Regenerative braking
The alternator recovers energy during braking and uses it to charge the battery. Reduces fuel use and brake-pad wear.
Regenerative braking is the ability of a vehicle to recover kinetic energy during braking by turning the alternator (or the electric motor, in hybrids/EVs) into a generator that charges the battery. In combustion cars fitted with a BMS (Battery Management System) — from the late 2000s in premium brands and from the mid-2010s in mainstream cars — the alternator raises its voltage during braking (typically up to 14.8-15 V) to inject a high current into the battery; during acceleration it lowers the voltage (12.8-13.5 V) reducing engine load and fuel consumption.
This subjects the battery to deep, repeated charge/discharge cycles that only AGM tolerates without degradation. In hybrids and pure EVs the system is much more sophisticated: the electric motor itself is reversed to brake the car and recharge the high-voltage traction battery (300-800 V). Driving that takes advantage of regenerative braking (anticipating stops, avoiding harsh braking) can further reduce consumption by 5-10%.