Let's make EV Batteries last longer

Life of Battery, a complicated subject. Well, it all depends on case to case basis and also on the system configuration, chemical composition, quality of raw materials used in manufacturing, manufacturing process, cell packaging, loads, charging pattern, thermal management system, vehicle design for cooling, DoD, aging, shock/vibrations etc, etc and etc.

So just to cut short, let's discuss about major factors we can consider to make our Battery packs last longer. All EV batteries will degrade over time, but avoiding the above situations can help you maximize your EV battery life. (This is not a complete list, it is just some introductory insight).

To protect customers, manufacturers of electric vehicles offer battery warranties. These warranties usually cover a certain amount of time and a certain number of miles driven, and end when the threshold of one or the other is met. The warranty typically guarantees that the battery won’t degrade past a certain percent of the original charge capacity during the warranty term. Below are few examples of popular cars

Battery aging is complex and not always predicable. Usage is a product of age, cycle count, charge speed, load levels and temperature.

Even though many examples are given in-terms of car platforms (as lot of real data is available there), the exactly same points and parameters are also applicable for 2 wh, 3 wh, Trucks and Buses also. I don’t see any specific exceptions applied in these use cases too.

Calendar Aging

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

 The largest permanent capacity losses are recorded at a high charge voltage, high SoC and elevated temperature.

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.

Effect on Loading

Figure below 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.

High losses when cycling Li-ion batteries at cool temperatures comes at a surprise. Figure above 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.

Temperature

It is very well known that temperature rise is the biggest enemy of the Lithium battery pack. A battery exposed to very hot temperatures will be prone to more damage, but by how much? Will an EV in Arizona have a different battery life than the same car driven in Norway?

To find out, GeoTab grouped the vehicles based on the following climate conditions: 

• Temperate (fewer than 5 days per year over 80°F (27°C) or under 23°F (-5°C))
• Hot (more than 5 days per year over 80°F (27°C))

As illustrated below, vehicles driven in hot climates showed a notably faster rate of decline than those driven in moderate climates. This is not great news off course.

From a thermal point of view, there are three main aspects to consider when using lithium-ion batteries in an EV:

1. At temperatures below 0°C (32°F), batteries lose charge due to slower chemical reactions taking place in the battery cells. The result is a significant loss in power, acceleration and driving range, and higher potential for battery damage during charging.
2. At temperatures above 30°C (86°F) the battery performance degrades, posing a real issue if a vehicle’s air conditioner is needed for passengers. The result is an impact on power density and reduced acceleration response.
3. Temperatures above 40°C (104°F) can lead to serious and irreversible damage in the battery. At even higher temperatures, e.g. 70-100°C, thermal runaway can occur. This is triggered when the runaway temperature is reached. The result is a self-heating chain reaction in a battery cell that causes its destruction while propagating to adjacent cell

Moreover we have also seen in the ‘Effect of Loading’ section, that the battery life also degrades when we operate is at cold temperatures. So it is very important to keep batteries in the safe Band of temperature, not very high, not very low.

The thermal management system also need to be smartly and properly designed. The employed heating and cooling method could create an uneven temperature distribution inside the battery pack, depending on the location of each stack or system, and external ambient conditions. This uneven temperature in the cells could trigger an uneven temperature distribution in the pack. Thus, the pack could lead to an unbalanced system. It restricts the optimum performance. Additionally, the lifetime of the battery pack is reduced

Charging

An intelligent charger should read battery state-of-health (SoH) and only apply as much charge current as the battery can reasonably absorb.

Though found lucrative due to time saving, Fast Charging actually results as major contribution to the battery life degradation due to its combined effect of faster chemical reactions and higher process temperature.

The electro-chemical reactions taking place in the battery have a direct impact on the lifetime of the battery. The rate at which the chemical reactions take place for each of the battery chemistry is well defined. Since the battery is subjected to such high voltage and current during fast charging, it will experience greater thermal degradation as a result of the comparatively higher temperatures generated during this process. The degradation of the active material on the cell plates is due to:

• The repeated processes of dissolution and re-crystallization results in the loss of active surface area on the plates
• Decrease in electrical contact between the metallic grids and active materials and
• Increase in the growth of inactive materials

Environmental conditions affect the deposition of lithium as follows:

• Lithium deposit grows when Li-ion is ultra-fast charged at low temperature
• Deposition develops if Li-ion is ultra-fast charged beyond a given state-of-charge level 
• The buildup is also said to increase as Li-ion cells age due to raised internal resistance. 

Most revealing finding with the GeoTab’s data from over 6,000 electric vehicles, is with how often the owner uses DC fast charging. Such chargers are becoming more popular as customers demand shorter and shorter charging times, but the effect they have on batteries may negate the benefit of the time saved. Take a look at the chart below.

All said and done, now let’s see how can we take control of the situation and try to maximize the Battery life from customer point of view (that’s more important than anything else)

Virtual Capacity

The secret of longevity in the EV battery is over-sizing 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. Over-sizing 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 on-board 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.

Figure below 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.

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 below 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.

Chevrolet also did same for the Volt plug-in hybrid. Of its 18.4. kWh battery pack, just 14 kWh of electricity is actually available to drivers – about 75% of the battery’s actual capacity. This means a charge from 0% to 100% in the car is more akin to a charge from 15% to 90% – a much less intense use-case for the battery pack. This allows for thousands of cycles before serious degradation begins to occur, compared to just hundreds.

SoC Considerations

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.

We 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.

Thermal Management

Battery thermal management system is used fundamentally to preserve the temperature of battery cells in a pack at an optimal range. It helps to enhance the lifetime while ensuring safe and secure operation of the battery pack. It is therefore inevitable that BTMS is typically associated with the process of retaining the operational temperature at an optimal level through keeping the temperature gradient within a relatively narrow range.

Depending on electrochemical-physical characteristics and corresponding reactions, the optimum operating range of different batteries will differ. The optimum range for most general batteries requires operating near room temperature (15–35 ?C). By keeping the temperature within a narrow optimum level, it helps to lengthen the battery pack lifetime. Since the performance of a battery pack depends on the performance of individual cells, the cooling scheme should be activated when the battery is exposed to the high rate of charge and discharge. Moreover, depending on altitude and geographical condition, the operation of BTMS varies.

Thermal insulation is needed in case of reducing the heat loss from high temperature either during the desired application’s operation and stand-by. Battery pack thermal management and control could be achieved by air or liquid systems, active or passive approaches. Increasing the insulation thickness was suggested for slowing the rate of temperature increase while parking in the summer time, although this also appears to be similarly beneficial for winter operations.

 EV battery pack thermal management should take care of following three basic points:

1. To ensure the pack operates in the desired temperature range for optimum performance and working life. A typical temperature range is 15-35°C.
2. To reduce uneven temperature distribution in the cells. Temperature differences should be less than 3-4C°.
3. To eliminate potential hazards related to uncontrolled temperature, e.g. thermal runaway.

Various cooling agents and methods are in use today as part of the thermal management of EV batteries. Among these are air cooling, the use of flowing liquid coolants, or direct immersion.

Air Cooling

The lowest cost method for EV battery cooling is with air. A passive air-cooling system uses outside air and the movement of the vehicle to cool the battery. Active air-cooling systems enhance this natural air with fans and blowers. Air cooling eliminates the need for cooling loops and any concerns about liquids leaking into the electronics. The added weight from using liquids, pumps and tubing is also avoided.

The trade-off is that air cooling, even with high-powered blowers, does not transport the same level of heat as a liquid system can. This has led to problems for EV in hot climates, including more temperature variation in battery pack cells. Blower noise can also be an issue.

Liquid Cooling

Piped liquid cooling systems provide better battery thermal management because they are better at conducting heat away from batteries than air-cooling systems. One downside is the limited supply of liquid in the system compared with the essentially limitless amount of air that can flow through a battery.

Tesla’s thermal management system (as well as GM’s) uses liquid glycol as a coolant. Both the GM and Tesla systems transfer heat via a refrigeration cycle. Glycol coolant is distributed throughout the battery pack to cool the cells. Considering that Tesla has 7,000 cells to cool, this is a challenge.

The Tesla Model S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery pack and carries a flow of water-glycol coolant; thermal contact with the cells is through their sides by thermal transfer material.

General Motor’s Chevrolet Volt features a liquid cooling system to manage battery heat. Each rectangular battery cell is about the size of a children’s book. Sandwiched between the cells is an aluminum cooling plate. There are five individual coolant paths passing thru the plate in parallel, not in series as the Tesla system does. Each battery pouch (cell) is housed in a plastic frame. The frames with coolant plates are then stacked longitudinally to make the entire pack.

Thermodynamic engineers at Porsche develop and optimize each vehicle’s entire cooling system. This includes the battery, of course, and one example is the liquid-filled cooling plate from the traction battery in the Boxster E.

Liquid Immersion

Instead of snaking coolant through lines and chambers within a battery pack’s case, XING Mobility takes a different approach by immersing its cells in a non-conductive fluid with a high boiling point. The coolant is 3M Novec 7200 Engineered Fluid, a non-conductive fluid designed for heat transfer applications, fire suppression and supercomputer cooling.

XING’s batteries take the form of 42 lithium-ion-cell modules that can be put together to build larger battery solutions. The complete XING battery houses 4,200 individual 18,650 lithium-ion cells encased in liquid-cooled module packs.

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.

If the hypothesis is correct, rejuvenating a faded Li-ion would also be possible by giving them a rest at a low SoC.

Other Simple Guidelines to prolonging the EV battery

• Avoid keeping your car sitting with a full or empty charge. Ideally, keep your SOC between 20-80% particularly when leaving it for longer periods, and only charge it fully for long distance trips.
• Minimize fast charging (DCFC). Some high-use duty cycles will need a faster charge, but if your vehicle sits overnight, level 2 should be sufficient for the majority of your charging needs. And especially when the battery is cold.
• Climate is out of an operator’s control, but do what you can to avoid extreme hot temperatures, such as choosing shade when parked on hot days.
• 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.
• Moderate the battery to room temperature in winter before charging and driving. The BMS may do this automatically.
• 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.

That's all for now. Enjoy. And have great time ahead !!!

In case you are looking for any Design / Consultancy Services in Electric Vehicle, please email me at [email protected]

Thank you, Jaywant Mahajan

(Large part this information is collected from various sources on internet and it remains copyright of the source and author does not hold any claim that information.)