Cooling System Design

Overview

As power levels of EV conversions increase, thermal management is becoming more important as a design consideration. Too often, this is treated as an afterthought, even by so-called "professional" shops. This guide is here to serve as a primer on the subject, and, while not exhaustive, it will address the major concepts needed and common mistakes seen in DIY builds.

For the moment, this guide will stick to the scope of ambient cooling (i.e. using a pump to circulate coolant through components, and then to a radiator to reject the heat). Sub-ambient cooling (i.e. using refrigerant to cool to temperatures below ambient) may eventually be added as the practice becomes more common in conversions.

Cooling System Concepts

Terminology (WIP):

• Head
• Pressure
• Flow
• Delta
• Prime/cavitation

Cooling System Components

Typical Component Layout

An established layout for a simple cooling loop is as follows:

1. The "start" of the loop is the header tank, which is mounted at the highest point in the system. By being the high point, the tank supplies "head" to the downstream components, and also serves as uppermost point for any air in the system to return to. The header tank will typically have two fittings: the one on the bottom will be the supply/output, the one on the side will be the return/inlet. There will also sometimes be a fitting next to the cap to allow for expansion/overflow conditions.
2. The pump should come directly after the header tank. Having the pump here assures that the pump will see coolant that is both cool and free of air, two important factors for the longevity of the pump. Also, having the pump primed directly by the header tank will allow it to "push" coolant (rather than pull it), which is how centrifugal pumps are meant to operate.
3. The load(s) comes next. In a simple loop, this might just be a single component (e.g. inverter), but it's not uncommon to have multiple components in a loop. The rule of thumb is that the most heat-sensitive components come first; so if you have an inverter and a motor in a coolant loop, the inverter always comes first because its operational range is <90°C, whereas a motor can see temperatures well over 100°C.
4. The heated coolant then goes into a radiator (sometimes called heat exchanger). In a radiator with top and bottom fittings, the top is the inlet and the bottom is the outlet. This is because warm coolant is less-dense and as it cools by passing though the radiator, it will get heavier and flow downwards naturally.
5. Lastly, after leaving the radiator, the cooled coolant returns to the header tank through the return/inlet on the side of the tank. Because the inlet is higher than the outlet, any air trapped in the system will separate out of the coolant when it flows through here.

Troubleshooting

Some important things to ask:

• Is your radiator getting enough airflow?
• Are your fans running?
• Where can air get trapped in your setup?

Because DIY coolant loops tend to be less than ideally-routed, an "airlift" type vacuum-based tool for filling your system is strongly recommended.