Regenerative braking systems By Pete Melville, Hybrid and Electric Vehicle Repair Alliance, HEVRA We’ve got two separate challenges when it comes to braking systems for electric vehicles. The first is brake assistance, normally provided by the suction of the engine, or by an engine-driven vacuum pump, and with the absence of an engine, or an engine that runs periodically, this isn’t going to work. The second is regenerative braking – if we can do some of our slowing down without generating brake dust or wearing out the pads, and get a bit of free fuel at the same time, this is our preferable option.
TECHNICAL
However, regenerative braking isn’t enough on its own. When the battery is too full, too hot, or too cold, it can’t absorb energy at the rate we might need it to. Regenerative braking is not very effective at very low speeds, or for holding a vehicle stationary once it has stopped. It also can’t slow us down fast enough when we really need to slam the anchors on, and it has no provision for individual wheel speeds, which means no ABS or ESP, and with most cars having a single motor and open differential, a loss of traction on one wheel can mean virtually no braking effort.
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So, we need a system that combines the friction braking with the regenerative braking and provides some sort of brake assistance. And like most things with electric vehicles, there’s not one-system-fits-all.
System One: Electric vacuum pump The first of our three contestants is the electric vacuum pump. This provides a simple, cheap solution and a nice consistent pedal feel, and is used on the cheap and cheerful Renault Kangoo ZE right up to the high-tech BMW i3. Typically, a
"The Tesla Model 3 doesn't have a single physical replaceable 12v fuse, which means we need to rethink some of the test procedures we've followed for decades " pressure sensor in the servo will report the current level of vacuum, and the pump will be triggered as required. The major downside of this system is it doesn’t allow for any regenerative braking via the brake pedal. Most manufacturers allow for this by providing strong regenerative braking when the accelerator is released – the driver tends to only press the brake pedal when harder braking is needed. These systems are straightforward to service as hydraulically they are similar to traditional cars. When replacing the vacuum pump, there is often a drain tube which must be at the lowest point to allow moisture to escape. Some models also monitor pump life, and this must be reset using diagnostic equipment if the pump is replaced.
