Active cooling provides substantial performance benefits—but it isn’t always necessary or cost-effective
Charging and discharging batteries generates heat, and heat hamstrings battery performance. Long-term operation at high battery temperatures can shorten cycle life, harm cell health, and create safety hazards. Effective temperature control is essential both for battery performance and to pass the various safety tests for regulatory compliance.
Passive cooling techniques, which use no power, can be very effective, and the technology is improving all the time. But there’s no contest—when it comes to bulk heat management, active measures like forced air and liquid loops are the best.
Yet, multitudes of battery-powered devices get along just fine without active cooling. That’s because the goal of cooling in battery pack design is not to make batteries as cool as possible, but to make batteries as effective and cost-effective as possible. Sometimes this requires active cooling, and sometimes passive cooling is sufficient. This article is about weighing the specific performance benefits of active cooling technology against the costs associated with complexity, bulk, weight, and parasitic power draw.
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Active cooling and improved battery performance
As we covered in our article on battery thermal management, batteries are electrochemical cells, and heat tends to speed up the chemical reactions taking place inside them—including the undesirable ones. Operation or storage in high temperatures chemically degrades batteries, causing effects like:
- Shortened cycle life
- Increased self-discharge
- The potential for failure and thermal runaway
In our discussion of the design considerations for high cell-count batteries, we also discussed the way that temperature differences between cells and modules can lead to charge imbalance and eventually to destructive events like overcharge or over-discharge.
All of these problems limit battery performance, but they can be mitigated very effectively with active cooling. When heat buildup is less of a problem, batteries can sustain more extreme conditions (like high power charge and discharge) that are very valuable in industries such as the electric vehicle (EV) market.
Transient and continuous high-power applications in electric vehicles
Many factors have slowed the widespread adoption of electric vehicles, but perhaps chief among them is “range anxiety.” Understandably, drivers worry about how far they can travel with a fully charged EV, and more still about whether they can find a charging station when their batteries do run low. Also baked into these anxieties are worries about how quickly they can get back on the road. For gas-powered cars, it takes a matter of minutes to fuel up. For EVs, it can take much longer. It takes 12 hours to charge a Tesla Model S at 240 V.
The electric vehicle industry has moved to alleviate this part of range anxiety by providing faster charging options. Part of this involves the design of batteries and battery chemistries that can sustain rapid charge dumps, but it also involves innovations in the charging process. The next generation of charging is extra-fast charging (XFC), which will involve upwards of 400 kW output at 800 V DC, allowing the full charge of electric vehicles in minutes.
The challenge is that this high-power DC current can also push the temperature of batteries over 500 F in a matter of minutes. The charging station, cable, and EV battery all require robust thermal design to withstand these conditions. One solution is a liquid cooling loop integrated with the charging cable. A liquid connection between the charging cable and the vehicle allows liquid flow to cool the cable and the battery during XFC.
This example of XFC illustrates the broader concepts of transient and continuous operations ranges for electric machines. Electric machines make certain windows of tolerance in which they can safely and reliably function. Some operational ranges can be maintained continuously without ill effect—like charging a battery at 240 V. Others are transient, meaning that they are possible and safe. XFC is an example of transient operation that will only be possible for a few minutes because of extremely powerful cooling.
Both continuous and transient operation ranges are thermally limited to a large degree. The simple and low-tech solution to thermal mitigation is to oversize the electric machine. A bigger motor and battery can provide continuous level of power and torque that a smaller machine could only provide transiently. Of course, this comes with the downsides of bulk, weight, and cost.
A core and specific benefit of active thermal management is the ability of machines to operate at higher power levels without oversizing and while minimizing the problems caused by heat buildup that would normally create thermal limits.
It’s not just about temperature—costs and economic considerations with active cooling
Boosted transient and continuous power capabilities, protecting batteries in extreme climates, and extending cycle life—these are all great benefits of active cooling. However, not all devices need any of these benefits. The cost-benefit analysis for an active cooling system involves battery performance and safety, true, but also takes into account return on investment and viability as a consumer product.
While there are many applications where performance could certainly be improved with better temperature regulation, market incentives can work against active cooling rather than for it.
Consider laptops, for instance. It’s possible that more robust cooling—such as bigger fans like those provided by cooling pads, or liquid cooling loops—could yield longer charge durations and cycle lives plus better computer performance. But consumers seem to prefer slimmer profiles and lighter weight for their laptops.
Or, to beat a dead horse in this article, consider the different choices made by different EV manufacturers. Tesla has used liquid cooling from its inception, and places great importance on the longevity and reliability of its batteries. The Chevy Volt also uses liquid cooling.
But Nissan went a different direction. The Nissan Leaf continues to rely only on conditioned air from the cabin—arguably a form of active cooling, but just barely. This design choice frustrates many consumers. But, contrary to some opinions, it’s unfair to say that the Leaf’s cooling system is just lazy engineering. Rather, it’s a business decision that illustrates the cost-benefit analysis that’s necessary with every battery pack design.
Let’s compare the some of the specs of Tesla and Nissan cars. According to Edmunds, 2020 Tesla Model S has a battery with a range of 373 miles and an MSRP is $79,990. The 2020 Nissan Leaf has a range of 149 miles and its MSRP is $34,190.
On paper, the Leaf’s battery performs worse than the Model S, but that may not matter to Nissan. Based on the price difference between the two, Nissan and Tesla aren’t competing for the same customers. Nissan has decided to pursue customers who are willing to spend less and who are content with lesser performance. Perhaps a liquid cooling system could improve the Leaf’s capabilities, but such technology clearly goes against the Nissan business strategy.
When designing a thermal management system for a battery, we must weigh the benefits of efficiency against the various downsides, which include actual material costs, the cost of weight and bulk, parasitic power usage, and the complexity and maintenance needs that come with more sophisticated systems.
Generally, the cooling choices follow this pattern: Passive cooling is the least efficient (though the technology is rapidly improving) but takes up the least space, adds the least weight, uses no battery power, and has the fewest points of failure. Forced air cooling is more efficient but consumes battery power and introduces a point of failure in the form of a fan motor. Liquid cooling is generally the most efficient option, but it also consumes power. Plus, the pump introduces complexity, and the circulating liquid is a potentially catastrophic point of failure.
Designing a cooling system, like any other part of a battery, is about picking the features that deliver the most useful and profitable battery, however, those facts are calculated.
Thermal management—it’s about performance and economics
Efficient temperature management is an undeniable boon for battery packs. Handling heat improves performance, safety, and lifespan. Active cooling is often the best way to manage heat—yet fans and liquid cooling loops are absent in many battery-powered devices.
That’s because the decision to include active cooling in a battery pack is about weighing the performance benefits against the costs. Complexity, weight, bulk, and parasitic power draw are all downsides to active cooling that could increase costs or simply make the end product unattractive to users. The Nissan Leaf is a prime example of this—it uses simplistic cooling technology compared to a vehicle like a Tesla Model S. But this seems to be part of Nissan’s business plan—to address a specific segment of the auto market sensitive to cost over range or efficiency.. From BMS to heat management, our design and engineering team can provide you with custom battery pack solutions optimized for your end product.
Contact us at Aved to learn more.