The electric motor is connected to the vehicle’s original factory transmission by means of an adaptor plate and hub. The plate (and usually a spacer ring) physically attaches the motor to the transmission and precisely aligns the shaft of each with the other. The hub mounts to the motor’s drive shaft and transmits the power to the transmission drive shaft.
From the transmission, gearbox, or pulley, the power goes to the drive wheels in the same way it does in an ICE car: through a differential, a device that splits the engine torque and allows the wheels to spin at different speeds on corners, and then through the axles to the wheels.
When you turn the key in an EV, nothing seems to happen. You don’t hear the engine turn over and catch. What does happen—silently—is that electricity flows from the battery pack to this contactor, which serves as a gateway to the speed controller. The car is now ready to roll. When your foot depresses the throttle pedal, the contactor closes, allowing the electricity to flow to the speed controller. While the potbox tells the controller how much electricity should go to the motor, the actual power flows through the contactor, once it closes and makes the connection.
Because running out of charge is even less fun than running out of gas, every EV should have some type of “fuel gauge.” As in an ICE vehicle, a fuel gauge in an EV usually reads from zero (“empty”) to 100% (“full”). A voltmeter shows the exact voltage of your battery pack at any given moment; a state-of-charge meter shows amp-hours or watt-hours.
Amp-hour and watt-hour meters do not actually measure the charge of the battery pack. They are initially calibrated to “full.” From there, they monitor the electricity drawn out of the batteries by driving, as well as the electricity put back in by charging and regenerative braking. With that information, the meters calculate the vehicle’s current state of charge.
Another useful instrument is an ammeter, which is essentially an efficiency gauge that tells you how much amperage the motor is drawing at a given moment. Once you become accustomed to reading it, the ammeter can fine-tune the efficiency of your driving style by helping you choose the most efficient (lowest current draw) gear for your speed. It can also alert you to possible problems, such as a slow leak in a tire or dragging brakes that will cause higher-than-normal current draw.
Safety is key when working with electricity. That’s why all EVs should have at least one emergency disconnect to break the circuit and disable the system in the event of a collision or other emergency. Disconnects also come in handy when you want added safety while working under the hood.
For extra safety, redundancy is always a good idea. Having more than one disconnect is advisable, since different types are designed to respond to different emergencies. The standard mix of disconnects includes fuses, circuit breakers, and a “panic button” that breaks the high-voltage circuit. Some disconnects work automatically, while others are activated manually.
While an EV’s main drive system runs on higher voltage, the vehicle’s accessories, such the horn, radio, lights, and windshield wipers, run on 12 volts. In an EV, the DC/DC converter takes over the job of an ICE car’s alternator. The high voltage of the battery pack is tapped at a low amperage and converted to low voltage at a slightly higher amperage to power the accessories. For example, the converter may initially draw 144 V from the main battery pack at 6 amps. (Compared to the 100 A or greater draw the vehicle uses for cruising at a steady speed, this is a trivial amount.) It then puts out a regulated 13.5 V to 14 V at 25 A or more—the same output you get from an ICE car’s alternator. The converter may power the accessory system directly or it may charge a 12 V battery that is separate from the main battery pack.