BU-1003a: Battery Aging in an Electric Vehicle (EV)

The electric vehicle (EV) puts new demands on the battery and this modern energy source performs amazingly well in this new responsibility. But we ask: “Why does the battery in our mobile phones only last three years while the battery in an EV is good for more than 10 years?” Not all answers are known and the battery industry is littered with broken promises. Progress is being made but storing electrical energy economically remains one of our yet unresolved challenges in modern society.

The secret of longevity in the EV battery is oversizing and only operating in mid-range with plenty of “grace capacity” as spare in the upper and lower bands. Partial use reduces battery stress, but leaves valuable energy storage under-utilized. Oversizing also adds cost and weight, but this spare capacity will eventually get used when the capacity fades.

Charging the battery to only 80% and discharging to 20%, as is typically done on a new EV battery, only utilizes 60% of the capacity. As charge acceptance fades with use and time, the onboard BMS demands a higher charge and a lower discharge to meet the driving range. This adjustment remains unnoticed by the driver until a reduction in driving range is noticed. This occurs when the “grace capacity” is consumed.

Theoretically, depletion requires a full charge and full discharge to meet the energy requirements. At this point, battery stress increases and capacity fade accelerates, resulting in reduced driving range. This change is predictable and evolves over a few years of driving. Once the battery capacity has dropped to 70%, the EV can still be used for short commuting and errands. In most cases, capacity fade only reduces the driving range while power remains strong.

The mobile phone does not apply grace capacity to prolong battery life. In the interest of small size and long runtime, the battery is fully utilized from the beginning. From the user standpoint, it best not to discharge the mobile phone too deeply but charge it more often. Battery life can also be prolonged by a partial charge, but most chargers do not have a provision to set the charge limit. End-of-charge will need to be controlled manually. Phone manufacturers build in obsolesces that often correspond with a broken touchscreen or the desire for new features.

Most EV batteries have an 8-year warranty or a 160,000km (100,000 mile) drive limit. Automakers in California are required to extend the warranty to 10 years or 240,000km (150,000 miles). The goal of USABC (United States Advanced Battery Consortium) is a 15 year battery life and 1,000 cycles by 2020. Research labs already report up to 2,000 EFC (equivalent full cycles). With 2,000 cycles at 250km (156 miles) each, an EV battery would be good for 500,000km (312,000 miles). But tests conducted in a lab tend to show better results than in real life.

Figure 1 limits the driving range of a new battery by adding grace capacity shown in green. After about 900 cycles, the upper grace capacity is being consumed. Software adjustment can prolong battery life by adding more grace capacity as shown in the graph, but this reduces the driving range.

When all grace capacity is consumed, the battery hypothetical needs a full charge and a deeper discharge to meet the driving range. This is when reduction in driving range becomes noticeable year by year(See also BU-1003: Electric Vehicle, Figure 5)

A new battery has plenty of grace capacity that is gradually being depleted. Higher charge levels and a deeper discharge maintain the driving range but stresses increase. For this study, capacity drop in the grace range is 5% per 75,000km at first. This increases as the grace capacity is consumed.

Historical data from Tesla shows capacity degradation of about 5% after 80,000km (50,000 miles). EV manufacturers keep a close eye on battery performance and make adjustments when needed to extend battery longevity. In some cases this involves adding grace capacity, but this reduces driving range. The adjustment is done by a software upgrade at a service center or online with modern Tesla models. Some upgrades are mandatory to retain warranty and prolong battery life.

Figure 2 illustrates the driving range of a Tesla EV model carrying an 85kWh battery as published on social media. Section 1 delivered a steady range up to 95,000 miles on the odometer reading. Section 2 demonstrates a 5% decrease in range, and Section 3 denotes a software upgrade at 130,000 miles. This reduces the driving range by about 10% by adding grace capacity.

The 38,800 mile odometer reading when records were first taken delivered a 247 mile range. After a software upgrade at 132,000 miles, the driving range is reduced to 218 miles. Software upgrade is sometimes needed to prolong battery life.

Battery Aging

Battery aging is complex and not always predicable. Usage is a product of age, cycle count, charge speed, load levels and temperature. The University of Munich (TUM) did extensive tests simulating batteries in an EV. The test battery is a NCA Li-ion in an 18650 package, the same cell found in a Tesla EV. The cathode material of this cell is nickel, cobalt and aluminum, the anode is graphite; 18640 outlines the cell size that is 16mm in diameter and 64mm in length(See BU-301a: Types of Battery Cells)

Calendar Aging

Figure 3 investigates capacity fade as part of calendar aging over 700 days at different state-of-charge (SoC) levels and temperatures.

Lower charge voltages and cooler temperatures preserve the Li-ion battery when not in use.

The largest permanent capacity losses are recorded at a high charge voltage, high SoC and elevated temperature. None of the Li-ion cells were charged to 4.20V/cell to reach full SoC, as done with a mobile phone battery because the capacity loss would be large(See also BU-702: How to Store Batteries) Additional information is in BU-808: How to Prolong Lithium-based Batteries, Table 3.

Reports reveal that under the right conditions capacity fade in storage can be kept below 10% in 15 years. Calendar aging and capacity fade by cycling are accumulative. Fading is not linear; the highest drop occurs at the beginning and fading slows with time. Experts believe that the high capacity loss at elevated temperatures is mainly caused by calendar aging rather than cycling.

Charging

Charging takes lithium from the cathode and intercalates it on the anode. The process is most effective when the battery has low charge; charge acceptance slows towards saturation. An analogy is gobbling up food quickly when we are hungry.

Ultra-fast charging, or boost charging, must be done under the right conditions. Charge efficiency varies according to SoC and battery temperature. As a battery ages, the internal resistance and the cell balance worsen and the charge rate must be slowed accordingly.

An intelligent charger should read battery state-of-health (SoH) and only apply as much charge current as the battery can reasonably absorb. (See BU-401a: Fast and Ultra-fast Chargers) In as similar way, an old man is perfectly fine to run a marathon as long as his exertion is controlled. Determining the maximum charging current is challenging and effective battery diagnostic technologies are still in development.

Energy cells should be charged at a C-rate below 1C (see BU-402: What Is C-rate?) At 1C, the Li-ion battery is charged to about 90% in one hour at a current that is equal to the battery’s Ah rating. Charging an 85kWh battery at 1C draws 85kW, the power five average households consume.

Going above 1C increases stress that reflects in rapid capacity degradation. Ultrafast charging is most effective between a SoC of 20–50%. The onboard BMS only applies full boosts in this level where charge acceptance is highest before lowering the current to a more moderate level.

The power cell is more rugged and can be charged faster than the energy cell. Power cells are commonly used for power tools. They deliver high current, have a wide temperature range but store less energy than the energy cell. See also BU-501a: Discharge Characteristics of Li-ion

If you ultra-fast charge the EV battery too often, the BMS may permanently lower the current by a few kilowatts. Instead of 120kW on a Supercharger, the charging power may drop to 90kW, prolonging the charge time by about 5 minutes. This software adjustment that an EV manufacturer may apply is not meant to discourage the use of Superchargers but to adjust to the battery conditions to maintain safety and prolong life.

Such close scrutiny may come as a surprise to EV owners. On one occasion a driver had charged his vehicle 245 times at a Supercharger, downloading 6,600kWh of energy. This is unusual because most EV owners use the Level 2 charger at home that takes about five hours for a recharge using about 7kW of power(See BU-1004: Charging an Electric Vehicle)

Battery temperature also governs how fast a battery can be charged. Figure 4 demonstrates fast-charging as a function of temperature. As milk stays fresh in a refrigerator for a long time, Li-ion also prefers a cool storage temperature, but charging and discharging get best results at an elevated room temperature. At 40°C (104°F), a battery charges in one hour compared to 1.5h at 5°C; however, the packs degrades more quickly than at a moderate 25°C;. At a high 50°C (122°F), however, the charger switches to half-power for safety. Charge power must also be reduced when charging below freezing because low temperature usage leads to anode degradation.

Li-ion performs best when warm but should be stored at cool temperature. At 40°C (orange),the battery fast-charges in 3,600 second (1h); and 5,400 seconds (1.5h) at 5°C. The charger switches to lower power at 50°C and at freezing temperatures.

There are concerns about the impact on the battery on regenerative braking. It has been demonstrated that short recharges during braking do not harm the battery even at low temperatures of 10°C to 0°C (50°F to 32°F).

Capacity loss is the product of high charge levels and deep discharges, and not the overall charge throughput. Regenerative breaking is beneficial. Supercapacitors to buffer load peaks are not necessary. The power created by regenerative breaking is commonly less than 1C.

Effect on Loading

Figure 5 demonstrates capacity fading during cycling at low, medium and high SoC and at different temperatures. These readings are demonstrated in colored solid lines. The graph also illustrates calendar aging that is represented in doted lines with less capacity loss that cycling.

The test battery had the highest capacity loss at 10°C (blue) with high SoC, but did well in calendar aging (Figure 5c) when kept cool. Here we have an opposite reactions.

High losses when cycling Li-ion batteries at cool temperatures comes at a surprise. Figure 5c only delivers 500 cycles when cycled at 10°C (50°F) with high SoC. Battery experts hint to lithium plating; cells charged with high currents suffered most. This phenomenon has been confirmed as a dominant aging mechanism affecting the anode. Li-ion should be warmed up to a comfortable temperature of about 25°C (77°F) with operating temperature of up to 40°C (104°F). Interestingly, the lithium plating exhibits some regeneration effects during idle periods.

Reversible Capacity Fade

Fast-charging a Li-ion battery beyond a given charge level causes lithium plating. Lithium is being removed and horded on the anode, creating a shortage that lowers capacity. Studies have shown that the loss of lithium is a major cause of capacity loss that is especially noticeable during fast charging at low temperatures. The lithium is parked in the overhand areas of the anode that has no cathode counterpart.

The longer a cell stays at high SoC, the more lithium plating occurs, and the more capacity is lost. But this horded capacity can in part be recovered. A given amount moves back into operation when the cell dwells at low to medium SoC for days and months. The recovery effect is not fully understood and needs further research.

Scientists believe that lithium, which was dislocated into non-active regions and has clogged pores on the anode, can be reinstated during one year of inactivity. The vanished lithium should dissolve again and made active by distribution, but the recovery mechanism is not fully understood and needs further research.

There appears to be similarity with the “memory” effect of the nickel-cadmium battery. The crystalline formation that formed when keeping a NiCd on full charge also results in a capacity loss that can be reversed by exercising the battery(See BU-807: How to Restore Nickel-based Batteries)

If the hypothesis is correct, rejuvenating a faded Li-ion would also be possible by giving them a rest at a low SoC. This, however, may not be practical because the battery needs to rest for a time. Users of battery-powered devices won’t give their beloved devices a deserved vacation to rejuvenate; however, a recovery will occur without user intervention under the right circumstances. Batteries do indeed reflect human qualities.

Rise of Internal Resistance

In addition to capacity fade, battery aging also involves rising internal resistance. Resistance and capacity fade do not correlate. This means that the SoH of a battery cannot be checked effectively by measuring resistance alone. Capacity is the leading health indicator, but capacity is more difficult to check on the fly than resistance.

Figure 6 illustrates the internal resistance of an 18650 Li-ion NCA cell when cycled at 40ºC (104ºF). Resistance measurements are done with AC and DC methods, two methods that provide different results.

The AC method typically uses 1,000 Hertz to measure the impedance and the resulting readings are reflected in the green frame of Figure 6. The numbers stay flat with cycling and do not reflect the true resistive state of a battery relating to delivering power in an EV. DC resistance is the more dependable method and is measured by observing the voltage drop under a load(See also BU-802a: How does Rising Internal Resistance affect Performance?)

AC resistance readings in green frame stay low; DC method gives true power performance. AC provides impedance while DC reflects true resistance.

Summary

Aging characteristics a Li-ion battery are complex and involve charge levels, charging speed, depth of discharge and temperature. Similar to a living organism, longevity is based on a combination of events that takes usage and environmental conditions into account.

SoC above 80% promotes capacity fade while a deep discharge increases the internal resistance. Li-ion must be shipped at 30% SoC; the recommended long-term storage is between 40–50%. Keeping Li-ion at high SoC affects battery life more than cycling in mid SoC range.

Future EVs may adjust battery charging to the user’s routine. Similar to an alarm clock, from Monday to Friday the EV is set in commute mode by only charging the battery to enable driving to work and back. The weekend follows the drive program as entered by an app on the EV owner’s smartphone.

The life of a Li-ion battery is prolonged when operating at a mild temperature. The EV battery should be warmed up to a comfortable temperature of around 25°C (77°F) for charging and driving. This is in contrast to storing or parking that should be at 10°C (50°F). Charging and operating Li-ion at low temperature causes stress, a phenomenon that does not apply equally to other chemistries.

The combination of low cycle-depth and low SoC leads to the longest battery life, but this does not fully utilize a large, heavy and expensive pack. To avoid resistance increase through deep a discharge, the onboard BMS always keeps some reserve capacity while indicating “empty” wrongly. Reserve capacity also protects the battery when charging at a high current because a completely discharged Li-ion cannot tolerate an ultrafast charge. For best results, charge more often without going full charge.

Simple Guidelines to prolonging the EV battery

• Limit ultra-fast charging, especially when the battery is cold. Use Level 2 when possible.

• Only charge the battery to the level needed for the daily routine. Full charge hastens capacity fade.

• Do not discharge the battery too low as this increases the internal resistance. Charge more often.

• Charge and use the battery at room temperature. Operating when cold reduces capacity.

• Store the battery in a cool place at partial charge. Usage and storage have different requirements.

• Moderate the battery to room temperature in winter before charging and driving. The BMS may do this automatically.

• Charge the EV after a sabbatical. Resting at low charge reverses capacity fade.

• It is best to let the battery rest at low SoC and only charge before use. Dwelling at low charge reduces calendar aging and may also reverse capacity fade.
  

Lab Observations

• At 40°C (104°F), battery efficiency is above 95%, but stress levels are high. At 25°C (77°F), the efficiency is between 93–95%, and at 10°C (50°F), the efficiency is only 89–92%.

• SoC above 80% hastens cathode degradation, discharging below 20% increases internal resistance.

• Capacity recovery is only possible at a SoC at 50% and lower over time.

• The EV fuel gauge is not absolute. The accuracy can be improved by an occasional full charge and deep discharge toe reset the flags. This exercise is similar to calibrating a smart battery(See BU-603: How to Calibrate a “Smart” Battery)

References

[1] Source: Technische Universität München (TUM) Document: Aging of Lithium-Ion Batteries in Electric Vehicles
[2] Source: Renault
[3] Source: Technische Universität München (TUM) Document: Aging of Lithium-Ion Batteries in Electric Vehicles
[4] Source: Technische Universität München (TUM)