversing the phasing of the motor but you'd have to disconnect it at near zero speed or you'd start going backwards. I can't imagine any cycle without some form of mechanical braking, even if it's only on one wheel.

 

[flecc]

They have a back brake but no other front brake than the electrically braked hub. Clearly the braking cannot be commenced by a near zero speed disconnect, so they've obviously devised some other way.

Being Panasonic, that's likely to be both unique and complex!
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[Deleted member 4366]

There's two technologies: Regenerative braking and electronically assisted braking system (EABS). EABS doesn't involve the battery.

 

[danielrlee]

But internally geared motors totally outclass DDs on hill climbing, if both are equally power rated legally. Also the many attempts at regenerative e-bikes have almost all failed, even when being from some of the most respected makers. There must be a reason for that.
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Slightly confused with that statement - ratings being almost meaningless nowadays. With a DD you can throw more power at it when needed. Also on the flip side, a DD totally outclasses geared when you're up to cruising speed.

 

[Sylvester]

They have a back brake but no other front brake than the electrically braked hub. Clearly the braking cannot be commenced by a near zero speed disconnect, so they've obviously devised some other way.

Being Panasonic, that's likely to be both unique and complex!
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This is brake-by-wire: the rider is not directly connected to the brakes. It's then a relatively easy step to add ABS....

 

[Sylvester]

Why can't you have regen braking with a geared hub motor? I can't see that it makes any difference.

 

[danielrlee]

Why can't you have regen braking with a geared hub motor? I can't see that it makes any difference.

You can if there is no active freewheel, although the composite planetaries would take too much of a hammering for my liking.

 

[Deleted member 4366]

Why can't you have regen braking with a geared hub motor? I can't see that it makes any difference.

You're going to kick yourself!

The motor doesn't turn when you go downhill. The clutch disengages. That's the whole point of them.

Actually, that's not quite true because you can get geared motors without a clutch like the Panasonic on the KTMs and some early Chinese ones, so your question is valid.

 

[Sylvester]

You can if there is no active freewheel, although the composite planetaries would take too much of a hammering for my liking.

The presence (or not) of a freewheel applies equally to geared and direct drive hub motors, and the durability of the gears is just a matter of appropriate design.

 

[flecc]

Slightly confused with that statement - ratings being almost meaningless nowadays. With a DD you can throw more power at it when needed. Also on the flip side, a DD totally outclasses geared when you're up to cruising speed.

Put it this way then, for a given current consumption a geared motor outclasses a DD on hill climbing.

I agree about the attractions of a DD when at cruising speed, and I think their advantages in a flat or unchallenging area make them the best choice then.
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