Battery pack
The battery pack contains the electrical potential energy used by the traction system and onboard electronics. The battery pack is often referred to in industry as the rechargeable energy storage system or RESS.
Contents
Introduction
Goals
Safety & reliability should rank pretty high in design criteria as it's impossible to win a competition with a car who caught fire.
Performance Goals
The battery pack capacity should be sufficient to complete the desired driving cycle, frequently this is based on the endurance circuit at competition or the expected cumulative drive cycle over the course of a test day.
The battery pack should be capable of safely discharging at the commanded current rate, both peak and continuous.
The highest rate of charge is often in regenerative braking. The pack should be designed to safely accommodate the peak and continuous charge rates from regen braking.
Design Goals
The pack may be required to meet other goals such as packaging requirements, mass, cost, material specification, etc. These can preclude meeting design goals, particularly capacity. Correctly prioritizing team and system goals will be critical in making these compromises.
Cell selection
The selection process of a lithium-ion/lithium-polymer cell is highly dependent on the application. Before even starting battery pack design, it is crucial to understand the operating conditions. A light-weight car with 2WD requires less power than a heavy 4WD car with a full aero package. Because 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 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
The most common form factors for lithium cells are the can (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 envelope 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).
Pouch cells can come in larger capacities than cylindrical cells. If the capacity is large enough you may not need any parallel cell connections at all.
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.
Arrangement of cells
Cells can either be connected in series (positive to negative) or parallel (positive to positive, negative to negative). Connecting cells in series raises the pack voltage, and connecting them in parallel increases the energy capacity. So if you have two 3.6V 3Ah cells, connecting them in series gets you a 7.2V 3Ah battery pack. Connecting them in parallel gets you a 3.6V 6Ah battery pack.
The cell arrangement is often labeled as "nSmP", where n is the number of cells in series and m is the number of cells in parallel. So a pack with 50 cells in series and 8 cells in parallel would be 50S8P. Multiplying n and m gives you the total number of cells in the battery pack.
Your maximum tractive system voltage will set how many cells in series you use, and you need enough energy capacity to make it through the endurance event. This is where simulations can help you decide how big of a pack you need.
Higher voltage, or higher current?
Simulations can tell you how many Wh your accumulator needs, which tells you how many cells you need. What the simulation can't tell you is whether to put more cells in series or more in parallel. A 6 kWh pack could be 600V and 10Ah, or it could be 60V and 100Ah. A higher voltage will also mean lower current (a 60kW draw at 600V is 100A, but at 60V it's 1000A).
Higher voltage/lower current pack
Pros:
- Lower current means smaller wires, less I²R heat losses.
- Smaller wires mean less weight, smaller wire bend radius
Cons:
- BMS needs to monitor more voltages
- Higher voltage means a bigger spark gap, so less safe
- Higher voltages are harder to find parts for
Lower voltage/higher current pack
Pros:
- Smaller BMS
- Parts rated to lower voltages can be easier to find
- Lower voltages are theoretically safer
Cons:
- Tractive system conductors (wires, etc) get bigger with the square of current (because of I²R heat losses)
- Bigger wires mean more weight and bigger bend radius
Other possible constraints
- The rules (max segment voltage, max segment Joules)
- Motor controller selection
- Motor selection
- Other misc. component selection (AIRs, DC-DC converter, etc.)
- Packaging constraints
- Any legacy designs from your team
- University safety policies
Packaging
A battery pack has to be designed to resists the mechanical loads and impacts it will be subjected to and the thermal conditions it may be exposed in case of a thermal runaway. Also, it must be designed considering electrical safety in mind.
Requirements may differ according to the cell type choice. Pouch cells use a thin flexible shell, which may easily by punctured. Hence, it requires an additional protection and more professional fabrication processes to protect it from being scratched, punctured, teared. Cylindrical and prismatic cells, on the other side, encapsulate the active materials in a hard shell. However, if the shell is made of a conductive material, it may require a dielectric barrier to avoid a short circuit when cells are packaged in series.
Mechanical loads
The battery pack must resists to impacts and vibration. Bolts need to have positive locking mechanism. The battery pack should be built strong enough in a way that cells are not crushed in case of a drop on one of its corners.
Thermal loads
Lithium-ion cells don't like heat. As per the rules, cells exterior casing shouldn't reach 60 Celsius. Although it's easy to circumvent the rules here, no one should do it. The heat sources may be external such as the motors, inverters, solar load, etc. or internal from heat generated by the module's internal resistance.
The internal resistances of the modules can come from the bus bars, fusing, contactors, or other power distribution components. Most likely, the largest source of internal heat generation will come from the internal resistance of the cells (DCIR). DCIR is dependent on temperature, state of health, state of charge, and discharge time [1]
Safety
Sharp edges inside the battery pack should be avoided. Appropriate clearance between wires should be maintained. Ensure that large voltage differences are far from each other. Positive locking mechanisms should ensure that screws don't loosen. It should be impossible to create a short circuit in case of the drop of tools. Think what could go wrong in case of an accident, and try to prevent it.
Materials
Dielectric materials and adhesives shall be fire retardant rated and be able to sustain to high temperatures.
Grounding
Conductive materials in the battery pack shall be grounded.
Segregation
As per the rules, modules need a fire retardant confinement to avoid a fire propagation within the battery pack.
Interconnection
Several techniques allow the cells interconnection. They are divided in two categories: the usage of mechanical fasteners and the usage of welding.
Fasteners
The concept behind the usage of fasteners is to compress the current tabs together. In this kind of joint, conduction increases with area and with pressure. Hence, the goal is to apply a load as uniform as possible on an area as big as possible.
Grease
With time, the interface resistance can increase, due to oxidation. One can slow down the oxidation rate with the usage of grease. Grease is actually non conductive but as little impact on overall conduction as it is squished away by the pressure. Some greases contains conductive particles in order to improve their conductivity. Examples are De-Ox or Penetrox.
Ultrasonic welding
Ultrasonic welding is one of the best available techniques to join different materials (as found in lithium cells). It's the recommended technique for pouch cells. However, this type of welding equipment is expensive.
Spot welding (also known as resistance welding)
Resistance welding is speedy and low cost. It doesn't transfer a lot of heat to cell and welding quality is good.However, conductive materials (aluminum, copper, ...) require more power to weld than resistive materials(nickel, ...). It's the recommended technique for cylindrical cells.
Brazing or Soldering
Brazing and soldering are generally not recommended, because the heat required to melt the brazing alloy can damage the cells components. They can also result in brittle connections that are prone to failure.
Crimping
Crimping cell terminal is rare but can be done with extremely simple tools.
Possible causes of a thermal runway
Thermal runway (i.e. battery fire) can be caused by an internal short that would be due to a poor cell design and manufacturing quality control. However, it is more likely that it is due to abuse conditions.
Abuse conditions
- Charging when too cold
- Charging when too hot
- Discharging when too hot
- Storage at elevated temperature
- Internal damage due to high heat when interconnecting (by soldering)
- Overcharge
- Overdischarge or charging an overdischarged cell
- Overcurrent
- Crush
- Impact or drop
- Nail penetration
- Sharp chips / dust
- Leakage current
What to do in case of a battery fire
Toxicity
Lithium batteries are toxic. Especially their electrolyte, which is flammable and contains poison salts.
What should I do?
In case of a fire, ensure that nobody is in danger of breathing in the smoke. If nobody is in danger, sound the fire alarm and get away. If somebody is in danger, try to save them as fast as possible so you won't get hurt yourself, as people are more important that objects. Lithium caused fires are not easy to put out, even for people that know how. Smother the fire with Sand. ABC extinguishers will not do much except give the cause the fire to slow. If it starts within your car, don't think that you can save it, as its outcome is not good after it starts. Better yet, don't let batteries you own ever get to the point of causing a fire. This is the idea when using Lithium, you must prevent the batteries from getting to that point. too much voltage or too little voltage, too much current running through, or the temperature reaching too high, each have particularly nasty consequences. For any person using lithium batteries, it is wise to read up on this technology before using, so that one can become aware of the challenges of putting out a fire caused by lithium-cell batteries.
How to properly use a battery pack
The proper usage of the battery shall be limited by the battery management system (BMS). Here's a list of advice:
- As per the rules, avoid exceeding 60 Celsius degree.
- Do not overcharge the cells (generally over 4.2V each), nor over discharge the cells(generally under 2.7V each).
- As per the rules, ensure that the battery pack design does not allow the drop of tools to create a short circuit.
- Ensure that all conductive materials in the battery pack is grounded.
- Ensure that all materials of the battery pack are fire retardant.
The proper storage of the battery shall be:
- In a fireproof container.
- At room temperature or cooler (< 25 Celsius degree).
- In a stable position.
- In a well ventilated area.
