Our mechanical engineer continues to talk about the nuts and volts of batterypowered automobiles in the second of a two-part article.
THE most suitable type of batteries for electric vehicles (EVs) today is lithium-ion.
They have the highest specific energy among all batteries, good hightemperature performance and easy recyclability of components.
Depending on the power control system, an EV’s batteries can be recharged (when the vehicle is stationary) using a pluggedin grid supply of singlephase 240V or three-phase 415V. Grid-based charging is necessary for EVs, except those that run on hydrogen fuel cells.
On the move, EVs have the capability to recharge their batteries to some extent. This is achieved by what is known as “regenerative charging”, which happens during braking or coasting/ slowing-down.
Unlike the internal combustion engine, whose energy flow is unidirectional (fuel to wheels), an EV can reverse the direction of its energy flow, from wheels to batteries.
This is because, when an electric motor is driven by any external power, it behaves like a generator to produce electricity. While coasting, for instance, the wheels are driving the motor.
The EV’s power controller enables this function every time the driver applies the brakes or releases the accelerator pedal.
Contrary to popular belief, there is no mechanical link between the braking system and the electric motor. Instead, a signal from the brake pedal commands the system to switch its role from motor to generator and start charging the batteries.
Any generator experiences “load” to produce electricity, and this gives the braking effect in an EV, akin to engine braking in a conventionally powered vehicle.
For acceleration and drive, the power controller manages the energy flow from battery to motor in proportion to the driver’s accelerator pedal input.
Therein lies another critical aspect of the electric motor: maximum torque is available from zero rpm.
Hence, a multiple-ratio reduction gearbox is not required. With the power controller modulating motor speed, an EV requires just one fixed ratio to provide the necessary road speed (motor speeds can reach 15,000rpm), since maximum torque is always available.
The engine of an EV is, of course, the electric motor.
Its construction is fascinatingly simple. The only moving part is the rotor, and the many “missing parts” include crankshaft, camshafts, valves, spark plugs and radiator. No engine lubricant is needed.
And since the motor stays dormant until the driver demands motion, there’s no need for a clutch either.
Any periodic/regular maintenance required is therefore centred around the suspension, which would be similar to any passenger car’s, and the brake pads/discs, which would wear down at a reduced rate because of the regenerative braking.
The batteries of an electric vehicle also don’t require periodic maintenance or any special treatment.
For electric cars, the electric motor is the “engine” and lithium-ion is the “fuel”.
