BlueSwordM
Regular
- Joined
- Jul 23, 2018
- Messages
- 586
After a considerable amount of thought, I’ve finally decided to write a post detailing a lot of my research, findings, experiments and general anecdotal information regarding battery pack thermal management.
As many of us know, keeping a battery pack at the right temperature is one of the best ways to keep each of the individual cells happy and living a long fruitful life : while achieving that goal with a single cell is rather simple, managing that within an entire battery pack filled with tens, to hundreds, to thousands of cells gets complicated rather quickly. Why? Well, as you scale up a battery pack, extracting the heat from an enclosed pack gets harder and harder; the center-most cells get stuck between other cells. Each additional row of cells increases thermal resistance, making the thermal situation worse.
At low charging or discharging speeds in normal environmental conditions, this doesn’t matter. However, as ambient temperatures drop or increase and as power density during these 2 events increase, temperature control starts becoming very important.
If the cells get too cold, charging speed, or safety, suffers; discharging becomes less efficient and you lose out on usable capacity until temperatures climb back up. If the cells get too hot, cycle life and longevity get worse. Too hot and safety issues start becoming a non-negligible issue.
If cell to cell temperatures vary too much, those temperature deltas will eventually result in cell to cell capacity and internal resistance (IR) differences will appear. If capacity and IR differences become severe enough, pack capacity utilisation drops and in the worst case scenario, the pack can become unusable.
All of these situations circle back to battery pack thermal conductivity or thermal resistance, its ability to conduct heat energy through the entire pack or resist temperature changes respectively. The more thermally conductive the pack is overall, the more thermally uniform it will be during moments of high thermal stress such as high power fast charging or regenerative breaking events. Our final goal would be minimize cell to cell variance and keeping cells in the optimal range as much as possible while minimizing energy input.
In larger vehicles like electric cars, the problem already has a solution : active liquid thermal management. While there are still many optimizatios that can be performed on top of that, which will be discussed further in the one of the next sections, it’s the best way to heat or cool down batteries.
The problem with liquid thermal management however is that you need a cooling loop, which increases system weight, cost and complexity. On a >=20kWh battery pack, this isn’t an issue. However, in most of the packs found in ebikes and other personal Light Electric Vehicles (LEVs) or Personal Electric Vehicles (PEVs) with battery capacities ranging from 0.5kWh to 10kWh, this kind of system is cumbersome, difficult to implement and rather annoying to deal with.
Therefore, we have to utilize other strategies to thermally manage our packs and keep them as thermally uniform as possible. Those require engineering skill, research and time to implement and this is why I’ve made this post : instead of us just talking sporadically in other threads on EndlessSphere and other forums, I’ve decided to talk about it here.
This semi-technical thread will be divided up into multiple thermal battery pack management techniques and optimizations. Each paper, finding or implementation will be numbered for easy documentation and research… ideally
1- Battery pack overbuilding.
2- Interconnect optimization.
3- Cell choice.
4- Thermal interface materials.
5- Thermal dissipation enhancements.
6- Phase Change Materials (PCMs).
7- Heating and cooling.
8- Charging algorithms.
9- Discharging algorithms.
10- Thermal runaway mitigation.
Make sure to contribute your findings as well, and document them with the proper number for ease of access and future references. Additionally, please add your sources for any papers, videos, websites or images from which you find new things to share. Thank you all and have a good end of the year.
As many of us know, keeping a battery pack at the right temperature is one of the best ways to keep each of the individual cells happy and living a long fruitful life : while achieving that goal with a single cell is rather simple, managing that within an entire battery pack filled with tens, to hundreds, to thousands of cells gets complicated rather quickly. Why? Well, as you scale up a battery pack, extracting the heat from an enclosed pack gets harder and harder; the center-most cells get stuck between other cells. Each additional row of cells increases thermal resistance, making the thermal situation worse.
At low charging or discharging speeds in normal environmental conditions, this doesn’t matter. However, as ambient temperatures drop or increase and as power density during these 2 events increase, temperature control starts becoming very important.
If the cells get too cold, charging speed, or safety, suffers; discharging becomes less efficient and you lose out on usable capacity until temperatures climb back up. If the cells get too hot, cycle life and longevity get worse. Too hot and safety issues start becoming a non-negligible issue.
If cell to cell temperatures vary too much, those temperature deltas will eventually result in cell to cell capacity and internal resistance (IR) differences will appear. If capacity and IR differences become severe enough, pack capacity utilisation drops and in the worst case scenario, the pack can become unusable.
All of these situations circle back to battery pack thermal conductivity or thermal resistance, its ability to conduct heat energy through the entire pack or resist temperature changes respectively. The more thermally conductive the pack is overall, the more thermally uniform it will be during moments of high thermal stress such as high power fast charging or regenerative breaking events. Our final goal would be minimize cell to cell variance and keeping cells in the optimal range as much as possible while minimizing energy input.
In larger vehicles like electric cars, the problem already has a solution : active liquid thermal management. While there are still many optimizatios that can be performed on top of that, which will be discussed further in the one of the next sections, it’s the best way to heat or cool down batteries.
The problem with liquid thermal management however is that you need a cooling loop, which increases system weight, cost and complexity. On a >=20kWh battery pack, this isn’t an issue. However, in most of the packs found in ebikes and other personal Light Electric Vehicles (LEVs) or Personal Electric Vehicles (PEVs) with battery capacities ranging from 0.5kWh to 10kWh, this kind of system is cumbersome, difficult to implement and rather annoying to deal with.
Therefore, we have to utilize other strategies to thermally manage our packs and keep them as thermally uniform as possible. Those require engineering skill, research and time to implement and this is why I’ve made this post : instead of us just talking sporadically in other threads on EndlessSphere and other forums, I’ve decided to talk about it here.
This semi-technical thread will be divided up into multiple thermal battery pack management techniques and optimizations. Each paper, finding or implementation will be numbered for easy documentation and research… ideally
1- Battery pack overbuilding.
2- Interconnect optimization.
3- Cell choice.
4- Thermal interface materials.
5- Thermal dissipation enhancements.
6- Phase Change Materials (PCMs).
7- Heating and cooling.
8- Charging algorithms.
9- Discharging algorithms.
10- Thermal runaway mitigation.
Make sure to contribute your findings as well, and document them with the proper number for ease of access and future references. Additionally, please add your sources for any papers, videos, websites or images from which you find new things to share. Thank you all and have a good end of the year.
