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Synergic cold-cranking of the 48 V – 12 V battery system in mild HEVs

IP.com Disclosure Number: IPCOM000245217D
Publication Date: 2016-Feb-18
Document File: 2 page(s) / 202K

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Synergic cold-cranking of the 48 V/12 V battery system in micro-HEVs

The new 48 V battery power-net provides the micro- or mild- hybrid vehicles (m-HEV) with improved fuel economy, drivability, and functionality at a minimum increase in costs, in comparison with full hybrid vehicles, which typically require a large battery pack with a high voltage of hundreds volts. Therefore, the electrical architecture and powertrain structure of the
48 V-based mHEV will be dramatically different from the prevalent high-voltage full hybrids.

Besides the conventional combustion engine and powertrain in mHEV, the 48 V battery pack works with an integrated-start-generator (ISG) and a secondary 12 V battery. The ISG is designed to absorb kinetic energy by converting braking heat into electricity and either stored in the 48 V battery, or used to propel the vehicle by the electric motor. Meanwhile, the ISG/battery system is expected to not only power other high electric loads, such as electric condenser, turbine, driveline stabilizer, but also re-start the engine. The 48 V battery pack is usually made with lithium-ion cells to meet the long-life and high-power requirements. Whereas the 12 V low- voltage battery is used to power lower loads such as, radio, lights, small auxiliary motors, etc. In order to reduce the overall cost, the 12 V battery is commonly automotive lead-acid type, and typically installed at under-hood locations, which is normally might be too hot for Li-ion cells. Due to the high cost, the Li-ion cells are expected to have the same life as the vehicle; while the
12 V lead-acid battery has to be replaced regularly because of its intrinsic internal corrosion.

One major feature of the mHEV system is that the 48 V battery replaces the traditional 12 V battery to crank the resting engine when the vehicle stops at traffic lights, and power other demanding electric loads. The new 48V battery is required to provide the power for engine cranking up to 150k start up cycles, and withstand millions of powerful regenerative braking and accelerating events without battery replacement over the mHEV life (i.e., 10 years or 150 k miles). At nominal operating temperature, the 48 V battery can meet the cranking/high power load requirements.

However, at extremely low temperatures, the 48 V Li-ion design has an inherent challenge, because the Li-ion battery may not be able to provide sufficient power capability to crank the engine, or recuperate braking energy, or boost acceleration. Extended operation at low- temperatures can also prematurely damaged the Li-ion cells. Secondly, the new ISG is expected to operate in an alternator mode for a significant amount of time during driving, similar to what regular 12 V alternators do. This alternator mode tends to keep the battery in a high state of charge (SOC) during driving. Lead-acid batteries prefer sitting at higher SOCs. On the contrary, sitting at higher SOCs can...