The increasing demand for electric vehicles (EVs) is seeing manufacturers developing and releasing more affordable models. The most important factor when designing EV batteries or battery management systems (BMSs) is safety. Safety is collectively pursued in industry via stringent regulation and certification and ensured through testing.
In this article, we’ll take a high-level view of which factors battery standard tests cover for electric vehicle battery systems.
EV batteries require thorough testing to ensure they’re safe enough for commercial use. Image used courtesy of UL (Underwriter Laboratories).
Why EV Battery Testing Matters
Since cars consume large amounts of power only battery technologies with high power densities are desirable and because cars need to be used on a daily basis the battery must be rechargeable. However, their greater energy storage capability (resulting from the use of reactive metals), means that they can be more destructive should they fail.
During failure, a lithium-based battery can release large quantities of hydrogen gas and the intense heat caused by an internal short-circuit will ignite the hydrogen effectively creating a flamethrower. Since cars are at risk of damage from collisions, it is essential that batteries contain multiple safety mechanisms to ensure that under no circumstance can the battery ignite.
“Safety strategies” for each EV system level, as laid out by a GM presentation for the United Nations Economic Commission for Europe in 2013
EV Testing Parameters
Like with any engineering project the first step is to always understand the environment that your product is expected to survive in.
The automotive industry is far from forgiving so what factors would need to be considered with regards to batteries?
Mechanical stress and effects are very profound in the automotive industry due to the constant motion of wheels, and uneven surfaces that the vehicle may be in contact with. Therefore, a battery system must be able to handle these vigorous vibrations for long periods of time. Vehicle collisions are also a real threat and any battery system that experiences such an event must be able to either survive or break graciously. It is essential that any impact or stress applied to the battery system does not cause a fire or leak explosive gasses.
While electric vehicles do not have engines, the large currents drawn from the batteries will lead to temperature increases. These batteries, however, may also be expected to survive very low temperatures depending on where the vehicle is used. But temperature swings can happen daily too with cold nights and hot days which could see frequent thermal stresses that will need to be considered. If the vehicle is left in the sun and not in use then designers may need to consider if automated cooling systems are required which are never disengaged.
Under normal conditions, designers will be able to determine the current draw from the batteries to the motors and thus make determinations on cable size and fuse settings. However, many other electrical factors need to be considered that may have serious impact on the battery. For one, the rate of change of voltage on the batteries during charging and discharging can cause overheating. Another factor to consider would be voltage spikes from the mains supply during grid faults which can cause serious damage to the battery system.
Types of Testing for Compliance
There are many standards in place for electric vehicles that are dictated by regulatory bodies. The ISO has over 40 standards published which cover safety from the level of individual components all the way up to V2X protocols. UL (Underwriter Laboratories) is another regulatory body that puts forth codified rules for battery testing, including UL 2580, which is concerned with electric vehicle battery safety and lays out requirements of how those batteries must be able to tolerate a wide spectrum of abuse.
While there are many standards and regulators, the tests designed for battery safety are all focused on ensuring that EV batteries can withstand various environments and conditions that could cause safety issues.
These conditions include:
- Standard thermal tests – Storage at various temperatures, etc.
- Thermal abuse – Withstand sudden high temperatures, etc.
- Loss of thermal cooling – Withstand lack of cooling
- Mechanical vibration – Withstand vehicle vibrations
- Mechanical shock – Withstand sudden shocks
- Mechanical impact and crush – Measure how the battery will cope with being crushed in a collision
- Mechanical penetration – Withstand being penetrated (i.e., short cells)
- Electrical short circuit – Handle being shorted
- Electrical overcharge – Overcharging does not cause damage to the battery or lead to a fault
- Electrical force discharge – Determine if rapid discharge causes damage via gas build-up etc.
- EMC susceptibility – Withstand sources of EM interference
- High voltage protection – Protection from sudden high voltages
- Electrical shutdown integrity – Ensure that shutdown separators function correctly
- Electrical imbalance charging – Ensure that the battery is safe during imbalanced charging
- Environmental – Survive altitude, moisture, humidity, and fire
- Water immersion – Ensure that the battery remains safe during water immersion
Standards are put in place to ensure that not only do designs use common compatible hardware and software solutions but to ensure that they meet a minimum level of safety. Electric battery systems are arguably more prone to fault and damage when compared to their engine/fuel counterparts as they are very sensitive to shock, vibration, temperature swings, and penetration. Therefore, it is essential that strict standards are followed when you incorporate electric battery systems to ensure that they remain safe in even the most hostile scenarios.