Start-Stop Batteries for Hybrid Electric Vehicles

Start-Stop Batteries for Hybrid Electric Vehicles

Overview of Mild Hybrid Vehicles and Start-Stop Batteries

The automotive market, which had been experiencing sustained sales growth, suddenly contracted after 2009. Production of electric vehicles began to increasingly focus on the “green” sector of hybrid electric vehicles and pure electric vehicles. Installing a start-stop system using conventional Starter-Lighting-Ignition (SLI) batteries demands exceptionally high charge acceptance and energy density, while delivering superior starting performance compared to standard SLI batteries. This approach reduces CO₂ emissions, yet such systems still fall short of meeting government emission targets. To approach an acceptable cost level, more functions are being transferred from the engine to the battery. This fundamentally demands batteries capable of delivering greater power and energy output reliably, while also operating at high rates of charge/discharge in a high rate partial state of charge (HRPSoC). Lead-acid batteries used in such conditions exhibit shorter lifespans, compelling EV manufacturers to adopt nickel-metal hydride (NiMH) and lithium-ion (Li-ion) batteries.

To maintain and expand lead-acid batteries' strong market position, new batteries with enhanced energy, power, and longevity are needed. The replacement trend for SLI batteries continues to grow, while vehicles equipped with start-stop systems gain an additional 3%-8% CO2 emission reduction benefit. “Mild hybrid” and “full hybrid” electric vehicles achieve greater CO2 reductions (15%-40%). Plug-in or “full hybrid” electric vehicles are more environmentally friendly. Projections indicate that sales of “mild hybrid” electric vehicles, capable of reducing CO2 emissions by 10%-20%, will continue to grow through 2020-2025. Conversely, “full hybrid” or plug-in hybrid electric vehicles will utilize nickel-metal hydride and lithium-ion batteries. Mild hybrid electric vehicles still primarily rely on lead-acid batteries. This presents a rare and significant business opportunity for high-performance lead-acid batteries. Over the next 30 years, whether advanced lead-acid batteries can become the primary energy source for electric vehicles hinges on their ability to deliver long lifespan and high performance.

Several meetings of the Advanced Lead-Acid Battery Alliance have discussed lead-acid batteries.The causes of short lifespan and failure mechanisms, along with effective industrial solutions to address this issue, have emerged over the past decade. The most significant approach involves adding carbon additives to the anode to prevent sulfation induced by HRPSoC conditions. Super batteries featuring carbon-based high-capacity negative electrode plates from Furukawa and East Penn have emerged. Manufacturers in Asia, Europe, and the United States have incorporated high-surface-area carbon powder into negative electrode active materials. While this approach extends lifespan, other performance parameters remain unchanged.

Lead-acid batteries utilize only 35%–40% of their theoretical active material capacity, which is the primary reason for their relatively low specific capacity and specific power. In advanced lead-acid batteries, increasing active material utilization offers significant potential to boost specific energy and specific power (W-h/kg, W/kg) by 2–3 times. Here, a successful example is provided: selecting advantageous grids and applying bipolar design to enhance specific energy and power.  

Performance Parameters of Bipolar Electrodes and Improved Grids  

Adding carbon additives to advanced lead-acid batteries extends beyond merely incorporating them into the negative electrode active material composition. Advanced forms of carbon can replace the grid metal lead. Carbon grids combined with suitable lead paste exhibit remarkably superior cycle stability and durability, comparable to nickel-metal hydride and lithium-ion batteries, as illustrated in the chart above. 

Lead-acid batteries used in hybrid electric vehicles exhibit capacity premature aging. Additionally, corrosion and maintenance incur extra costs. The primary cause is the operating mode: batteries are used not in float charge but in a partially charged state with high-rate charge/discharge cycles. This leads to sulfation of the negative plates, causing capacity decay and reduced lifespan. Nevertheless, it is certain that lead-acid battery performance is steadily improving. Many new designs can meet the most demanding modern application requirements. Further research will ensure lead-acid batteries remain the best-selling chemical power source.