VS - Electrical Systems From Stop-Start to Hybridization

The early stage of regenerative braking in the main 12v battery

> 14 V

Recovery charge

14 V

Conventional charge

Recover

Optimize torque demand

Charge

12 V

Low charge

Coasting / Braking

Acceleration

Driving

On the early generations of i-StARS some energy recovery features were implemented to improve the battery State of Charge (SoC) and enlarge the Stop- Start function availability. Though this feature was limited to some particular conditions to avoid too high currents and stress to the battery. The aim of the system was based on the adaptation of the alternator charging voltage in order to: ●● Gain in battery charge level during coasting and braking phases ●● Reduce fuel consumption in acceleration phases To do so, the alternator setup is tuned according to the situation: ●● In acceleration: the charge voltage is reduced to limit the torque demand on the belt drive and thus on the engine. As a second effect to fuel consumption reduction this permits some better acceleration. ●● In coasting and braking: the charge voltage is increased to recover more energy in the battery benefiting from the “offered” torque on the belt drive. ●● In driving: the charge voltage is set to a nominal value.

Conventional lead-acid SLI batteries are not compatible with energy recovery systems

Recovering energy during coasting or braking situations generate transient peaks of power that must be absorbed by the storage element (battery or alternative device), conventional batteries are not designed for this purpose and would overheat if submitted to such recovery charging cycles. Improved lead–acid batteries as Enhanced Flooded Battery (EFB) or Absorbent Glass Mat (AGM) allow modest energy recovery, they are used for Stop- Start functions, nevertheless they cannot be used for higher energy recovery cycles, their charge impedance (internal resistance) is still too high limiting the ability to absorb and store transient energy peaks. Energy storage is one of the most challenging topics in recent automotive electrical architectures, storage packs evolved from conventional SLI batteries (Starting-Lighting-Ignition) to EFB, AGM, then to higher voltage in NiMH, Lithium-ion batteries and forthcoming technologies.

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