Regenerative braking [tour 8]

Regenerative braking is a way of diverting the kinetic energy of a moving motor / vehicle back into the battery so that it re-charges the battery and provides a braking effect.

1, 2 and 4 ‘quadrant’ controllers

Some controllers can only supply (source) current to power the motor. These are ‘single-quadrant’ controllers.

Other controllers can also sink current which is generated by the motor when it runs faster than the speed set by the controller. These are ‘2 quadrant’ controllers.

A ‘4 quadrant’ controller is like a two quadrant but can also reverse the motor to give the extra quadrants.


What is regeneration

If the controller can sink, as well as source current, then it will give regenerative braking (or regen for short) – regenerative since the braking energy is fed back – re-generated – to the battery.

Of course, for the controller to sink current the motor must be a type that can source it – some motor types cannot.


When does regeneration occur

The process of regeneration is entirely automatic and occurs when the motor over-runs (or the demand speed is reduced) so that the motor’s back EMF is greater than the output voltage from the controller.

It is best treated as braking (which just happens to feed the braking energy back to the battery) rather than a means of saving lots of energy. This is because most motors and drive systems are inefficient and you won’t save a lot of battery! However there are applications – such as Go-Karts with the Lynch motor – where the energy re-generated can make a significant difference to the battery life.

It is actually the battery which is doing the braking, not the controller, since the braking energy gets dumped into the battery. For this reason regen braking should not be considered as a safety or emergency braking system. A properly designed vehicle will also have an adequate mechanical brake, for emergencies.


When does regen braking not work

There is one unusual situation where you have to be wary with regen braking: what happens if, for instance, you live at the top of a hill and charge your battery there so that it is full when you start off? The controller can only overcharge the battery and its operation in this condition will not be guaranteed. However – lead acid batteries do in practise stand significant overcharging. However overcharging is is not good for the battery, so such situations should generally be avoided!


I don’t understand!

The best way of thinking about 2 quadrant control is that, normally, the controller is supplying a voltage to the motor to drive it, and the motor is generating a back emf, proportional to its speed, which mostly cancels out the drive voltage.

If the motor now goes faster, its back emf rises and the current (caused by the difference between the controller’s output voltage and the motor’s back emf) falls. If the motor rotates fast enough, the motor current falls to zero as the back emf then equals the controller’s output. Now, if the motor rotates even faster, the current must go negative (feeding back into the controller) as the back emf is now greater than the controller’s output voltage. If the controller can accept this current being fed back into it, then braking starts to occur. The controller has to do something with this current. Crude designs simply dump it as resistive heating but it is more efficient (and not difficult) to feed the current back into the battery.

If you are technical enough to understand circuit diagrams, there is a detailed description of how regen braking works in our circuits archive.


Disabling regen braking ‘on the fly’.

We sometimes get asked for a controller where regen braking can be switched on and off. In most applications there is little benefit in disabling braking. However some controllers can be supplied with the feature disabled. On some controllers it can be enabled/disabled as a user on-board option.

This feature must not be switched on/off during active use:

  • Switch regen OFF when running – no problem.
  • Switch regen ON when running. This is not permissible because you would need to switch regen back on when the motor’s back emf was exactly equal to the controller’s output or a large jerk would result. It is simply not practical to do this by hand!

Can I vary the regen braking?

Regen braking is indeed variable. In the same way as you adjust the acceleration (power to the motor) by advancing the speed control, you adjust the braking (power from the motor) by reducing the speed.

There are adjustable acceleration and deceleration ramps which vary the maximum acceleration and deceleration rates.


Can I coast?

If you hold the speed so you are neither applying power to the motor nor braking it, you will indeed coast: motor current will be zero and the controller will be doing no work. Of course, if you want to coast down a hill, you will need to gradually speed up the motor to match road speed. Conversely to coast up a hill you would need to reduce speed to keep the motor current zero, but why would you want to do this? Why does a small amount of drive or braking matter?

What the controller does not have (as it does not require it) is separate acceleration and brake controls. To do this would actually be possible but would add greatly to the controller’s complexity – because it would have to measure the motor current and hold the output voltage exactly to keep the motor current at zero. A somewhat pointless expense as you can do very much the same thing on the single speed control!


Can I disable regen braking?

Some controllers (see controllers list below) can be altered so regen braking does not occur. However in generally we do not encourage this as there are very few applications where regen braking is actually a disadvantage. Experience has shown that when a user wants to disable the feature it is because they do not properly understand how it works!


Can I control the battery re-charging?

No, you cannot.

In theory, a controller could either control the braking level or the battery charging level. However – braking is (in most applications) essential and must be controlled. Charging is secondary and occurs because the controller has to dump the energy somewhere. So there can be no control of the charging, only of the braking (which is under driver control anyway!).

In most applications, braking will only occur after energy has been already taken from the battery and, as there are always inefficiencies, more energy will be taken that will be recovered. Exceptions might be where the vehicle user lives at the top of a hill and starts downhill with fully charged batteries, or where solar, wind or other powered battery charging is also used. The first involves over-charging the batteries. The others would all involve bespoke design – of a charger that did not fully charge the batteries!

There is more on this subject in our faq section – Recharge under sail


How much energy will regenerative brakingrecover?

As far as the controller is concerned, virtually all of the energy it can get has to be returned to the battery – the controller simply has no way of dissipating this energy so must dump it.

However many gear trains are not efficient when driven backwards, so they will waste a significant part of the energy the motor generates.

The motor, also, isn’t designed as a generator, so may not be so efficient. Then charging a battery is never 100% efficient.

So sorry, we can’t give you an accurate answer to the question, but energy recovery can be quite significant.


What happens if there is no battery connected?

In this case, the controller still attempts to feed braking energy back to the battery. It does this by using the motor’s inductance to step up the voltage at the battery terminals. If there is no battery connected this voltage can rise to quite a high level (typically 47v for lower voltage controllers and about 68v for 48v controllers) at which point the controller stops regenerating. So there is no braking.


Controllers

  • The following controllers have regen braking.
    • Porter
    • Pro-150
  • The following controllers have regen braking (but it can be can be disabled by an on-board option – if required)
    • 4QD series
    • Pro-120 Mk 2
    • DNO series
    • Uni series
  • The following controllers have regen braking but can be can be supplied without (subject to minimum order quantity).
    • 2QD series details of change
  • The following controllers do not include regenerative braking.
    • 1QD series

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