From time to time, we get customers calling up asking for a health check for their EV battery, or more often, an EV they are planning to buy. Most consumers and many professionals believe health checks are simple “plug and analyse” checks that provide accurate and reliable results. But this simply isn’t the case.
Measuring state of health is a relatively straightforward process but takes time. Typically, more than 24 hours. Some EVs like the Tesla Model 3 have an on-board feature that performs this function, but many do not.
Basic concepts
Before we dive into the detail, it pays to understand a few basic concepts:
- Battery pack. An EV battery pack consists of many thousands of battery cells. The number and capacity of each cell determines the capacity of the battery pack.
- Lithium-ion battery charging characteristics. These batteries use a constant current constant voltage (or CCCV) charging strategy. When the battery reaches its maximum cell voltage capacity of around 4.5VDC (typically at around 80% capacity) charging current steadily falls away as the battery approaches its fully charged state.
- Fully charged. A lithium-ion battery cell is fully charged when the charging current reaches a pre-determined minimum.
- Fully discharged. A lithium-ion battery cell is fully discharged when the voltage drops to a pre-determined minimum, usually around 3.0VDC.
- Battery capacity. Battery pack capacity is the amount of energy released during discharge, or the amount of energy stored during charging. Typically expressed in kilowatt-hours or kWh.
- State of health. Actual battery capacity compared to the nameplate or “as new” capacity. For example, a new Model 3 2WD has a 57.5kWh battery capacity. If we measure actual capacity at 55kWh, then state of health will be 55/57.5 x 100, or 95.65%.
Measuring state of health
The only reliable way to measure state of health (or actual battery capacity) is to measure the amount of energy discharged from the battery pack from its fully charged state to its fully discharged state, or the amount of energy supplied when charging from fully discharged to fully charged.
For EVs that provide a measured state of health function, like the Tesla Model 3, the car will use a high energy on-board device like a cooling fan or the A/C system to fully discharge the battery. Once fully discharged, the car will then fully charge the battery pack and measure the amount of energy supplied by the charger. The energy accepted by the battery pack is the actual capacity.
The Mitsubishi Outlander PHEV has a similar function, sometimes referred to as a battery reset.
State of health apps and other readouts
There are plenty of apps and other devices on the market that allow users to plug into their EV and access a state of health readout. While this information is useful it’s important to remember that these readouts are calculated using an algorithm, not measured. These algorithms are derived from testing and represent typical degradation rates based on usage and charging characteristics.
Therefore, a state of health readout represents a typical battery pack, not the one fitted to the car. All being well, measured and calculated will be roughly similar but if the pack in your car has some issues actual capacity may be substantially different to the state of health readout.
Conclusion
EV battery health checks require more than just a quick plug-and-play assessment. While apps and readouts provide useful insights, they rely on algorithms and may not reflect the actual capacity of your battery. Accurate measurement involves a full charge and discharge cycle, which takes time but provides reliable results.
By understanding these nuances, you can make more informed decisions, whether maintaining your EV or evaluating one to purchase.
