Difference between revisions of "Battery pack"
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| − | =Introduction= | + | ==Introduction== |
Safety & reliability should rank pretty high in design criteria as it's impossible to win a competition with a car who caught fire. | Safety & reliability should rank pretty high in design criteria as it's impossible to win a competition with a car who caught fire. | ||
| − | =Cell selection= | + | ==Cell selection== |
The selection process of a lithium-ion/lithium-polymer cell is highly dependent on the application it is being used for. Thus, before even starting battery design, it is crucial to understand the battery operating conditions. A light car with 2WD would requires less power than a heavy 4WD with wings. Generally speaking, there is a trade-off between cell energy density (Wh/kg) and cell power density (W/kg). It is then possible to compare the performance of two cell chemistries using lap simulation: there are cases where a higher specific energy cell is optimal, and others where high specific power is. | The selection process of a lithium-ion/lithium-polymer cell is highly dependent on the application it is being used for. Thus, before even starting battery design, it is crucial to understand the battery operating conditions. A light car with 2WD would requires less power than a heavy 4WD with wings. Generally speaking, there is a trade-off between cell energy density (Wh/kg) and cell power density (W/kg). It is then possible to compare the performance of two cell chemistries using lap simulation: there are cases where a higher specific energy cell is optimal, and others where high specific power is. | ||
| − | ==Cans vs pouches== | + | ===Cans vs pouches=== |
Lithium cells come the most often in cylindrical cans format (i.e. 18650) or in soft metal-plastic pouches. It is easier to design a safe battery pack with cylindrical cells as each cells are protected in a hard shell that protects them from accidental perforation. Cylindrical cells tend to offer higher energy density, while pouches tend to offer higher power density. Cylindrical cells can be spot welded together with fairly inexpensive equipment, while pouches can be connected together by ultrasonic welding, laser welding, or mechanical assembly (rivets, nuts&bolts, ...). While cylindrical cells are typically found in small capacities ( < 4 Ah ), pouches can be found in high capacities ( > 10 Ah), greatly reducing the complexity of assembly of a battery pack, since fewer cells are required to meet the battery pack energy requirement. | Lithium cells come the most often in cylindrical cans format (i.e. 18650) or in soft metal-plastic pouches. It is easier to design a safe battery pack with cylindrical cells as each cells are protected in a hard shell that protects them from accidental perforation. Cylindrical cells tend to offer higher energy density, while pouches tend to offer higher power density. Cylindrical cells can be spot welded together with fairly inexpensive equipment, while pouches can be connected together by ultrasonic welding, laser welding, or mechanical assembly (rivets, nuts&bolts, ...). While cylindrical cells are typically found in small capacities ( < 4 Ah ), pouches can be found in high capacities ( > 10 Ah), greatly reducing the complexity of assembly of a battery pack, since fewer cells are required to meet the battery pack energy requirement. | ||
| − | ===Pouches=== | + | ====Pouches==== |
Pouches offer better heat dissipation than other type of cells, but their flexible and thin enveloppe make them prone to perforation or crushing. Some pouches have tabs on the same edge, some, on opposite edges. Opposite edge tabs allow better heat dissipation, but make packaging more complex. They stack easily, using most of the available space. The cells edges are fragile: treat them with care. It is preferable to ask the cell manufacturer to have the edges taped. Most pouches will ignite in case of a nail penetration. One way to reduce the risk it would ignite is to improve the cooling of the cell. As an example, using sheets of metal between cells would spread the heat (and evacuate it out of the cell). | Pouches offer better heat dissipation than other type of cells, but their flexible and thin enveloppe make them prone to perforation or crushing. Some pouches have tabs on the same edge, some, on opposite edges. Opposite edge tabs allow better heat dissipation, but make packaging more complex. They stack easily, using most of the available space. The cells edges are fragile: treat them with care. It is preferable to ask the cell manufacturer to have the edges taped. Most pouches will ignite in case of a nail penetration. One way to reduce the risk it would ignite is to improve the cooling of the cell. As an example, using sheets of metal between cells would spread the heat (and evacuate it out of the cell). | ||
| − | === | + | ====Cans==== |
Cans can be cylindrical or prismatic. They are typically safer than pouches, since they are are valve regulated and benefits from a hard shell, reducing their perforation and crushing risk. However, it is generally harder to cool them down. Cylindrical cans also have a lower packing factor than prismatic cells leading to an effectively reduced volumetric density. However, the process of cylindrical cell fabrication is so well honed that battery packs made with state of the art cylindrical cans compare in terms of metrics with battery packs made with state of the art pouches. Cans usually also contain a positive thermal coefficient (PTC) resistor that can protect the cell in case of a short circuit. However, this protection is usually not rated for high voltage packs, can could actually ignite a fire, in a high voltage pack. Cans also can ignite when perforated. Wrapping cells with a phase change material (PCM) is one of the solutions<br />that have been developed to absorb heat in a thermal event, reducing the odds to create a cascade event. | Cans can be cylindrical or prismatic. They are typically safer than pouches, since they are are valve regulated and benefits from a hard shell, reducing their perforation and crushing risk. However, it is generally harder to cool them down. Cylindrical cans also have a lower packing factor than prismatic cells leading to an effectively reduced volumetric density. However, the process of cylindrical cell fabrication is so well honed that battery packs made with state of the art cylindrical cans compare in terms of metrics with battery packs made with state of the art pouches. Cans usually also contain a positive thermal coefficient (PTC) resistor that can protect the cell in case of a short circuit. However, this protection is usually not rated for high voltage packs, can could actually ignite a fire, in a high voltage pack. Cans also can ignite when perforated. Wrapping cells with a phase change material (PCM) is one of the solutions<br />that have been developed to absorb heat in a thermal event, reducing the odds to create a cascade event. | ||
| − | ==Cells capacity vs safety== | + | ===Cells capacity vs safety=== |
At the cell level, a smaller capacity cell is safer than a higher capacity cell: a small capacity cell may not store enough energy to generate a thermal runaway. However, it is false to assume that a huge capacity pouch is more dangerous than a couple of low capacity cells put in parallel. High capacity cells are made under extremely high quality control, while low capacity 18650s are not: it's more cash efficient to discard under performing these cells. Discarding a 240 Ah cell would be extremely costly for a manufacturer. Also, high capacity cells include various safety techniques (including internal self fusing) which intent is to prevent a problematic layer of the cell to make the cell enter a thermal runway. | At the cell level, a smaller capacity cell is safer than a higher capacity cell: a small capacity cell may not store enough energy to generate a thermal runaway. However, it is false to assume that a huge capacity pouch is more dangerous than a couple of low capacity cells put in parallel. High capacity cells are made under extremely high quality control, while low capacity 18650s are not: it's more cash efficient to discard under performing these cells. Discarding a 240 Ah cell would be extremely costly for a manufacturer. Also, high capacity cells include various safety techniques (including internal self fusing) which intent is to prevent a problematic layer of the cell to make the cell enter a thermal runway. | ||
| − | =Packaging= | + | ==Packaging== |
| − | =Possible causes of a thermal runway= | + | ==Possible causes of a thermal runway== |
| − | =What to do in case of a battery fire= | + | ==What to do in case of a battery fire== |
| − | =How to properly use a battery pack= | + | ==How to properly use a battery pack== |
[[Category:Electric Vehicle]] | [[Category:Electric Vehicle]] | ||
Revision as of 19:12, 19 May 2020
Contents
Introduction
Safety & reliability should rank pretty high in design criteria as it's impossible to win a competition with a car who caught fire.
Cell selection
The selection process of a lithium-ion/lithium-polymer cell is highly dependent on the application it is being used for. Thus, before even starting battery design, it is crucial to understand the battery operating conditions. A light car with 2WD would requires less power than a heavy 4WD with wings. Generally speaking, there is a trade-off between cell energy density (Wh/kg) and cell power density (W/kg). It is then possible to compare the performance of two cell chemistries using lap simulation: there are cases where a higher specific energy cell is optimal, and others where high specific power is.
Cans vs pouches
Lithium cells come the most often in cylindrical cans format (i.e. 18650) or in soft metal-plastic pouches. It is easier to design a safe battery pack with cylindrical cells as each cells are protected in a hard shell that protects them from accidental perforation. Cylindrical cells tend to offer higher energy density, while pouches tend to offer higher power density. Cylindrical cells can be spot welded together with fairly inexpensive equipment, while pouches can be connected together by ultrasonic welding, laser welding, or mechanical assembly (rivets, nuts&bolts, ...). While cylindrical cells are typically found in small capacities ( < 4 Ah ), pouches can be found in high capacities ( > 10 Ah), greatly reducing the complexity of assembly of a battery pack, since fewer cells are required to meet the battery pack energy requirement.
Pouches
Pouches offer better heat dissipation than other type of cells, but their flexible and thin enveloppe make them prone to perforation or crushing. Some pouches have tabs on the same edge, some, on opposite edges. Opposite edge tabs allow better heat dissipation, but make packaging more complex. They stack easily, using most of the available space. The cells edges are fragile: treat them with care. It is preferable to ask the cell manufacturer to have the edges taped. Most pouches will ignite in case of a nail penetration. One way to reduce the risk it would ignite is to improve the cooling of the cell. As an example, using sheets of metal between cells would spread the heat (and evacuate it out of the cell).
Cans
Cans can be cylindrical or prismatic. They are typically safer than pouches, since they are are valve regulated and benefits from a hard shell, reducing their perforation and crushing risk. However, it is generally harder to cool them down. Cylindrical cans also have a lower packing factor than prismatic cells leading to an effectively reduced volumetric density. However, the process of cylindrical cell fabrication is so well honed that battery packs made with state of the art cylindrical cans compare in terms of metrics with battery packs made with state of the art pouches. Cans usually also contain a positive thermal coefficient (PTC) resistor that can protect the cell in case of a short circuit. However, this protection is usually not rated for high voltage packs, can could actually ignite a fire, in a high voltage pack. Cans also can ignite when perforated. Wrapping cells with a phase change material (PCM) is one of the solutions
that have been developed to absorb heat in a thermal event, reducing the odds to create a cascade event.
Cells capacity vs safety
At the cell level, a smaller capacity cell is safer than a higher capacity cell: a small capacity cell may not store enough energy to generate a thermal runaway. However, it is false to assume that a huge capacity pouch is more dangerous than a couple of low capacity cells put in parallel. High capacity cells are made under extremely high quality control, while low capacity 18650s are not: it's more cash efficient to discard under performing these cells. Discarding a 240 Ah cell would be extremely costly for a manufacturer. Also, high capacity cells include various safety techniques (including internal self fusing) which intent is to prevent a problematic layer of the cell to make the cell enter a thermal runway.
