EV
Battery electric vehicles replace the engine with batteries, inverters and motors. Learn EV architecture, road-load and energy consumption, battery packs (cells, series/parallel, kWh), motors and inverters, charging, thermal management and range estimation.
Electric vehicles convert 85–90 % of battery energy into wheel work, compared with 25–35 % for combustion engines. They deliver instant torque, regenerate braking energy, and have far fewer moving parts.
Mechanical engineers design EV packaging, battery structures and cooling, drive units, NVH and vehicle efficiency — every watt matters for range.
An analogy
An EV is like a big, rechargeable power tool on wheels: battery → controller → motor → gearbox → wheels.
EV architecture
Battery pack (hundreds to thousands of lithium-ion cells) → battery management system (BMS) → inverter (DC→AC) → electric motor (PMSM or induction) → single-speed reducer and differential → wheels. On-board charger for AC, DC fast-charging directly to the pack, DC-DC converter for 12 V systems, and a thermal system with coolant loops and heat pump.
Where the energy goes
At city speeds rolling resistance and acceleration dominate (regen recovers much of the latter). At highway speeds aerodynamic drag (∝ v²; power ∝ v³) dominates — why EVs have smooth underbodies, low C_d (≈ 0.2–0.3) and why range drops on motorways. Heating and cooling can take 1–5 kW.
Battery packs
Cells (cylindrical, prismatic, pouch) connect in series for voltage (400 V or 800 V class) and parallel for capacity. The BMS balances cells, estimates state of charge/health and protects against over-voltage, over-current and over-temperature. Packs are also structural (cell-to-pack, cell-to-body) and crash-protected.
Charging and thermal management
DC fast charging (50–350 kW) is limited by cell chemistry, temperature and voltage — charge curves taper above ~60–80 % SoC. 800 V architectures halve current for the same power. Liquid-cooled plates keep cells at ~20–40 °C; heat pumps recover waste heat for cabin heating, preserving winter range. Thermal runaway protection is critical for safety.
Formulas
- Road load:
F = m g C_rr + ½ ρ C_d A v² + m g sin θ + m a - Power at wheels:
P = F · v - Consumption:
E/km = F · 1000 / (3.6 × 10⁶ · η_dt) [kWh/km] - Range:
range = E_b / (E/km) - Pack energy:
E = n_s V_cell × n_p Ah_cell / 1000 [kWh]
Worked example: Consumption and range at 100 km/h
Given: m = 1800 kg, C_rr = 0.01; C_d = 0.28, A = 2.3 m², ρ = 1.2; η = 0.85, usable 75 kWh
Find: Road load, power, consumption and range.
- Rolling = 176.6 N; drag = ½ × 1.2 × 0.644 × 27.8² = 298.1 N → F = 474.7 N
- P = 474.7 × 27.8 = 13.2 kW at the wheels
- E/km = 474.7 × 1000/3.6 × 10⁶/0.85 = 0.155 kWh/km
- Range = 75/0.155 = 483 km
Answer: ≈ 15.5 kWh/100 km, ≈ 480 km
Worked example: Pack sizing
Given: 96s46p of 3.7 V, 4.8 Ah cells
Find: Voltage and energy.
- V = 96 × 3.7 = 355 V
- Ah = 46 × 4.8 = 220.8 Ah
- E = 355 × 220.8 = 78.4 kWh (≈ 4416 cells)
Answer: 355 V, 78.4 kWh
Common mistakes
- Wrong: Quoting nominal instead of usable battery capacity. — Right: Use usable kWh (buffers at top/bottom SoC).
- Wrong: Assuming range is independent of speed and temperature. — Right: Drag (∝ v²) and heating loads reduce range substantially.
Applications
- Passenger EVs: Skateboard platforms with structural packs and integrated drive units.
- Commercial & buses: Urban buses, vans and trucks with depot or megawatt charging.
- Two/three-wheelers: The largest EV segment by numbers in many countries.
Key takeaways
- Road load = rolling + drag + grade + inertia.
- Consumption ≈ F/η; range = usable kWh / consumption.
- Series for voltage, parallel for capacity.
- Thermal management governs charging speed and life.
Going further
- Cell chemistries: NMC for energy density, LFP for cost and cycle life, sodium-ion and solid-state as emerging options.
- Motor types: PMSM (efficient), induction (no magnets, robust), wound-rotor synchronous (rare-earth-free).
- Vehicle-to-grid: Bidirectional charging lets EVs support homes and the grid.