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)

Battery thermal management in PEV Batteries

Most of the methods that EVs use to control temperature of batteries don't work well on PEVs, and many times it's not necessary to do this; especially if the designer considered thermal management already when designing batteries.

Here's some reasons on why:

1. PEV batteries are much smaller in volume & size.

This enables PEV batteries to dissipate heat much faster, provided it has good contact with the outer casing.

2. PEV batteries are often placed in locations exposed to atmosphere.

This further helps to cool PEV batteries to ambient temperature.

3. PEV batteries (due to profit margin & competition) often leaves no room for active cooling features.

This is a deterrence factor, and consumer education can help defeat this issue.

However, with battery makers (who often do not know in-depth knowledge about consequences of improper BTMS, or simply neglect the importance) who sell batteries in e-commerce sites such as Amazon, Ebay do not equip their batteries to be suitable for long term use.

This goes back to the primary purpose of this article - to educate readers on potential consequences of improperly thermal managed batteries. Since e-bike batteries are passive cooled, e-bike batteries are especially susceptible to this.

Consequences of passive cooling batteries

Imagine I have 100 18650 batteries, and I want to make a battery pack with it. I can manipulate them quite a bit to get different shapes; DIY-battery takes about this to some degree.

Generally, you can make something thicker (more layers in between outer cells), or thinner. The internal resistance of each cell generates ohmic heat to heat the cells.

Now, imagine which battery would have more even temperature. A thicker battery or a thinner battery? <Insert cell designs here for visualization>

The best way to visualization would be using CFD simulations; But you can also intuitively guess as well (mainly because almost nobody reading this probably has access to a CFD, or even CAD software).

Imagine a heatsink copper pipe with multiple CPUs connected in a row in series to the pipe. What would happen if you were to give all of those CPUs same power (let's say, 10W) with only fan & fins on the end? You could deduce that the copper section furthest away from the fan would be hottest, because there are other CPUs generating enthalpy to the system next to one another.

What about a same setup but each CPUs are given their own heatsink independent from one another and has their own heat fins & fans?

Odds are, the CPUs would have more uniform temperatures.