SO DANGEROUS!! Regen Braking Explained

I’d normally film this sort of video in my car, but with the insane temperatures outside, I think I’ll stick to the safety of indoors. Speaking of safety, driving the Mercedes EQS I can’t say I felt all that safe. The biggest reason for that was the brake pedal, because, and I wish I was kidding here, IT MOVES ON ITS OWN. You can be gently pressing on the pedal and IT WILL JUST MOVE AWAY FROM YOUR FOOT! I can’t describe how terrifying that is when you first feel it happen, especially since I was hooning the thing around Millbrook’s hill route. The reason for it moving is based in sanity, just about, and it’s called regen braking.

Regenerative braking isn’t exactly new, or all that complicated. BMW has had “efficient dynamics” on their internal combustion engine cars for a decade or more, and anyone who drives is plenty familiar with engine braking. In simple terms, when driving along in an electric vehicle, energy is flowing from the battery to the motor to make it spin, which in turn spins the wheels and the car moves forward. When you stop accelerating, the wheels start spinning the motor freely, which acts like a generator to send power back into the battery.

You can vary how strong the braking feel you get, and therefore how much energy you harvest, with a number of manufacturers offering flappy paddles on steering wheels – not for gears but for regen braking levels. Some manufacturers though have taken this a step further, and are using the brake pedal as a controller for how much regen braking you want. This is where the problem lies – so let’s talk about brake pedal travel.

In a normal, say, petrol or diesel based car, when you press on the brake pedal there is some amount of dead space before the brake pads start gripping the discs. You press the pedal until you feel the “biting point”, then depending on how far you press the pedal, the more pressure the pads put on the discs, slowing the car. What EV makers have start doing is modifying the pedal travel, so all that “dead space” (and a bit more) now works as a controller for how much regen braking you want, then once you pass the halfway mark the friction brakes kick in like normal, on top of the regen braking.

That means if you only press the pedal a quarter of the way in, you are getting no friction braking, and about half of the available regen braking. Just lifting off the accelerator only slows the car by what the manufacturer decides is a “normal” amount of engine braking – to get it to regen brake you have to press the brake pedal, but not so far you waste energy by using the friction brakes instead.

And this is the problem Mercedes have. They decided that, since the first half of the brake pedal travel is just regen control, if you have the car in a high regen mode already they would move the pedal to the bite point of the friction brakes so you don’t have a load of wasted travel on the pedal before actually braking. See, it comes from a place of logic, even if the result is absolute insanity.

This sort of design is created as a halfway house between just letting you use the regen braking as if it was strong engine braking, and keeping everything the same so in theory there is no learning curve. Personally I think this is a pretty horrible approach, especially since using EVs in their “braked” modes – the “one pedal driving” modes many vehicles tout – is not only easier but more efficient.

The other catch is the automakers seem to be under-sizing the friction brakes – that EQS felt like the second you stepped on the actual friction brakes, ABS had to kick in to stop them from just locking up rather than providing effective braking force. They felt worryingly weak, which isn’t what you want to feel when hurtling at a tight bend in a 2 tonne £150,000 monster. Not all makers are doing that, the BMW i4 M50 felt a lot more ‘conventional’ in its brake feel.

And I get why they are trying to shrink the brakes – they don’t get used nearly as much, plus the smaller the discs the less rotating mass needs to be spun up making for a much more efficient drive, plus that’s less unsprung mass too so even better handling and control. The catch is those friction brakes are badly needed for emergencies.

So, what’s the solution? In short: LEAVE THE DAMN BRAKE PEDAL ALONE! The learning curve for regenerative braking is not steep, and it’s one I feel people should learn to more effectively, safely and efficiently drive this new mode of transport. One pedal driving modes are easier to use, and if that means you leave the brake pedal alone, then that’s even better. Keeping the brake pedal for the friction braking alone simplifies the driving experience and I’d argue makes it safer.

Before anyone pipes up with the comment about the brake lights not illuminating when using one pedal driving modes, I have a simple solution for that. Your car already has multiple accelerometers onboard. Have the in car control module monitor the accelerometer looking for braking force, and when the braking force exceeds a set limit turn on the brake lights. Easy.

So in short regen braking is great, but EV makers are unsure if you can handle the slight change in driving style so are complicating the control system in a, I think, misguided effort to simplify it. I hope that sort of complication dwindles and the easier one pedal driving modes win out. With that said, those are my thoughts, but I’d love to know yours in the comments down below!