Among the many technologies being developed to replace traditional fossil fuels, lithium-ion batteries make the strongest case as a market-ready option for various reasons. Increases in energy density, faster charging times, and lower total cost of ownership are among the chief reasons for the rise in electric vehicle manufacture and ownership. And now that off-highway OEMs are embracing these benefits, serious inroads are also being made in the non-road mobile machinery (NRMM) market.
One of the key developments that has seen this increased uptake across these two markets is the improvement in thermal management system technology. Thermal runaway is a significant concern when it comes to lithium-ion batteries, as it can pose severe threats to vehicle operators and bystanders. The built-in solutions at a battery management system level are designed to prevent such events from happening at all, running alongside containment and fire extinguishing features.
Nowadays there are state-of-the-art thermal management systems available that make lithium-ion batteries a safer proposition than diesel engines. In this article, we’ll explore how good thermal management affects battery performance, what happens to lithium-ion batteries at opposite ends of the temperature spectrum, and the various thermal management systems available today. We’ll then wrap it up with a dive into the inner workings of Xerotech’s market-leading thermal management system.
Benefits of a high-performance TMS
Since battery performance suffers in high heat or extremely low temperatures, it’s critical to have a thermal management system that can maintain operating conditions within a 15-30°C range regardless of external ambient conditions. This is particularly important for off-highway vehicles used in freezing conditions in the woodlands or sweltering heat at the bottom of an underground mine.
Maintaining this temperature range prevents the risk of thermal runaway that arise when repeatedly exposing lithium-ion batteries to such conditions. It also ensures the battery’s lifespan is maximized and capacity and performance do not suffer. Thermal management systems are designed to enhance energy density and improve performance at maximum and minimum temperatures, with many factors coming into play.
One of the reasons we opt for cylindrical cells here at Xerotech is the enhanced heat dissipation the shape offers. It also facilitates the passing of our coolant manifolds between the cells, helping maintain the 15-30°C temperature range.
High-performance thermal management systems will also be essential to unlock the full potential of extreme fast charging (XFC). Given the high temperatures battery platforms will face during fast charging, the TMS must be capable of dealing with these conditions. An extreme fast-charging (XFC) report compiled by the U.S. Department of Energy states that a 10-minute extreme fast charge can result in a 270°C temperature rise in the pack.
Xerotech’s cylindrical cells within the thermal system
Effectively handling these temperatures makes EV ownership a far more attractive proposition. In the off-highway market, less time is spent at charging points, whereas for off-highway vehicles, there’s less idle time spent waiting for the vehicle to charge.
An additional benefit of a high-performance thermal management system is that cell chemistry can be chosen based on the application’s required performance rather than for safety purposes alone. Initially, one might be drawn towards LFP since it’s cell chemistry is inherently safer than most of its counterparts. However, with an advanced thermal management system in place, the choice is based on what best suits the customer’s application.
Controlling the thermal management system is the battery management system (BMS), essentially the brains behind the battery platform. The BMS communicates the safe current, voltage and temperature operating limits within the battery. Xerotech’s Nexus BMS monitors for over and under voltage at cell, pack and link levels, and temperature monitoring at module level, operating at a maximum of 1,000V and 500A current.
What happens at opposite ends of the temperature scale?
At very low temperatures, Li-ion batteries can still be discharged, albeit at much lower rates. However, no charging can take place below 0°C; in a Xerotech platform, the battery management system will actively prevent charging from taking place if it reads such temperatures.
The reason for this is that charging in these conditions can cause irreparable damage by way of liquid plating, which is when ions flow across the separator. Because of the freezing conditions, these ions will plate on the anode’s surface, causing a massive drop in battery capacity.
Persistent charging at sub-zero temperatures can also cause the growth of lithium metals called dendrites. In some cases, these can punch through the separator, contact the cathode on the other side, and cause an internal short circuit. The resultant heat could melt the plastic separator inside the cell and put the whole battery at risk of a thermal event.
In hot environments, batteries experience lower resistance, which pushes the battery’s capacity beyond what it would typically be able to achieve. So, up to certain temperatures, batteries will outperform their expected metrics, though doing so regularly will result in reduced capacity (after a while) and lifetime. Also, if it gets too hot and the separator is damaged or melts, it creates grounds for a short circuit and potential thermal event.
In cases of high or low temperatures, it’s the battery management system (BMS) that will regulate the battery’s internal temperature. The ideal temperature tends to be around 25°C, so the BMS will respond to temperature fluctuations and regulate accordingly to ensure that the cells are protected from thermal events while ensuring performance levels and cycle life are unaffected.
Types of systems
Put simply, an active thermal management system requires electricity from the vehicle’s power system to function. In contrast, passive systems use natural or external forces without the vehicle or application’s energy source. Instead, they would use gravity, latent heat, or vehicle motion to control the battery’s temperature. In some cold environment cases, the thermal management system can circulate the battery’s self-heating to maintain temperature across the platform.
While true to some extent that an active thermal management system translates to ancillary losses since it draws power from the vehicle, modern systems are highly efficient and are far more adept at maintaining optimum temperatures than passive systems. The most prominent thermal management systems in operation now are air cooling, liquid cooling, phase change material (PCM) cooling, heat pipe cooling, and hybrid cooling.
The table below explains some of the features, advantages, and disadvantages of each system.
|Air cooling||• Simple design |
• Adaptable, lightweight
|• Weak temperature uniformity and control when under high magnification |
• Active air-cooling methods consume more energy, lowering pack density
• Passive air-cooling results in weak temperature uniformity and control when under high magnification
|Liquid cooling (indirect)||• Greater specific heat capacity and thermal conductivity compared to air-cooling |
• Can be enhanced by coolant flow, channel design, material properties
• Uniformity improved with ducts, cooling plated in contact with cell sides
• Technology is still developing, can yet improve
|• Increased complexity, weight, cost |
• Indirect contact can inhibit cooling effect slightly
• Pipe or cooling plate thermal conductivity is high, which won’t slow down thermal runaway
|Liquid cooling (direct)||• Simple, light, inexpensive, and compact structure |
• Better heat transfer
• Liquid medium can help prevent short circuit and thermal runaway spread
|• Higher sealing requirements needed |
• Pumps and other systems required to circulate coolant and control temperature
|Phase Change Material cooling||• Easy-to-alter shape leading to simpler system arrangement and better temperature uniformity |
• Good insulation that reduces short circuit risks
|• PCM volume likely to change after a phase change, increasing leakage potential |
• Low thermal conductivity makes it less sensitive to temperature changes
• Continuous circulation reduces cooling effect, meaning another cooling system would be needed to shift heat absorbed by PCM
|Heat pipe cooling||• Exceptional thermal conductivity |
• Sensitive to temperature changes, controlling temperature in real-time without increasing power requirements
|• Complex system that’s difficult and expensive to manufacture |
• Risk of leakage, small contact area with battery
|Hybrid cooling||• Mixed approach can mitigate disadvantages, enhance strengths, and reduce energy consumption||• Larger, more complex system since two or more are being integrated, increasing manufacturing and maintenance costs|
The Xerotech approach
As mentioned earlier, one of the benefits of using cylindrical cells is the slightly enhanced heat dissipation the shape offers. Though this characteristic does prevent one from using all the space available, it does contribute to better thermal management. However, what else is behind Xerotech’s industry-leading approach to thermal management?
Using small units of energy per battery cell and fusing them individually, thermal propagation and short circuit events can be limited to single cells instead of risking the entire module or pack. Our Xerotherm™ active cooling liquid (water/glycol) does everything a robust TMS should; it prevents thermal runaway events, extends battery life, and improves battery performance across a broad temperature spectrum.
Furthermore, each cell is fully enclosed in fire-retardant foam. This insulates each cell from neighboring cells, so even if there is a thermal event across multiple cells, the foam limits failure to these cells alone, protecting the rest of the module and pack.
Backing this up is a market-leading fire suppression system that doesn’t require active monitoring as it’s an always-on system. The ability to immediately extinguish cell-level fires prevents thermal events from spreading, as the threat can be quashed in seconds.
Xerotech’s proprietary fire-retardant foam
Having all the above in place is all well and good; however, any thermal management system must undergo continuous rigorous testing. This way, not only is the current system guaranteed to operate at the optimum levels, but it’s also how we can find new ways to improve and reach new levels.
Our videos below illustrate how all the pieces of our thermal management system come together to create an incredibly safe and robust fire and thermal propagation prevention system. The first video shows what happens in a projectile penetration on a fully charged high-energy cell (NMC) while the second video illustrates what happens when a cell is forced into thermal runaway within the center of a module.
This video demonstrates the difference our safety features make in the unlikely event of a catastrophic incident, keeping fire and thermal runaway suppressed to the cell level vs total failure of the control pack.
This video demonstrates the difference our safety features make in the unlikely event of a catastrophic incident, keeping fire and thermal runaway suppressed to the cell level vs total failure of the control module. This thermal runaway test has been conducted in a dry module without active fire suppression via our liquid-filled thermal management system and using the highest energy EnerCore NMC cells.
For more information on our safety systems, click HERE or visit our online catalog, where you can search thousands of battery options in seconds to see if there’s a battery ready to integrate into your application.