Regenerative braking is not only a car term, but a method of braking an electric motor and “regenerating” the battery.
It’s used in cars, saws, trams, trains, and almost anything else with an electric motor and battery system. Energy can’t always be fed back into a battery, and that’s what’s called dynamic (or rheostatic) braking.
We’ll get to that and more soon, though. Keep reading to find out the whats, hows, and whys of regenerative braking!
The Law of Conservation of Energy and Regenerative Braking
To know what regenerative braking really is, we need to delve into the law of conservation of energy.
No, it isn’t talking about green energy. It does help to conserve wasting usable energy, though. Often, being more efficient does save or even recover waste, which does lead to being green.
The law of conservation of energy is the main reason that we can exploit regenerative braking. This law basically boils down to the fact that energy is not created or destroyed. Instead, whatever energy is being “used up” is really only converting into another form.
There aren’t many methods for motor braking, but regenerative braking is best used in transportation. That’s because the kinetic energy in your wheel that’s generated by the electric motor goes back through the system as you brake.
It’s Getting Hot in Here
Unfortunately, whenever you attempt to harvest energy as you convert it, it isn’t a 100% efficient process.
Taking kinetic energy and transforming it into electricity is the basis behind hydropower, wind power, nuclear power, or even steam power. They are all based on a turbine pushing against an electric motor.
This creates electricity and heat. Heat increases resistance in an electrical system, and resistance causes a build-up of heat.
So, then, the three types of energy we’re dealing with are kinetic, thermal, and electrical. The trick we’re going for is to maximize the transfer from kinetic to electrical energy when braking and electrical to kinetic when accelerating. Heat is the enemy.
How Regenerative Braking Works
When a force goes against the direction of the force of the motor and exceeds the power of the motor, that force has turned your motor into a generator.
Remember how we said resistance generates heat, and that’s called “dynamic braking”? That’s how you deal with your motor turning into a generator if you don’t have a battery or other electrical sinking system.
When you brake using brakepads, your disc rotor and brakepad press against each other. That pressure converts the kinetic energy in the wheel into heat. That disc cools in the air, especially as the wheel spins, to dissipate the energy into the atmosphere.
In a dynamic braking system heat is released through heatsinks, resistors, or both. A regenerative system may not be able to completely recharge the batteries at the same speed they are generating electricity. In that case, the excess electricity is passed through to — you guessed it — heatsinks and resistors.
Some regenerative systems will pass electricity into a capacitor or into a battery, depending on the intent of the reclaimed energy. In stop and go traffic a hybrid vehicle may get better mileage from capacitors because the car needs to overcome its inertia of rest
Limitations of Batteries in Regenerative Braking
Batteries often have a higher discharge C-rate than charging C-rate. 1C is a one-hour discharge. This isn’t to be confused with Amperes, but rather is the discharged compared with the battery’s capacity.
Li-ion batteries will break down at high temperatures, storage in high voltage, or both. As you charge batteries internal resistance builds up (but not as high as in lead-acid batteries) which, as you remember builds heat, which increases resistance.
Li-ion battery cells can safely discharge to about 80% but can absolutely not accept overcharge. Most LFP Li-ion batteries can handle a .5 to .8C charge rate. That’s two or more hours. A 1C charge rate would be 0% to 100% in one hour.
It would be an absolutely wonderful technology if NMC Li-ion (Nickel Magnesium Cobalt Oxide) batteries could be in every vehicle. Unfortunately, it’s a technology that is just emerging in the market.
When that happens, less electricity will be released as heat and can be recycled. However, conversion will never be 100%.
That being said, Tesla superchargers recharge at a rate of 150 kWh. If two Teslas hook up at the same charger, it shares that load across both cars. If we assume the average Tesla has an 85 kWh battery, it needs 68 kWh to fill (20% up to 100%) which can be accomplished in 27 minutes.
That’s actually a 1.75C rate of charge. The battery at full load (pedal to the floor) drains the batteries at about 4.5C. The magic is in the way Tesla battery systems distribute electricity across the roughly 7100 Li-ion cells in the battery pack.
That being said, Tesla is at the forefront of battery technology, including “tabless” batteries. Tabless batteries are in actuality batteries with hundreds of tabs.
A Car Term That Matters For More Than Cars
When it comes to a car term you can use almost anywhere, regenerative braking will be one of them. It isn’t just used for braking a car, but any electric motor.
A typical EV battery can handle regenerative braking (depending on the vehicle power, braking speeds, etc.). Not all systems, however, can. In those cases, dynamic braking may be an option.
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