Knowing the State of Health (SoH) and safety status of lithium-ion batteries is fundamental for managing electric vehicle safety, maintenance, and battery warranty. Yet, keeping a battery system safe and reliable is not as easy as just operating the battery within specified thresholds. It needs far more advanced solutions to ensure seamless, safe, and profitable operation. For battery health and safety, key performance indicators such as SoH or State of Charge (SoC), are important values to keep track of. However, determining accurate battery SoH remains a considerable challenge. There is a lack of consensus in the industry about how SoH should be calculated, meaning there is no standardized approach. Different manufacturers also calculate SoH differently. This creates complexity, particularly for fleet operators as various parties might refer to their very own SoH calculation. On-board battery management systems (BMS) sometimes, but not always, provide an estimation of SoH. However, the accuracy of the estimations decreases over the lifetime of a battery, meaning that on-board BMS alone are not sufficient in assessing a battery’s health over its entire lifetime. These barriers to predicting battery health can be overcome by deploying battery analytics.
Why is it Important to Assess Battery Health?
Lithium-ion batteries age over time, depending on how they are used. They lose capacity and increase their resistance, which, when used in vehicles, results in reduced driving performance and electric range. The warranty provided by an electric car manufacturer usually ends when the battery reaches 70-80% of its original capacity. The time and mileage until the battery is considered ‘dead’ must therefore be maximized, so that an electric vehicle can be operated for as many years as possible. This is necessary to increase financial returns, as well as improve the climate footprint of the battery. At any given time, the electric vehicle owner, the car and battery manufacturer, the fleet operator, or the insurance provider may want to understand how the battery is used, and how it performs in terms of its SoH. Doing so not only allows these stakeholders to gain transparency into the health of the battery, it also enables owners and manufacturers to streamline their daily operations and protect their long-term business case. For example, fleet operators need to know when a battery’s performance has decreased to such an extent that it will not be able to complete certain routes. SoH data is also crucial in terms of understanding when warranties can be claimed from the manufacturer. Providing SoH data is not a standard approach, but it can be enabled by transmitting battery data from the vehicle to a cloud platform. From such a platform, the SoH can be calculated using algorithms, and can be accessed by all stakeholders if and when desired.Why Should You Care About Battery Safety?
As well as accurately determining a battery’s SoH, ensuring safe operation should also be a top priority. If a battery malfunctions, it poses a risk to the vehicle occupants, bystanders, and potentially emergency response personnel in the case of a serious accident. Safety issues are also closely related to long-term battery performance and reliability, as such issues will often accelerate wear and degradation. In terms of reliability, it is not uncommon for a BMS to shut down a battery at a threshold that falls below a severe safety incident. This leads to unnecessary downtime, due to the BMS’s inability to sufficiently assess the issue in question. Limiting this downtime, in addition to preventing severe safety issues, is an essential aspect of efficient battery management. So accurately assessing battery safety and reliability is very important, but unfortunately doing so can be quite complex, as a battery breakdown or fire is usually caused by an accumulation of events over time. Eventually, these events lead to some sort of battery failure. Thermal runaway, a situation where the battery enters a rapid self-heating state, which leads to overheating, fire, or sometimes even explosion, is a common example of such a failure. Internal short circuits are another, caused in various ways by lithium plating and dendrite growth. Because determining the exact cause of such issues can be quite complicated, relying solely on BMS to monitor battery safety is not advised.How Can Battery Health be Assessed?
SoC and SoH cannot be accurately physically measured in a vehicle. Estimation models hosted by the on-board BMS estimate these by a combination of physical measurements and models, which are limited to only measuring temperature, voltage, and current. The BMS consists of a hardware element and a software component, which keeps the battery in a ‘happy state’ at any given time during vehicle operation. It limits minimum and maximum cell voltage, current, and temperature, and is developed towards the specific battery cell chemistry, and the electrical arrangement and cooling of the modules and packs. One of the primary objectives for the BMS is the safe and reliable operation of the battery at any given time, not the optimization over time. During operation, the BMS determines the SoC and (potentially) the SoH of the battery.- State of Charge: The typical SoC estimation algorithm integrated into the on-board BMS requires prior battery cell testing. It is accurate at the beginning of life but loses its accuracy over time if not updated.
- State of Health: Not every car manufacturer has an on-board SoH model integrated in their BMS. Traditional SoH models require extensive cell testing as validation ‘on-board only’ is impossible.
Why Relying on BMS is Not Enough
For numerous reasons, relying solely on BMS does not guarantee battery safety, or allow for accurate SoH measurements.For Battery Safety
- From a battery safety perspective, to begin with, BMS can themselves malfunction. And a malfunctioning BMS can lead to battery over-charging or deep discharging, causing batteries to exceed safe voltage, current, and temperature thresholds.
- BMS also have limited access to historical data as they are rarely built to log information. Combined with limited computer power, this means BMS do not analyze historical data, something that is crucial for detecting complex safety issues that arise over time. Due to an inability to detect or prevent reactions occurring inside cells, common causes of battery fire such as short circuits caused by lithium plating and dendrite growth also go undetected by BMS.
- Adding to this, as BMS function is to control only one battery, it is not possible to gain an overview of an entire fleet of vehicles with a BMS.
