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Clever Cooling and BMSes Drive Longer Range EVs

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Last year, when Mercedes-Benz’ VISION EQXX electric vehicle (EV) completed a 1,202 km (747 mile) journey on a single battery charge, it was a demonstration of how far designers could push an EV’s range with good thermal and battery management.

The car completed a trip from Stuttgart, Germany, to Silverstone, England, amid summer temperatures of up to 30 degrees Celsius (80 degrees Fahrenheit), and increased traffic density (impacting energy usage) around Stuttgart and in the southeast of England.

The 1,202 km route taken by Mercedes-Benz’ VISION EQXX electric vehicle (EV) on a single battery charge, from Stuttgart, Germany, to Silverstone, England. (Source: Mercedes-Benz)

Mercedes said the innovation that helped them achieve this was exceptional efficiency of the electric drive unit (EDU), which means it generates only minimal waste heat. This is a result of carefully engineered interaction of aero-shutters, coolant valves and pumps to ensure the EDU maintains the most efficient temperature balance at minimum energy cost.

A combination of innovative air-flow management and a cooling plate installed in the vehicle floor meant it was able to take advantage of the air flowing along the underside of the car. This, Mercedes said, is the most aerodynamically efficient way of keeping the EDU cool under normal conditions, allowing an increase in range of around 2% in the most aerodynamic mode.

Another aspect was clever cooling of the electric drivetrain and passenger cabin to take into account the high ambient temperatures and stop-and-go motorway traffic along the journey. This is where the on-demand cooling system helped—it meant there was no significant impact on range. That is because the multi-source heat pump in the VISION EQXX was efficient at keeping the cabin temperature cool in the warm summer weather. During the 14 hours and 30 minutes of driving time, the air conditioning was operational for just over eight hours, yet had a minimal negative impact on overall energy consumption.

Targeting 95% powertrain efficiency

Simon Cawthorne, head of hybrid technology at Mercedes AMG High Performance Powertrains (HPP), explained last year at the Goodwood Festival of Speed some of the challenges in achieving the efficiency that enabled the VISION EQXX to get to an average consumption figure of 8.3 kWh per 100 km on a single charge. Their target powertrain efficiency was to achieve better than 95% from battery to drive shaft.

The Mercedes-Benz VISION EQXX at the Goodwood Festival of Speed 2022. (Source: Nitin Dahad)

This was achieved by tapping into Mercedes’ Formula E experience and AMG Project One and Formula One, combining it with new technology the company hadn’t used before, and packaging them all together to achieve that 95% increase in efficiency. A key part of this was the collaboration with partners, such as onsemi and Analog Devices (ADI), for the powertrain and battery management, respectively.

The VE-Trac silicon carbide (SiC) modules from onsemi increase the efficiency and lower the weight of the VISION EQXX’s traction inverter, extending the vehicle’s range by up to 10%. Onsemi said the high efficiency of its SiC solutions allow customers to avoid trade-offs between the cost of the battery and the range of the vehicle.

“We worked on optimizing die parametrics all the way to process parameters so that Mercedes could get the most out of their device [the SiC module],” Asif Jakwani, senior vice president and general manager at onsemi, told EE Times. A core piece of differentiation that onsemi provided was its expertise in packaging, which is critical for improving heat dissipation and increasing power output at a smaller footprint than other devices, as well as reducing the weight and cost of a power module.

Earlier this year, onsemi said its SiC power modules were also selected for Kia Corporation’s EV6 GT model—an EV that accelerates from zero to 60 mph in 3.4 seconds and reaches top speeds of 161 mph. Within the traction inverter of a high-performance EV, the onsemi EliteSiC power module enables high-efficiency power conversion from the DC 800 V of the battery to the AC drive for the rear axle. The high-density power module’s packaging technology minimizes parasitics and thermal resistance, as well as offers package reliability using innovative interconnects. This leads to reduced power losses associated with DC to AC conversions, along with reduced size and weight of the traction inverter, increasing performance and EV range by 5%.

Active balancing for better battery health

Mercedes’ Cawthorne added that active balancing technology in the battery system was also a key enabler of the efficiency achieved. “EQXX is the first car to run active balancing, and we’ve had some really good benefits seen already with that system. Beyond just getting it working,” he said.

When we asked ADI about their partnership with Mercedes, a company spokesperson said the company didn’t disclose which solutions it offers for specific customers, but very often these are customized solutions. The company did, however, give an overview of its solutions for thermal management systems for more efficient EVs—more specifically, active balancing.

A high-accuracy, high-efficiency battery management system (BMS) is an integral part of enabling superior thermal management capabilities, including addressing both the charging and the discharging aspects. This can help to deliver both longer range per charge, as well as longer battery life and improved battery pack health. In turn, this can lead to drastic improvements to the efficiency of the thermal management system, helping push the limits of powertrain efficiency and maximize driving range under various driving conditions.

For this, ADI offers its integrated active cell balancer technology. The company describes this as enabling benefits analogous to avoiding fuel spillage while filling up the gas tank. The active cell balancer technology manages the flow of energy from the battery using a bi-directional flyback converter. It takes energy out of the healthy cells and redistributes it across the less healthy cells in a very efficient manner.

The mechanism thus enables all cells to be discharged at the same rate, without sacrificing vehicle range. Since the batteries are not balanced by burning excess energy as heat, they don’t need to be charged as much, so the batteries don’t age as quickly. This compares to more traditional passive balancing that results in all battery cells having a similar state of charge (SOC) by simply dissipating excess charge in a bleed resistor—it does not however, extend system run time.

Both active and passive cell balancing are effective ways to improve system health by monitoring and matching the SOC of each cell, and the selection depends upon the application.

Another way of looking at active balancing is this: a typical battery stack will start with all cells starting at full capacity, but over time, some cells will become weaker than others, resulting in an uneven discharge profile. Moreover, even though there may be quite a bit of capacity left in several batteries, the weak batteries limit the runtime of the system.

A battery mismatch of 5% can result in 5% of the capacity unused. With large batteries, this can be an excessive amount of energy left unused. This becomes critical in remote systems and systems that are difficult to access since it results in an increase in the number of battery charge and discharge cycles, which reduces the lifetime of the battery, leading to higher costs associated with more frequent battery replacement.

Over time, some cells will become weaker than others, resulting in an uneven discharge profile. A battery mismatch of 5% can result in 5% of the capacity unused. (Source: Analog Devices)

With active balancing, charge is redistributed from the stronger cells to the weaker cells, resulting in a fully depleted battery stack profile.

Range is key to EV adoption

From a consumer perspective, one of the biggest challenges for adoption of EVs over recent years has been range anxiety. The powertrain teams on the Mercedes Vision EQXX, along with their semiconductor solution partners, have managed to demonstrate how they can work together to optimize thermal management and battery management to hit both targets, in terms of efficiency and thermal management.

As these technologies and techniques trickle down into more mass market vehicles, they could go a long way to helping overcome consumers’ range anxiety.

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