At some point, you’ve probably been told, “Water and electricity don’t mix.” While this is a wise rule to follow in general, it does not tell the complete story. Pure water is, in fact, an excellent insulator of electricity.
Table 1. The electrical conductivities of some common materials. Note that deionized water is 3 orders of magnitude and 6 orders of magnitude less conductive than tap and sea water, respectively.
However, pure water is seldom encountered in everyday life. Water, the “universal solvent,” has unique chemical properties that make it a superior thermal management fluid and good at dissolving a range of substances — including positively charged, negatively charged, and neutral species.
These dissolved components, particularly the charged or ionic materials, create the electrical highways, allowing electrons to move through fluid and leading to shorting, fire, and electrocution warnings that typically accompany the aforementioned “water and electricity don’t mix” adage.
When battery electric vehicles (EVs) gained traction as alternatives to gas and diesel internal combustion engines (ICEs) — offering a partial solution to reduce global greenhouse gas emissions — it became obvious that EV’s traction battery, power electronics, and electric motors would benefit from active cooling systems. Engineers wasted little time discovering that indirect cooling fluids, circulating through cold plates, jackets, and heat exchangers, were far superior at managing EV temperatures compared to air cooling.
One might think the fluid for these systems would be the tried-and-true formulations developed for ICEs and used effectively for nearly a century in similar thermal management systems. To understand why borrowing coolant technology directly from ICE vehicles may be risky, we must first examine what components make up an ICE coolant.
In addition to a base blend of water and ethylene glycol found in virtually all modern engine coolants, proprietary blends of special additives, often ionic in nature, are added to the formulation. These additives are primarily used to address corrosion. Without additives, water will accelerate the corrosion of certain metals, leading to deposit formation, premature wear, reduced heat transfer, and system blockages.
Figure 1. The proper formulation of ionic corrosion inhibitors can greatly increase the lifetime of ICE system components.
The additives inhibit the unwanted corrosion process through a passivation mechanism on the metal surfaces. This is why certain coolants on the market can carry a 350k-mile guarantee, which is only possible with the careful formulation of these ionic, anti-corrosion additives.
These ionic corrosion inhibitors significantly increase the electrical conductivity of water-based ICE coolants. While the coolant in EV thermal systems is designed to be isolated from the high-voltage (HV) electronic components, the risk of a faulty seal, structural failure, or damage during a crash, could lead to exposure of the HV components to the fluid.
There have been reports of EVs, BEV heavy-duty trucks, and stationary lithium-ion energy storage centers catching fire, with high conductivity of the indirect coolant being identified as a contributing factor to these thermal runaway events. In some instances, there was a pre-existing coolant leak. In others, the coolant leak resulted from a vehicle crash, with a three-week delay between the accident and the fire.
As a result, automakers, standards committees, and government agencies are discussing how EV coolant specifications should differ from those developed for conventional ICE coolants. While the details of proposed specifications vary between organizations, there are points of parity between them:
1. Lower electrical conductivity. A primary driver for EV-specific thermal management fluid specifications is safety. Lower fluid electrical conductivity means a lower risk of electrical shorting, less heat generation, and a lower probability of fire in the off chance the fluid contacts the HV components.
Figure 2. In the case of a leak or impact, the conductivity of thermal management fluid is directly related to the compatibility with the battery and the HV components.
Traditional ICE coolants typically have between 2000 and 5000 µS/cm or higher electrical conductivity. Most new EV coolant specifications, including from the American Society for Testing Materials (ASTM), the Chinese Guobiao Standard (GB), and large automotive OEMs, are converging around 100 µS/cm as a safe limit to prevent battery thermal runaway and fire.
2. EV-specific material corrosion protection. While EVs share many construction materials with ICE vehicles, including copper, brass, and stainless steel, some metallurgies (such as specific grades of aluminum) are gaining widespread adoption in EVs. For this reason, new EV-specific thermal management fluids should be engineered to protect these materials.
However, lowering electrical conductivity and improving corrosion protection somewhat oppose one another. Traditional ICE coolant corrosion inhibitors increase conductivity due to their ionic nature. Removing them from formulations lowers conductivity but at the cost of corrosion protection.
Today’s EV formulators are challenged with developing formulations that rely on non-ionic corrosion inhibitors or unique combinations of ionic corrosion inhibitors that contribute less to electrical conductivity than conventional additives.
Figure 3 presents data from the GB29743.2 glassware corrosion test, a crucial evaluation method. In this test, metal specimens are exposed to coolant in the presence of corrosive salts and heated for two weeks. The specimens are then evaluated and weighed. A passing test must show no more than 10 mg of weight change from the test start. A traditional ICE coolant with high amounts of ionic corrosion inhibitors can pass this test relatively easily, underscoring the importance of effective formulations.
Figure 3. Glassware corrosion data per GB29743.2 comparing the performance of conventional coolant, well-formulated low conductivity coolant, and low-conductivity coolants without sufficient corrosion protection.
However, products marketed as EV coolants have typically sacrificed corrosion protection to obtain low conductivity and will not pass the test. It takes careful formulation consideration to achieve low conductivity and passing corrosion results.
In summary, when choosing a thermal management fluid for your EV application, it’s corrosion protection and safety are a must. Although many of today’s EV thermal management fluid offerings force a choice between corrosion protection and low conductivity, coolant innovators are developing the products to ensure there are no compromises.
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Filed Under: FAQs, Featured Contributions, Thermal Management System