Thermal Issues In Battery Packs For Electric Vehicles

Guest Commentary August 2019 Thermal Issues Battery Packs Electric Vehicles

KARTIK GOPAL is an Independent Consultant – Electric Vehicles

India is poised to be one of the first countries with a tropical climate to adopt electric vehicles (EVs) with lithium-ion battery packs, on a hopefully large scale. Countries currently adopting EVs such as China, Norway and US lie in temperate regions. High ambient temperatures have adverse effects on safety, durability and performance of lithium-ion batteries.

In India, we are likely to see a sizeable adoption of two- and three-wheeler EVs, which pose constraints on cost, weight and volume available to package the battery packs. Lastly, EVs are also expected to be adopted in fleet operations, where daily distances are high and fleet operators expect EVs to offer a lower total cost of ownership. Batteries need to last for a large number of charge-discharge cycles, or in other words, have a low cost per “extracted kWh” (upfront cost divided by the total energy discharged from the pack over its useful cycle life).

The challenge of designing safe, durable, cost-effective, high performance packs under such conditions throws up opportunities for innovative pack designs and battery thermal management solutions (BTMS). This article, intended for a non-technical audience, aims to provide a brief overview of the issues and current solutions available and some considerations in the BTMS design space.

IMPACT OF HIGH TEMPERATURE ON LITHIUM-ION BATTERIES

The “ideal” temperature for using lithium-ion batteries is in the range between 15-35 °C. Higher than this, the cell’s cycle life starts to degrade rapidly. One of the published studies shows that at 15 °C, a lithium-ion ferrous phosphate cell lost ~ 7 % capacity after 2,628 cycles, while at 45 °C, it lost 22 % capacity after only 1,376 cycles. Loss of capacity reduces range per full charge and hastens battery replacement, adversely impacting the financials for fleet operations.

Secondly, at high temperatures, to prevent further temperature build-up and thermal runaways, the charging rate may need to be reduced, thereby increasing time for charging and reducing vehicle uptime. Likewise, the discharge power may also need to be curtailed at high temperatures, thereby reducing top speed and acceleration. Most cell manufacturers would allow operations only up to around 50 °C or 55 °C to reduce the risk of thermal runaway. At temperatures close to this, the vehicle may need to be ‘shut down’ temporarily till the pack cools down.

Thirdly, cathodes of current lithium-ion cells contain a metal-oxide (e.g. ferrous phosphate, nickel-manganese-cobalt oxide). A fire, once kindled in a cell on account of high temperatures, tends to sustain itself using the oxygen contained in the cathode. This can result in serious safety issues.

High summer temperatures in most urban areas in India limit the possibility of using forced natural air for cooling. Hence, more effective battery thermal management systems (BTMS) are needed to ensure safety, longevity and functional robustness of EV battery packs.

FUNCTIONS OF A BTMS

A BTMS ensures the temperature of battery cells does not exceed specified limits – the variation in temperature between all the cells and modules in the pack is within an optimum range that ensures all relevant parameters are monitored and controlled and there are appropriate safety systems to prevent thermal runaways. A ‘good’ BTMS design is one that – achieves very high levels of safety, and optimises performance, cost, weight and volume, complexity (manufacturability, reliability & maintainability) and parasitic energy consumption without compromising on safety.

TYPES OF BTMS

There is a vast research literature on BTMS design options. A brief overview of BTMS design options is as follows:

1. BTMS with a Vapour Condensation Cycle (VCC)
i. Cabin HVAC based-air cooling: The cabin HVAC in cars can be used to circulate cool air through the battery pack. This is a relatively simpler and lower cost system, but cooling effectiveness is usually lower than liquid cooling.
ii. Liquid cooling coupled with HVAC: Metal plates with built-in piping through which liquid coolant can flow through are placed at appropriate locations on the battery pack. The coolant evacuates battery heat and exchanges it with a refrigerant flowing through a chiller, which is in turn, cooled using the “evaporator – condenser – radiator” system of conventional HVACs. Alternatively, the coolant may directly exchange heat through the radiator. This system offers a fair balance of cost and effectiveness, but is more complex and expensive than cabin air cooling.
iii. Liquid cooling with radiator: A simpler system using the coolant to pass through the metal plate attached to the battery.
iv. Refrigerant-based cooling: Rather than a coolant, the refrigerant used in HVAC systems is directly passed through the metal plates. This is simpler than (ii) above, but may involve higher power consumption since the compressor may need to be active at all times, even if a cabin HVAC is off.

2. BTMS without VCC
i. Phase Change Materials (PCM): These are materials that change phase at specific temperatures and absorb (or emit) heat in the process. These materials are placed around cells ensuring uniform heat absorption, but add weight and volume. These may work for brief operating hours in hot conditions (once the PCM has melted, it stays so until cooled and cannot absorb heat any further). It does not require any additional energy to operate (unless cooling it back takes up energy from the battery).
ii. Heat pipes: These use phase change heat transfers using pipes with an evaporator, an adiabatic section and a condenser. These are still under study for use in EVs.
iii. Thermoelectric element cooling system: These use the Peltier effect to convert thermal energy into electrical energy (and vice versa) to heat and cool batteries. These are also being studied for use in EVs.
iv. Metallic heat sinks with forced air cooling: This is the conventional approach to cooling an entity enclosed in metal (usually aluminium) heat sinks with natural or forced air circulating over it.

Several approaches that combine two or more options are also being tried. In the Indian context, given that two- and three-wheelers do not have “cabin HVAC”, any of the VCC implementations will add cost and complexity to the vehicle relative to status quo. These challenges require India-centric innovations to create effective and cost-effective BTMS. It is heartening to note that the innovations are being carried out by Indian two-wheeler companies like Ultraviolette, GoGreen BOV, and Greenfuel Energy, among others in this space.

OTHER CONSIDERATIONS

A robust thermal runaway prevention strategy is foundational to pack design. The pack and BTMS design should factor in the ambient environment of operations, driving and charging duty cycles, range & power requirements, vehicle integration constraints and sources of mechanical and thermal stresses.

Cell selection (a cell with a higher C-rating will heat up less than one rated lower at a given current), pack sizing, provisioning ‘eco’ driving modes to limit current drawn, pre-cooling of pack during charging, high quality welds & contacts and selecting appropriate pack construction materials can all assist the BTMS in addressing thermal issues. Thermal modelling and simulations are essential to understand heat generation & flows and evaluate design options. Finally, robust cell, pack and vehicle level validation are a must.

In a nascent EV market like India, a single, highly-publicised safety incident could shake the confidence of customers in EVs. Hence, a good BTMS is critical towards ensuring safety and durability. Literally, the uptake of EVs in India depends on being able to create really ‘cool’ EVs!