Battery Thermal Management Methods

Intent: To educate readers on common PEV (Personal Electric Vehicle - EVs below size/carry capacity of a car) battery thermal management methods, and consequences of improper thermal management on rechargeable batteries.

Introduction

Battery thermal management will always have a form of heat exchange system.

What is a heat exchanger? This is a glorified term for anything that can move heat from one place to another. For example, the copper pipes on your laptop/desktop CPU/GPU where it connects to heat sink/fin to a fan is a type of heat exchanger i.e. radiative, using a fan. Since this system requires power to operate, It can be considered as an active type of heat exchange.

Therefore, the definition of Active management in this scope of article (and generally, outside the article) means that the system transfers enthalpy from one place to another while using power. For example, a fan would be transferring electrical energy to ohmic heat (from coils of the fan) + Kinetic energy (to drive the fan itself), pulling/pushing air (containing heat, aka, enthalpy) from one place to another.

Passive management, in our scope, would be type of system that does not use energy to transfer enthalpy from one place to another: Heat sink/fins would be good example of this (and probably the only one, in the scope of this article). Since the system does not have a form of active device to block enthalpy transfer, this system, in theory, even out the temperature (enthalpy) of two system.

Practically speaking, most (if not all) e-bike batteries that are commercial uses passive management. Electric vehicles, however, often (if not always) employ some form of active management to manage enthalpy of batteries.

Active vs Passive

So why do Electric Vehicles use active method? Here are some following reasons.

1. Active temperature management of batteries take up less space, on larger batteries.

Imagine an EV that has battery mounted to the car. A typical EV, at minimum, starts around 40kWh worth of batteries: This is about 70 times more energy stored than a typical 48V12Ah Hailong battery that we put on e-bikes.

Often times, these batteries are mounted underneath the car, because that's pretty much the only practical place (and only) place to put batteries. (A primary reason of this is that it creates lower center of mass, improving stability of the car! this also applies to ebikes and motorcycles as well)

Now, we have to cool these batteries, so they don't cook off and become fireworks. Well, so, where do I put the heatsink? Underneath the battery?

Well, the heat fin wouldn't do well there, and it's most likely going to get damaged. Above? people sit on there. Sides? that's outside the car, and we won't be able to utilize much space.

What about watercooling like we do with computers? We can have copper pipes inside a plate that touches the battery; the cold (or hot) water can flow through this plate, and then cool/warm the battery. The waste heat can have a centralized radiator that is located on front of the car, and it can be passively cooled when the car is driving.

Needlessly, this type of system would cost more & a lot more complicated than simply putting fins on side of the battery.

2. Active temperature management can control the flow of enthalpy on a battery.

This is crucial. Let's say you can't put your ebike indoors, and you can't take the battery out of the bicycle. Also, it's winter, and you live in Canada. since most ebikes don't have a way to control enthalpy, the enthalpy outside the battery will slowly cool down the battery to ambient temperature.

Now, Typically, discharging batteries in cold is not terrible, per se; but cold cells have higher internal resistance, leading to more waste heat (also causing a self-heating effect).

But, what if you tried to charge the battery while the battery is cold? Well, that can be reader's homework. (Hint: Nothing good happens to it)