|Examples of degradation effects causing Li-ion battery power or capacity fading. Source: Roland Matthé, GM. Click to enlarge.|
With the Volt extended range electric vehicle and the Leaf battery electric vehicle now on the market, joining an ever increasing array of hybrids, and with next-generation versions of all of these already in the works, automakers and battery manufacturers provided some insight at the recent Automotive Advanced Battery Conference (AABC) in Pasadena into their learnings over the requirements for and development of advanced lithium-ion battery packs targeted at the different automotive applications.
Roland Matthé, GM technical manager for the Voltec battery system, provided an overview of GM’s views on the requirements and challenges for batteries specifically for extended range electric vehicles—i.e., the Volt—but also more broadly for batteries and electrified vehicle applications in general. Different applications require different types of cells, he noted.
|Key metrics of electrified propulsion systems (GM)|
(e.g., LaCrosse w/ e-Assist)
(E.g., Tahoe two-mode)
|Fuel cell hybrid electric|
|Pure battery-electric range||na||up to 2 km at speed < 50 km/h & low acceleration||up to 20 km at speed < 100 km/h & low acceleration||40 to 80 km, all speeds, full acceleration||> 100 km||up to 2 km at speed < 50 km/h & low acceleration|
|Total range||> 500 km||> 500 km||> 500 km||> 500 km||< 200 km||400-500 km|
|Battery energy||< 1 kWh||1 to 3 kWh||5 to 10 kWh||> 10 kWh||> 20 kWh||1 to 3 kWh|
|Battery power||< 20 kW||20 kW to 40 kW||> 50 kW||> 100 kW||> 100 kW||20 to 40 kW|
|Power to energy ratio||~20||~20||~7||~7||~4||~20|
|SoC window||< 20%||< 20%||< 70%||< 70%||< 90%||< 20%|
|Recharge time||na||na||1 to 4h||4 to 10h||10 to 20h
|Refuel time||< 5m||< 5m||< 5m||< 5m||na||< 5m|
In an earlier talk describing GM’s battery life estimation process, Joe LoGrasso, an engineering manager also with the GM’s Global Battery Systems Engineering Group, like Matthé, noted that customer expectations are an important factor to consider in establishing specifications relating to battery life and battery safety. In short, he said:
- Customers expect that batteries will last the normal life of the vehicle, that expensive replacements will be minimized, and that such service will be delayed until at least 10 years of battery life have elapsed, assuming normal usage.
- Customers expect xEVs with advanced batteries and high voltage systems to provide a level of safety comparable to that present in today’s vehicles.
Achieving the first requires predictive life models and adaptive vehicle control, he noted. Achieving the second requires a comprehensive system approach to battery safety at system, pack and cell level.
The battery pack for an extended range electric vehicle such as the Volt—which runs in an all-battery powered charge-depleting mode with full speed and acceleration up to the point at which it switches to operate in charge sustaining mode faces a number of challenges based on this mixed duty cycle—i.e., part EV, part hybrid. The challenges include:
- High number of full operation charge/discharge cycles
- High discharge power during charge sustaining mode (at a low state of charge, SOC)
- High discharge power requirement for acceleration performance
- High charge power requirement for regenerative braking and charge sustaining mode (transients at both high and low SoC)
- Temperature conditions
The factors all interplay, complicating demands placed on both battery and driver. For example, the depth of pack discharge in daily use will vary, Matthé noted. With public charging or charging at work, two or more cycles per day are possible; he said that he (driving a Volt) sometimes charges 3 times day. Because of the differences in how you can use the car, he said, you have to accommodate for that in your battery life.
There are also a number of factors—high charge/discharge rate; high or low State of Charge (SoC); hot or cold temperatures—that affect degradation of power and capacity. As examples, high charge rate and cold temperatures can result in metallic lithium plating and electrolyte decomposition. Low discharge rates and low temperatures can result in a corrosion risk on the current collector. High charge rates at warm temperatures can result in electrolyte decomposition and impedance rise. Low discharge rate at warm temperatures can result in metal dissolution and the loss of active material, with an accompanying fade in capacity. Designers must strive to keep the battery functioning in the minimal cell degradation area—essentially balanced in the center between these different extremes.
You have a wide range of matrix of conditions you have to consider. Every cell is differently sensitive to that kind of behavior. So first of all, you have to understand how sensitive is your cell to that [particular] degradation mechanism. As long as you do not have fully developed physical models, ground up...have full understanding of what happens inside the cell, you have to characterize what you have in front of you.
Now we have a very deep relationship with our cell supplier. That is important to do such an endeavor. If you just take a cell you don’t know, the vendor will not tell you...might not even in a very new cell know exactly, you have to characterize it. To characterize it, you have to think about the power levels you might face in your applications, you have to think about distribution of discharge cycles you face, and you have to consider the cold and warm exposure. You develop a test matrix for your battery to get to know you battery. And in time to understand what your cell is all about.
In general what you learn is the deeper your discharge cycles the less energy you can put through over life. If you do only little cycling, total accumulated energy is 2.5 times [that possible with high levels of cycling]. The next thing is temperature. Temperature requires a sophisticated model. The effect of battery temperature on battery cycle should not be underestimated.
If you want to have consistent performance, when you discharge your battery too low, your vehicle gets slower. The problem with an extended range electric vehicle is that you still need to do passing, so you want predictable power. On the other hand, you want to maximize efficiency.
You do not do only that power profile and discharge cycling, you also have to think about that your battery might have degradation effects you haven’t dreamed about.—Roland Matthé