Get charged up! Simple ways to maximize the health of your electric vehicle’s battery.
Congratulations on being an electric vehicle (EV) driver. How about that handling and acceleration! Not having to pay for gasoline, the lower environmental impact, the list of benefits goes on… but you may also be inundated with “advice” from others on how best to charge your EV. You also may be wondering how you can maximize the lifespan of your vehicle. Fortunately, EVs are much easier to maintain compared to an internal combustion engine vehicle. Because the battery has the most impact on the efficiency of the EV, most recommendations will focus on keeping the battery as healthy as possible.
The good news is that with improved battery technology and battery management systems (BMS), there are a few simple guidelines that you can follow when driving and charging your EV. Overall, unless you are driving long distances daily or charging regularly in extreme temperatures, you can relax. Just charge and enjoy your EV while following these guidelines as a general rule.
- Those in the know store it low
Don’t store your EV at 100% state of charge (SOC, the percentage of available energy at a given point in time) for prolonged periods. A study looked at a battery stored at 100% SOC at both 50 and 30 degrees Celsius (122 and 86 degrees Fahrenheit, respectively). The 50 degree battery degraded to 60% of its capacity (the total amount of electricity produced by the battery’s electrochemical reactions) in less than 200 days while the 30 degree battery degraded to 85% in about 400 days. The study’s conclusion is that both heat and voltage are bad for the battery. If you are going on vacation, discharge the battery before you store the vehicle. There will be more battery degradation at a higher SOC because there will be a higher voltage in the battery and more heat. Higher temperatures can lead to faster battery degradation. 30% SOC is a good target level. If you need to charge your battery to 100% SOC prior to a trip, do it the night before so the battery is not at 100% SOC for an extended period. Interestingly, you do not have to worry about the SOC when storing in cold temperatures. By slowing down chemical reactions in your battery, the cold actually slows the degradation.
- Say yes to less
How much should you typically discharge your battery (known as “depth of discharge”, DOD)? Less is better. The previously mentioned study also showed that when a battery is charged/discharged constantly over the entire range from 0% to 100%, the battery capacity decreased to 50% in less than 100 days. When a battery was charged/discharged over a smaller range of 20%, it still had around 85% capacity after 400 days. Dr. Jeff Dahn, one of the pioneering developers of the lithium-ion battery, noted that would be the equivalent of 3200 full cycles over the 400 days and if the battery range is 250 miles at full charge, then this would be the equivalent of 800,000 miles before the battery decreased to 85% capacity. So ideally you charge regularly with a small DOD of 20% (this would be 50 miles of a battery with a range of 250 miles – the majority of US drivers average about 30-40 miles/day).
- Fill ’er up… to 75%
Don’t charge regularly to 100%. When you get above 75% SOC, there is a process called “microcracking” that can be accelerated in the cathode (electrical conductor in the battery), which leads to faster battery degradation. This mainly applies to EV batteries with a large nickel content, which is a large majority of batteries on the road currently.
- To DC or not to DC
Does direct current fast charging (DCFC) degrade the battery faster than Level I or Level II charging? Recurrent, which studies EV battery health, notes that both in the lab and theoretically, high voltage charging should cause battery degradation. They looked at 13,000 Teslas that are currently in use. They did not find a statistically significant difference in range between vehicles that fast charged more than 70% of the time compared to those that did less than 30% of the time. Of note, the majority of the vehicles studied were from 2018 on, so the study looks at just a small time period. We do not know if there will be a significant long-term effect further down the road.
A test by the Idaho National Laboratory also did not find significant effects of repeated DCFC. In Phoenix, they tested four 2012 Nissan Leaf EVs. Two were DC fast charged and the other two were Level 2 charged. They were driven down to 5 miles remaining of battery range and were then recharged (this process was done twice daily) for 50,000 miles. The battery capacity was measured at 10,000 mile intervals. They found the capacity loss for the DCFC EVs was larger than for the Level 2-charged EVs, though the difference was small compared to overall capacity loss. Moreover, they were charged twice daily year round in a hot environment, so this is likely the upper end for capacity loss in EVs with similar batteries.
However, Geotab found that, specifically in hot weather, the use of DCFC does appear to significantly impact the rate at which batteries degrade. They studied 3 groups of EVs that varied in the frequency of DCFC. After 48 months of charging they found a significantly lower battery state of health (difference for a particular battery compared to its new counterpart) for the group that frequently used DCFC.
In summary, avoid DCFC at high and low battery temperatures. The BMS should control the current if the battery is too hot or too cold to avoid damage to the battery. As long-term effects are not clear, it is best to try to limit regular DCFC when possible. Also avoid DCFC when the battery is at the extremes of SOC (10 or 90%), as this can put more stress on the battery. Most EVs have software that slow charging once you reach 80% to protect the battery.
- No haste to replace
Will you have to replace the battery eventually? The chances are low. Recurrent looked at this question and noted across all years and models, outside of big recalls, only 2.5% have been replaced. For cars older than 2015, replacement rates are 13%, but under 1% for cars from 2016 and newer. As the EV battery is still a relatively new technology, if your battery does need to be replaced, it is likely going to happen early on and will be covered under the battery warranty (typical is 8 years or 100,000 miles). It is likely the battery will outlast the rest of the vehicle.
So there you go. Now that you know how to take charge of the health of your EV battery, do you know of someone who is thinking about purchasing an EV or are you interested in spreading the word about the benefits of driving an EV? Acterra runs financial incentive clinics for Bay Area residents. Consider becoming one of our EV ambassadors! These are both part of Acterra’s Karl Knapp GoEV Program, which is helping accelerate the shift from fossil fuels to a renewable energy economy by providing local residents with hands-on opportunities to experience the benefits of EVs.