|Brake Vacuum Reservoir, mounts to bottom of battery rack.|
Almost any modern car has power brakes, and this system needs to be preserved when the car is converted to an EV, since the net weight of the car will go up during the conversion. Obviously, the brakes in a car need to be absolutely reliable and up to the challenge. In an EV this presents a problem due to the added weight, and due to the fact that the power brake's original source of assist energy (engine vacuum) is gone. Note that working brakes are especially important with an EV since one source of emergency braking (engine compression, such as what you experience when you downshift and then let off on the gas) is gone with an EV, you get no such thing from an electric motor, unless you have a regenerative braking system.
Another thing to think about is that since the car is heavier as an EV, if it is possible to install larger brake pads or other heavy duty or performance brake components, it would not be a bad idea. Bigger brakes often came on cars or trucks with tow packages, or with heavy duty options, or bigger engines. Do your research, and figure out what might be available for your car.
The braking system in my Toyota MR2 EV is unmodified from the original gas configuration, with the exception of the fact that I have installed "performance" rotors and brake pads. These are common fare on Ebay, and don't cost any more than the stock items do from the local auto parts store. These supposedly run much cooler than stock rotors do, increasing braking performance.
What is different about the power braking system on a typical EV is not the system itself, but instead where it derives the power for the brake booster. The source of power for the brake booster on virtually any modern car is engine vacuum. This is not available on an EV, but it can be easily replaced by installing an electric vacuum pump which will feed the brake booster.
A vacuum pump is basically the same thing as an air compressor pump, but it is valved to suck air, instead of blowing it. Ones that are appropriate for EVs can be found off-the-shelf from EV parts suppliers for a few hundred dollars. (See the EV Reference Material for more details.) In a typical car, the power brakes work off of a vacuum level of about 16 inches of mercury, so a similar level of vacuum must be provided by the pump.
It is possible to find a vacuum pump on diesel cars and trucks, since diesel engines do not provide enough vacuum to run power brakes. Unfortunately most diesel vacuum pumps are designed to be driven via a belt, but some are electronic.
Some Volvos, and probably other cars, have vacuum pumps that may be useable as well. In fact, the MR2 had one as part of its cruise control system, but it was too small to be of use for the brakes.
Finally, Hot Rod/Performance cars use them since their supercharged/turbocharged engines also produce very little vacuum. Websites that cater to the performance/racing crowd offer vacuum pumps, including some electronic models.
If you go the route of trying to scrounge a pump, make darn sure it has sufficient capacity to feed a power brake booster, (At least 0.1 horsepower motor (6 amps at 12v) for a small car) and adequate duty cycle (percent of time running) to avoid early failure. If the pump was designed to run a braking system to begin with, you should be fine. Avoid pumps that were intended only for intermittent use.
In an EV, the vacuum pump will typically have to run for several seconds for every few times you pump the brake pedal. The exact time and duration of the run will depend on how hard and often you brake, the capacity of the vacuum pump, and also on the size of the vacuum reservior that you install.
The original engine in a car created a lot of vacuum very quickly. This meant that it could very rapidly replenish the brake booster, which in a normal car only holds enough vacuum for one good stop. Without installing a very large vacuum pump, it is not possible to achieve the same amount of vacuum as quickly in an EV. Instead of doing that, what is typically done is to install a vacuum reservoir between the pump and the brake booster. This allows the pump to "charge up" the reservoir with enough vacuum to power the brake booster for several stops. This provides a safety margin so you can pump the brakes repeatedly without losing power, and it prevents the vacuum pump from having to run every time you tap the brakes. A larger reservoir means that the vacuum pump will not have to run as often, but when it does run, it will stay on longer.
A vacuum reservoir can be bought from EV parts suppliers, but is easy (and way cheaper) to make. Typically it will consist of a section of ABS (black plastic) sewer pipe with two end caps. A good size for a smaller car is 1-foot section of 4" pipe. For a truck or larger car, a reservoir of about double that size would be better.
The one in my car is an 18" section of 4" pipe. There is an inlet, outlet, and third hose fitting for a vacuum switch or to run any vacuum-operated accessories that the car has. I have chosen to install mine in the tub area of the front trunk of the car, hanging underneath one of the EV Battery Racks. This is a good, out-of-the-way place for it.
The vacuum switch is simply a sensor that turns on the vacuum pump when the level of vacuum in the reservior drops too low, and turns the pump back off once the vacuum reaches a higher level.
A good EV vacuum switch will turn on at a low level of vacuum such as 15 inches Hg, and turn off at a somewhat higher level, such as 20 inches Hg. Note that if the switch has no "hysteresis" (gap) between the off and on transitions, then the pump will turn on and off very rapidly and for very short durations. This is bad for the pump motor, and would be very annoying to listen to. Note also that sometimes a vacuum pump will have the switch built in. In this case, you need to be sure that the switch on and off settings are in the correct range for the application, or they need to be adjustable.
The only other consideration to worry about with the vacuum switch is whether or not it is rated to handle the motor load from the vacuum pump. If the units were sold together, then it is not anything to worry about, but if you found the switch in one place and the pump in another, then you need to make sure that the switch is rated to handle the voltage and current load (with adequate safety margin) from the vacuum pump motor. If it is not, you can always use the switch to control a relay that is big enough to turn the motor on and off. If your car is anything like mine, there won't be any shortage of extra relays to retask for such a purpose.
I am using an SSBC (Stainless Steel Brakes Corporate) electric vacuum pump kit for braking systems. The kit cost about $300 and included the pump, mounting hardware, vacuum switch, and relay. Refer to the EV Electronics page for the circuit diagram. The one issue I had was that clearance was tight enough that I could not fit the pump in the location I wanted while using the supplied rubber shock mounts, so I just bolted the pump directly to the car body. This will result in increased noise when the pump runs. If this becomes annoying, I will rework the mounting bracket on the pump motor to allow installation of shock mounts.
The emergency brake should still be there after you perform a conversion, and it ABSOLUTELY MUST be in good working order. An electric motor has no 'engine compression' so if your regular braking system fails, there will be no way at all to slow the car down. With low rolling resistance tires and an aerodynamic body, even a slight downhill slope could keep a car like the MR2 rolling at high speed.
Fancier production electric vehicles, such as the GM EV1 and modern high performance models all have regenerative braking capability. So do homebuilt conversions that use the more-expensive AC drive systems. What this is, is simply a system that uses the electric motor as a generator, to recharge the batteries as you slow down, or roll down a hill. It can also be considered a safety feature since it can be used as another method of slowing or stopping the car. It is possible to retrofit such a system to a DC system such as what I am installing in my Toyota MR2 EV, but I am not going to do this for now.