Battery pack

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.

Packaging

Pouch Cell Module Design Example. Credits: Poly eRacing 2014 (Polytechnique Montreal)

Cylindrical Cell Battery Pack Design Example. Credits: Poly eRacing 2016 (Polytechnique Montreal)

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.

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 joint different materials (as found in lithium cells). However, these welding equipment is expensive. It's the recommended technique for pouch cells.

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

Brazing (soldering) is generally not recommended, because the heat required to melt the brazing alloy can damage the cells components.

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