I have frequently seen a Regenerative Braking Limited warning message pop up shortly after starting my electric car and wondered just why it’s being limited. I set out to research exactly what are the reasons why regenerative braking was limited and what we can do to prevent it.
Why is regenerative braking limited? The root cause of regenerative braking being limited is your battery. The limitation can be caused by:
- Cold Temperatures/conditions
- A fully charged battery, with no more capacity for a charge
- Too much energy to convert that would overload the battery.
To help fully understand why it is limited, we also need to look at what regenerative braking really is, and how it works concerning your battery.
How Does Regenerative Braking Work and When Does Limiting Come Into Play?
Regenerative braking is a component used by many electric and hybrid cars that takes kinetic energy from braking and slowing your vehicle and converts it into a voltage that can recharge your car battery.
At a basic level, we apply torque to a vehicle to make it move, and with regenerative braking, we apply reverse torque to slow the vehicle, in some cases, it’s possible to use that reverse torque to bring the vehicle to a complete stop.
In digging into the research on the mechanics of regenerative braking, I came across several sources where they show the actual amount of kinetic energy expressed as an equation E = ½mv². If, like me, you aren’t an electrical engineer, I needed to see that a little more visually and broken down to help make it a little easier to understand.
Looking at the equation broken down we have:
- E in the equation is your kinetic energy,
- M being mass or weight, and
- V being velocity or speed.
Without breaking this down into a physics class, the general idea is that more mass and add more speed, means more kinetic energy.
Car Weight (doubled) = two times the amount of produced kinetic energy. If we think about it from the standpoint of a vehicle with just the driver, compared to the vehicle with the driver and four adult-sized passengers. That additional weight means more kinetic energy produced in slowing the vehicle when the vehicle is full. Meaning the production of energy of a full car for regenerative braking is greater than that from a car with just the driver.
The other component is car speed. Car Speed (doubled with the weight doubled) = four times the kinetic energy. To apply that to our example, a vehicle that was moving at 20 mph will produce a smaller amount of kinetic energy than a vehicle that is moving 80 mph. Higher speeds do produce higher amounts of kinetic energy, but we will find that where regenerative braking is involved, this can sometimes be a reason that it limits our regeneration because it’s too much energy.
How is Regenrative Braking Different From Traditional Braking
In a traditional vehicle, the process of slowing down takes that kinetic energy produced and dissipates it to the brake pads, the road, and the environment as heat. In a vehicle that uses regenerative braking, that kinetic energy that is produced from slowing down is converted by the components of your car into an electrical charge for the battery mainly using applied torque.
If you haven’t experienced driving a vehicle with regenerative braking, you may wonder what it feels like. It can range depending on the manufacturer, to be honest. Some differences you’ll notice may be:
- Some high-end manufacturers apply regenerative braking in a way where you can basically drive the car with just the accelerator.
- Some newer manufacturers applying the mechanics behind the process may not feel quite as smooth and has more pull.
- Some manufacturers even allow you to switch between multiple settings to find your preference.
Based on how regenerative braking works, there are a couple different reasons that this process can be limited. Next, we’ll look closer at some of the reasons we looked at that regenerative braking may be limited.
Cold Weather Limits Regenerative Braking
I was disappointed to find that electric cars could have limitations in cold weather, much like my old traditional vehicle. However; when it comes to batteries and how they work, there is something to be said for making sure that your battery is a little warmer.
Cold Batteries struggle to recharge because the transition that is required to happen at the anode cannot happen in cold conditions. You can also encounter frozen condensation that can change how well the battery functions.
So, if you live in an area with freezing weather, how can you get around cold weather, limiting your regenerative braking options on your vehicle? While this is a known limitation with electric and hybrid vehicles, there are some ways that manufacturers are helping us solve the problem or that we can do on our own:
- Allow a lead time charge before leaving in cold weather, and make sure that you’re heating the cabin of the car.
- Heating the cabin area can help raise the battery temperature enough to help your regenerative braking kick in, it also has the added benefit of making your car much more comfortable to get in.
- Some models have the option for battery pack heaters, and some will use applied waste heat from the engine to help heat the battery area.
- Another feature that may be available is scheduling your departure so that you’ve had a chance to let your vehicle reach the optimal charge and temperature before leaving for your trip.
Too Much Charge for your Battery to Keep Up With
At a basic level, the type of battery that is used in most of these environmentally-friendly vehicles is lithium-ion batteries. One of the main limitations of this regenerative braking is that you cannot charge a lithium-ion battery faster than it discharges energy. Most of the batteries that are used in these types of vehicles charge at a rate of half that they discharge.
When we look at driving at a high rate of speed, for example, on the interstate at 80 mph, and when you encounter a sudden road hazard and need to break immediately, a significant amount of kinetic energy will be produced based on the rapid deceleration.
This amount of kinetic energy would be far too much for the regenerative braking system to handle and is the main reason that even on the environmentally friendly cars that have regenerative braking systems, traditional braking systems are also installed.
If, like me, you prefer the feel of the regenerative braking, one way to ensure you’re maximizing it is to make sure that you’re driving in a way that allows you to slow to a stop rather than slamming on the breaks.
We all know that it’s unrealistic to say you’ll never have a need to stop quickly, but doing things like leaving extra car lengths between you and other drivers can help reduce the number of times you’re having to slam on your breaks to slow your vehicle. Keeping the number of times you slam on your brakes limited helps you maximize your regenerative braking.
Your Battery is Already Fully Charged
Voltages in any battery have constraints, within the vehicle and use of lithium-ion batteries, that level of voltage will be something the manufacturer builds in and will often apply without any way for you to bypass for the obvious safety reasons. This is one of the main reasons anything using these batteries discourages overcharging.
Like most drivers who have experienced driving with and without regenerative braking, I prefer the feel and control of the car when it’s engaged. One way to help kick in the regenerative braking and avoid over-charging your battery is leaving enough room in your charge, something like a 10-20% buffer could be enough to kick in the regenerative braking.
Hopefully, with the knowledge of how regenerative braking works and a couple of ways to avoid those instances where regenerative braking is limited, we can make sure that we get the most out of our driving experience.