LFP, or "Lithium Iron Phosphate batteries," is becoming increasingly common in car manufacturers' "standard range models." One reason for this is that this battery type has a cost-effective manufacturing technique, although with the downside of lower energy density and higher sensitivity to cold temperatures compared to equivalent lithium-ion batteries like NMC. Joakim Hansson, Quality and HSE Manager at DEKRA Automotive AB, explains:
– “Standard range” models often have smaller batteries than 'long range' models, which means you need to cycle the battery more to cover longer distances. LFP is suitable for this purpose as they are relatively insensitive to large cycles and can literally handle thousands of charge and discharge cycles. Another advantage of LFP compared to other lithium-based batteries is that the risk of a battery fire is significantly lower in the event of damage.
LFP batteries and BMS
The primary function of a Battery Management System (BMS) is to monitor the battery's State of Charge (SOC) by measuring its resting voltage. The BMS uses a built-in table of voltage levels that the system adapts to, and when the BMS measures the resting voltage, it also determines the SOC.
– During driving, it's not possible to measure the resting voltage. Therefore, the car relies on the last known SOC and subtracts consumed energy while displaying a "preliminary" SOC on the car's screen. After parking, when the resting voltage can be measured again, the SOC is updated.
– LFP batteries have a lower voltage than NCA and NMC batteries, but they also have a very flat voltage curve between 25 and 85 percent SOC. This makes it challenging for the BMS to calculate the exact SOC even when the battery is at rest. The BMS can calculate energy gains and losses, but in practice, this only works for a short time before the calculation starts to deviate from reality.
Joakim Hansson explains that energy is lost through heat in the battery, which varies with the battery's temperature, power output, and wear. The car cannot measure this energy loss in its BMS, even though many manufacturers likely have estimates of heat losses.
– As a result, if the BMS is accurate on day one, the system will likely start to have some errors on day two and even more on day three, and so on. One day, you may find that your car doesn't reach its destination because the battery suddenly runs out of power. To avoid this, car manufacturers recommend fully charging LFP cars at least once a week.
When you charge fully, the charger maintains a constant voltage until the battery stops receiving power, at which point the BMS knows the battery is full. From that point, the system has a fixed reference point to calculate energy gains and losses in the next battery cycle.
Dynamic Battery Buffer
To compensate for the increasing divergence between the BMS calculation and the actual state of charge (SOC) in LFP batteries, some automakers like Tesla have introduced dynamic buffers. This means the portion of the battery's capacity below zero percent becomes adaptable.
– As the BMS encounters uncertainty, the buffer can be expanded to ensure there's an extra margin to prevent drivers from getting stranded with an empty battery. When the buffer expands, there's less energy available in the 100 to 0 percent range. If you're using a "km display" instead of a percentage, each kilometer will contain less energy, resulting in a shorter range even if the car shows 100 percent.
– If you frequently charge your LFP battery to full, there will be shorter intervals between the fixed points where the BMS knows the exact SOC. It's also reasonable to expect that the buffer will decrease, providing more energy to use between 100 and 0 percent.
Joakim Hansson explains that this is why it's recommended to charge an LFP battery to full at least once a week, and many car manufacturers suggest keeping the setting at 100 percent. By following the guidelines for your specific car model, you maximize the range and improve the car's ability to determine SOC accurately.
Degradation in LFP Batteries
LFP batteries essentially work like other lithium-based batteries, meaning they perform best at a low state of charge (SOC). Research results on battery wear based on age always lag a few years behind because they need to be tested for one to two years before there's reliable data to review.
– One might think that the latest LFP batteries have aged less, but it's unlikely that the fundamental chemical principles governing their operation have been magically altered. Therefore, we likely have roughly the same degradation principles as other battery types.
According to the battery manufacturer CATL, their LFP batteries can withstand 4,000 to 8,000 full FCE cycles before losing 20 percent of their original capacity.
– If the batteries were to endure 5,000 FCE cycles, it would equate to roughly 5,000 multiplied by 300 kilometers, which is 1,500,000 kilometers before they degrade by 20 percent. This means the batteries would lose approximately 0.13 percent per ten thousand kilometers if you were driving full cycles.
When an electric car equipped with an LFP battery has covered just over 50,000 kilometers, it's reasonable to assume the battery has lost a maximum of 0.67 percent. However, in reality, it has degraded by 6.6 percent. What's the reason for this? Joakim Hansson explains:
– The remaining part beyond the 0.67 percent is wear due to aging. To minimize degradation in LFP batteries, it would be optimal to keep a low SOC. But in practice, it becomes a bit tricky as you should charge the battery fully relatively often for the car's BMS to maintain its accuracy.
– Battery degradation decreases over time, no matter how you look at it. If you've lost six percent after three years, it will take about six more years to double that. While driving a lot, it might be possible to optimize the battery by fully charging it for a longer trip every week and then maintaining a lower SOC. However, in practice, it's probably easiest to accept that some battery degradation will occur.
- BMS = Battery Management System
- NMC = Nickel Manganese Cobalt
- NCA = Nickel Cobalt Aluminum
- LFP = Lithium Iron Phosphate
- SOC = State of Charge