The EV charging curve is a crucial concept for electric vehicle (EV) owners and enthusiasts. It describes the varying rate at which an EV battery charges over time, influenced by factors such as battery capacity, state of charge (SOC), temperature, and the type of charger used. Here’s a detailed look at how the charging process works and why the 80% rule is recommended for optimal battery health.
The Phases of the Charging Curve
- Initial Phase: Rapid Charging. When an EV battery is in a low SOC, typically between 0% and 20%, it charges at its fastest rate capability. This phase, often referred to as the “bulk” phase, can quickly replenish a significant portion of the battery. The vehicle’s onboard systems allow maximum power from the charger, optimizing efficiency and reducing charge time.
- Mid-Phase: Slowing Down. As the battery reaches anywhere between 60-80% SOC, the charging rate begins to slow down. This reduction occurs due to thermal management and to prevent overloading the battery cells. The charging system reduces the current to manage heat and maintain battery health. This phase is crucial for ensuring longevity and preventing battery degradation.
- Final Phase: Tapering Off. The final 20% of the battery’s charge is the slowest. Known as the “tapering” phase, the charging power decreases significantly to balance the cells and ensure the battery reaches full capacity safely. This phase can take as long or longer than the initial rapid charging phase, which is why many drivers prefer to charge up to 80% during short stops.
Factors Influencing the Charging Curve
- Charger Type: AC and DC Chargers each impact the charging curve differently. DC chargers offer rapid charging but may be limited by the vehicle’s maximum charging rate, while AC Chargers are more traditionally associated with slower home charging.
- Temperature: Battery temperature plays a significant role. Cold temperatures can slow down the charging rate, while too much heat requires the system to reduce power to manage thermal conditions. Many of the latest EV’s will pre-condition the battery, in advance of a planned charge, to optimise charge time. This preconditioning is enabled via use of the EV Sat Nav/Maps feature when planning a charging session during a road trip.
- Battery Technology: Different EVs use varying battery chemistries, affecting their charging curves.
- The vehicles EV Architecture, 400V or 800V. The higher the cars voltage, the higher potential for passing electricity around a system; in a nutshell, a 800v system has the capacity of handling more ‘pressure’ than a 400V system, and can charge at a higher rate for longer.
The 80% Rule
The primary reason for the 80% rule is to prolong the health and longevity of your EV’s battery. Lithium-ion batteries, which are commonly used in electric cars, operate most efficiently and have a longer lifespan when kept within a charge range of 20% to 80%. Charging to 100% is perfectly safe and sometimes necessary, especially for long trips, but consistently doing so can strain the battery over time, reducing its overall lifespan and efficiency.
By only charging up to 80%, you not only help maintain your battery’s health but also save significant time. For instance, the time taken to charge from 80% to 100% can often be nearly as long as charging from 10% to 80 (at DC chargers only). For daily commutes, charging to 80% provides ample range while allowing for faster, more efficient charging sessions.
It is perfectly OK to charge a car to 100% if you plan on using it soon after, keeping a battery at max state of charge for long periods of time can be detrimental to the battery packs lifespan.
Optimizing the Charging Experience
Understanding the charging curve helps EV owners plan their charging sessions more effectively. Here are some tips:
- Home Charging: Use AC home chargers for overnight charging to maximize convenience and cost savings.
- Charge During Off-Peak Hours: To avoid high demand and potential wait times at public chargers.
- Utilize Fast Chargers for Quick Top-Ups: Especially when needing a rapid charge up to 80%.
- Avoid DC Charging when battery in a high SOC: Try to limit use of DC Charging to instances when SOC is below 40%, thus ensuring you benefit from the higher rate of charging your EV is capable of taking.
Etiquette and Practicality
Charging etiquette is another factor to consider. When using public charging stations, occupying a spot to charge from 80% to 100% can inconvenience others, especially given the slower charging rates at higher capacities. By sticking to the 80% rule, you help ensure that chargers are available for others to use more promptly.
The EV charging curve is an essential aspect of electric vehicle ownership, impacting how quickly and efficiently a vehicle can be charged. By understanding the different phases of charging and the factors that influence them, EV owners can better manage their charging habits, ensure battery longevity, and optimize their driving experience.
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