What Is Coolant?

Coolant is the stuff running through your battery pack keeping cells from cooking themselves. Pretty simple concept but a lot of people get it wrong.

In EV and ESS applications we're talking about a glycol-water mix circulating through cold plates or cooling channels machined into the pack structure. The fluid picks up heat from cells, carries it to a chiller or radiator, dumps the heat, comes back for more. Loop keeps running as long as the pack is active.

The Fluid Itself

Forget whatever you know about automotive coolant. That green Prestone from AutoZone will destroy a battery pack. The corrosion inhibitors in conventional antifreeze are ionic compounds-they conduct electricity. You run that through a 400V pack, you're asking for ground faults.

Battery coolant has to meet two requirements that regular antifreeze doesn't care about:

Electrical resistivity above 100 kΩ·cm minimum. Most OEMs spec 150 kΩ·cm or higher. Tesla's spec calls for >200 kΩ·cm on fresh fill.

Ion content below 25 ppm. Some specs say 10 ppm. This is why you can't just use distilled water and mix your own-the glycol itself has to be ultra-pure.

BASF Glysantin G40 and Arteco Havoline XLC are the two you'll see most often in the field. Chevron also makes a battery-specific formulation. These run about $40-60 per gallon versus $12-15 for regular OAT coolant. Worth it.

Mix Ratio

50/50 ethylene glycol to deionized water is standard. Some guys run 60/40 in cold climates. Going above 60% glycol actually hurts you-viscosity goes up, heat transfer goes down, pump works harder.

The freezing point math:

50/50 mix freezes around -37°C

60/40 gets you to maybe -52°C

Pure glycol freezes at -12°C (yes, adding water lowers the freeze point, chemistry is weird)

For most of North America 50/50 is fine. If you're deploying packs in Minnesota or Alberta, talk to your thermal engineer about going richer.

Flow Rate

Here's where I see the most confusion. People either way oversize pumps or undersize them.

The calculation isn't complicated. You need to know three things: how much heat the pack generates, what temperature rise you can tolerate across the circuit, and your coolant's specific heat.

For a 50/50 EG/water mix, specific heat is about 3.3 kJ/kg·K. Density around 1.07 kg/L.

Say you've got a 75 kWh pack doing 2C discharge. That's 150 kW output. Figure 5-6% heat generation at that rate-call it 8 kW thermal load. If you want to keep inlet-to-outlet delta under 4°C:

8000 W ÷ (1.07 kg/L × 3300 J/kg·K × 4 K) × 60 = 34 L/min

That's a real number from a real project. The actual pump we spec'd was rated 40 L/min to give us headroom.

Most passenger EV packs run 8-15 L/min at normal driving loads. Bumps up during fast charging or sustained high power.

What Goes Wrong

Seen a few failure modes over the years.

Wrong coolant type. Shop tops off with conventional antifreeze during service. Conductivity spikes. BMS sees isolation fault, pack shuts down. If you're lucky it just throws codes. If you're not lucky you get internal arcing.

Contamination. Water quality matters. One site I worked used tap water for the initial fill because "it's going through the deionizer anyway." Except they bypassed the deionizer. Chloride content was 180 ppm. Had to flush the whole system and replace the cold plates-$40k job on a $200k installation.

Degradation over time. Glycol oxidizes. Forms glycolic acid, then oxalic acid. pH drops. Corrosion inhibitors get used up. OEMs say 4-5 year service interval but I've seen coolant go bad in 2 years in high-temp environments. Test annually.

Air in the system. Bleeding EV cooling loops is finicky. Air pockets cause hot spots. The cell directly under the bubble runs 8-10°C hotter than its neighbors. Accelerated aging, capacity fade, eventually a warranty claim.

Testing Coolant Condition

You need three measurements:

pH-fresh coolant runs 7.8 to 8.5 depending on formulation. Below 7.0 means the inhibitor package is depleted. Change it.

Resistivity-portable meters exist but they're expensive. Send a sample to a lab if you don't have one. Anything under 50 kΩ·cm is getting marginal.

Glycol concentration-refractometer works. $30 tool, takes 10 seconds. You're checking for dilution from condensation or someone adding straight water.

Some fleets do oil analysis style programs, send samples to Polaris Labs or Blackstone every 6 months. Overkill for most applications but makes sense for large installations.

Direct Immersion vs Indirect Cooling

Everything above assumes indirect cooling-fluid runs through channels, never touches cells. That's 95% of what's out there.

Direct immersion is different. Cells sit in a bath of dielectric fluid. No cold plates, no thermal interface material, fluid contacts the can directly. Heat transfer is better. Complexity is lower in some ways, higher in others.

The fluids are different too. 3M Novec and Solvay Galden are the big names. These are fluorinated compounds, extremely low conductivity, chemically inert. Also extremely expensive-$300-400 per liter for some formulations.

Direct immersion is showing up in grid storage and some high-performance vehicle applications. Not mainstream yet but the thermal performance is hard to argue with.

Spec Sheet

Quick reference for what to look for when sourcing battery coolant:

Get the MSDS and technical data sheet from your supplier. If they can't provide resistivity data, find a different supplier.