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Lithium-ion batteries are the most common battery in consumer electronics. They are used in everything from cellphones to power tools to electric cars and more. However, they have well defined characteristics that cause them to wear out, and understanding these characteristics can help you to double the life of your batteries — or more. This is especially useful for products that do not have replaceable batteries.
Battery wear is loss of capacity and/or increased internal resistance. The latter is not a well-known concept, but over time the battery is able to put out less amperage as the battery ages, and eventually the battery is unable to generate power quickly enough to operate the appliance at all even though the battery is not empty.
The standard disclaimers apply, all advice is for informational purposes only, CleanTechnica is not responsible for any damages caused by inaccurate information or following any advice provided. Also, new technology may change the characteristics spoken about, making them less or more relevant in the future or even rendering them obsolete.
These articles explain each facet in detail and are worth reviewing if you’re interested in understanding the logic behind the following recommendations.
Try to buy batteries when you need them, because lithium ion ages from the moment it leaves the assembly line. However, by following the recommendations below you can get a longer lifetime from the batteries you own. If possible, look for the date stamp on any battery powered item you intend to buy and try get the newest one. Often you will find it on there, either on the outside of the package or on the item itself.
One cycle is fully charging the battery and then fully draining it. Lithium-ion batteries are often rated to last from 300-15,000 full cycles. However, often you don’t know which brand/model of battery is in the item you buy.
Partial cycles will give you many more cycles before the battery wears out, so when possible do partial discharges and then recharge. Don’t intentionally drain a battery before recharging for lithium-ion batteries.
For some equipment this is not realistic, in electric lawnmowers and other outdoor tools for example, but the manufacturer will hopefully have selected a battery chemistry designed for this use case.
Try to keep your batteries cool whenever possible. Don’t store a cellphone or other portable lithium battery in a car on a hot day, and keep them cool when not in use (bring your portable tool batteries inside instead of leaving them in an unconditioned shed/garage). Park an electric vehicle in the shade or a reasonable temperature garage when possible. Many EVs have active cooling of batteries so that will take care of this for you, although you still save battery power by parking in the shade or a conditioned garage.
Also, your pocket is about 30ºC, so store your cellphone on a desk and out of direct sunlight if you’re in the office or at home when practical.
Charge your battery at a slow rate when possible. For a cellphone, use a charger that is rated for about 1/4 of the battery capacity if you can. Avoid quick charging except for rare instances when you absolutely need the most juice as quickly as possible. Charging at 1/2 its capacity per hour is acceptable but chargers that can charge a phone in under 1.5 hours from empty can be very hard on the battery.
For power tools, try to get a slow charger instead of the quick chargers many of them come with. This is not always possible, but often is.
Don’t leave any device connected to the charger once charging is complete. In fact, you should aim to charge to a maximum of 80% (more on that below).
Try not to abuse your battery by pulling as much power as quickly from it as possible. For an EV, flooring the acceleration pedal on a regular basis is not good for the battery. Similarly, power hungry games can drain cellphone batteries quite quickly as well. If your phone gets hot from high power use (and not the sun or high room temperature), it is an indication that you are punishing the battery.
Sometimes taking it easy on batteries is not always possible because some products, such as lithium-ion powered tools, are hard on the battery by design (drills, lawnmower, snowblowers, etc.). In these cases, manufacturers will typically use batteries designed for high drain rates (but have lower capacity), but anything you can do to be gentle on even these batteries will pay dividends in longer life. For power banks, try to use the power at a moderate rate. USB models can be tricky to limit your current draw rate as a phone or tablet will draw what it wants up to the bank limit, but for non-USB items you can often try to limit how quickly it’s drawing power.
Also you can “hack” this issue by buying and using a larger capacity battery if your device can handle it. For the same power draw, a larger capacity battery will have a lower percent drain per hour. This also reduces cycle count.
For items you don’t use daily, check on your batteries from time to time in case they are draining themselves when not in use. For EVs and cellphones, this is not a noticeable problem, but for power tools and power banks it is a good idea to check on the battery every few months (or weeks if it drains itself quickly) and top it up to 50%-ish for storage.
Unlike most other battery types (especially lead acid), lithium-ion batteries do not like being stored at high charge levels. Charging and then storing them above 80% hastens capacity loss. So charge the battery to 80% or a bit less if that will get you through the day/week. Most EVs have the ability to select a percentage to charge up to in the software.
Charging above 80% is not a big problem if you intend to draw it down quickly and need the full capacity. Of course, try not to do this regularly if you don’t have to. Avoid overnight charging of your phone unless it has a smart charging feature, such as some Apple phones. For Android phones, use Accubattery software or similar, which will beep at 80% charge as a reminder to unplug the cord. Charge to full in the morning if needed to get through the day.
Similarly, for your EV if you have a long driving day planned, setting the software to charge to full by morning (not storing the vehicle overnight at full) and driving until you are below 80% rather quickly will not cause much extra wear to your batteries.
In general, it’s the storage time above 75-80% that causes most of the extra high charge wear.
For storing batteries long term, charge them to about 50% and check on them every now and then.
According to many sources, lithium-ion doesn’t like being fully discharged. So try to avoid draining your batteries below about 25% when possible. If unavoidable, then charge it back up to above 25% as soon as possible so the time spent near empty is minimized.
End of life for a lithium-ion battery typically occurs when the battery can no longer perform the function the user requires of it. Commercially, when a battery (pack) has reached 80% of its design capacity it is considered EOL, but for end users, it’s typically looked at as when the device (or battery pack) becomes unusable.
When your battery starts acting funny, it can mean it’s ready to be retired. Some Apple phones have the ability to calculate capacity remaining (it is buried in the settings) and Accubattery for Android can do the same thing if installed and used for at least a week.
These are some of the strange quirks you may run into that can occur with worn out lithium-ion batteries:
Be sure to recycle all batteries at the end of their life as they contain valuable materials that can be recycled into new batteries.
Charging algorithm = Battery is charged at Constant Current, then near full charge (typically over 80%) the charger switches to Constant Voltage. The charging rate slows until the battery reaches 100% charge. Many EVs modify this algorithm.
C = Capacity of the battery
Series = Multiple batteries linked in a chain to increase the total voltage of the pack.
Parallel = Multiple batteries linked side by side to increase amperage instead of voltage.
(x)S(x)P configuration = explains how multiple batteries are linked. 4S2P for example means 8 cells, four in Series and two Parallel rows
Volts (V) = Electric potential. Power outlets are measured in volts.
Amps (A)= Number of Coulombs of electrons carrying those volts.
Watts (W)= Volts x Amps. Energy/Power usage is often measured in watts. A kilowatt is 1000 watts. kWh is Kilowatts per hour.
Energy is measured in Joules and is convertible to Watts/second if you have a time component.
Power = Energy over Time. Typically measured in Watts. One Joule per second is 1 watt. The same number of Joules or Watts in half the time is twice the power.
Nominal voltage = Voltage used to calculate Watts of a battery.
Battery capacity = How many Ah of power the battery can output (when new).
Load = Device that uses the power from the battery.
Internal resistance of a battery affects its Power output. Increased internal resistance is the reduction in rate of Power output the battery can deliver. Energy output is affected somewhat by increased internal resistance.
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Battery research is focusing on lithium chemistries so much that one could imagine that the battery future lies solely in lithium. There are good reasons to be optimistic as lithium-ion is, in many ways, superior to other chemistries. Applications are growing and are encroaching into markets that previously were solidly held by lead acid, such as standby and load leveling. Many satellites are also powered by Li-ion.
Lithium-ion has not yet fully matured and is still improving. Notable advancements have been made in longevity and safety while the capacity is increasing incrementally. Today, Li-ion meets the expectations of most consumer devices but applications for the EV need further development before this power source will become the accepted norm. BU-104c: The Octagon Battery – What makes a Battery a Battery, describes the stringent requirements a battery must meet.
As battery care-giver, you have choices in how to prolong battery life. Each battery system has unique needs in terms of charging speed, depth of discharge, loading and exposure to adverse temperature. Check what causes capacity loss, how does rising internal resistance affect performance, what does elevated self-discharge do and how low can a battery be discharged? You may also be interested in the fundamentals of battery testing.
The lithium-ion battery works on ion movement between the positive and negative electrodes. In theory such a mechanism should work forever, but cycling, elevated temperature and aging decrease the performance over time. Manufacturers take a conservative approach and specify the life of Li-ion in most consumer products as being between 300 and 500 discharge/charge cycles.
In 2020, small wearable batteries deliver about 300 cycles whereas modern smartphones have a cycle life requirement is 800 cycles and more. The largest advancements are made in EV batteries with talk about the one-million-mile battery representing 5,000 cycles.
Evaluating battery life on counting cycles is not conclusive because a discharge may vary in depth and there are no clearly defined standards of what constitutes a cycle(See BU-501: Basics About Discharging). In lieu of cycle count, some device manufacturers suggest battery replacement on a date stamp, but this method does not take usage into account. A battery may fail within the allotted time due to heavy use or unfavorable temperature conditions; however, most packs last considerably longer than what the stamp indicates.
The performance of a battery is measured in capacity, a leading health indicator. Internal resistance and self-discharge also play roles, but these are less significant in predicting the end of battery life with modern Li-ion.
Figure 1 illustrates the capacity drop of 11 Li-polymer batteries that have been cycled at a Cadex laboratory. The 1,500mAh pouch cells for mobile phones were first charged at a current of 1,500mA (1C) to 4.20V/cell and then allowed to saturate to 0.05C (75mA) as part of the full charge saturation. The batteries were then discharged at 1,500mA to 3.0V/cell, and the cycle was repeated. The expected capacity loss of Li-ion batteries was uniform over the delivered 250 cycles and the batteries performed as expected.
Eleven new Li-ion were tested on a Cadex C7400 battery analyzer. All packs started at a capacity of 88–94% and decreased to 73–84% after 250 full discharge cycles. The 1500mAh pouch packs are used in mobile phones.
Although a battery should deliver 100 percent capacity during the first year of service, it is common to see lower than specified capacities, and shelf life may contribute to this loss. In addition, manufacturers tend to overrate their batteries, knowing that very few users will do spot-checks and complain if low. Not having to match single cells in mobile phones and tablets, as is required in multi-cell packs, opens the floodgates for a much broader performance acceptance. Cells with lower capacities may slip through cracks without the consumer knowing.
Similar to a mechanical device that wears out faster with heavy use, the depth of discharge (DoD) determines the cycle count of the battery. The smaller the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses. Partial discharge on Li-ion is fine. There is no memory and the battery does not need periodic full discharge cycles to prolong life. The exception may be a periodic calibration of the fuel gauge on a smart battery or intelligent device(See BU-603: How to Calibrate a “Smart” Battery)
The following tables indicate stress related capacity losses on cobalt-based lithium-ion. The voltages of lithium iron phosphate and lithium titanate are lower and do not apply to the voltage references given.
Note:
Tables 2, 3 and 4 indicate general aging trends of common cobalt-based Li-ion batteries on depth-of-discharge, temperature and charge levels, Table 6 further looks at capacity loss when operating within given and discharge bandwidths. The tables do not address ultra-fast charging and high load discharges that will shorten battery life. No all batteries behave the same.Table 2 estimates the number of discharge/charge cycles Li-ion can deliver at various DoD levels before the battery capacity drops to 70 percent. DoD constitutes a full charge followed by a discharge to the indicated state-of-charge (SoC) level in the table.
Depth of Discharge
Discharge cycles
NMC
LiPO4
100% DoD
~300
~600
80% DoD
~400
~900
60% DoD
~600
~1,500
40% DoD
~1,000
~3,000
20% DoD
~2,000
~9,000
10% DoD
~6,000
~15,000
Table 2: Cycle life as a function ofdepth of discharge** 100% DoD is a full cycle; 10% is very brief. Cycling in mid-state-of-charge would have best longevity.
Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table 3 demonstrates capacity loss as a function of temperature and SoC.
Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 4.0V/cell should deliver 1,200–2,000 and 3.90V/cell should provide 2,400–4,000 cycles.
On the negative side, a lower peak charge voltage reduces the capacity the battery stores. As a simple guideline, every 70mV reduction in charge voltage lowers the overall capacity by 10 percent. Applying the peak charge voltage on a subsequent charge will restore the full capacity.
In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses; going lower may not gain further benefits but induce other symptoms(See BU-808b: What causes Li-ion to die?) Table 4 summarizes the capacity as a function of charge levels. (All values are estimated; Energy Cells with higher voltage thresholds may deviate.)
Every 0.10V drop below 4.20V/cell doubles the cycle but holds less capacity. Raising the voltage above 4.20V/cell would shorten the life. The readings reflect regular Li-ion charging to 4.20V/cell.
Guideline: Every 70mV drop in charge voltage lowers the usable capacity by about 10%.
Note: Partial charging negates the benefit of Li-ion in terms of high specific energy.
* Similar life cycles apply for batteries with different voltage levels on full charge.
** Based on a new battery with 100% capacity when charged to the full voltage.
Experiment: Chalmers University of Technology, Sweden, reports that using a reduced charge level of 50% SOC increases the lifetime expectancy of the vehicle Li-ion battery by 44–130%.
Most chargers for mobile phones, laptops, tablets and digital cameras charge Li-ion to 4.20V/cell. This allows maximum capacity, because the consumer wants nothing less than optimal runtime. Industry, on the other hand, is more concerned about longevity and may choose lower voltage thresholds. Satellites and electric vehicles are such examples.
For safety reasons, many lithium-ions cannot exceed 4.20V/cell. (Some NMC are the exception.) While a higher voltage boosts capacity, exceeding the voltage shortens service life and compromises safety. Figure 5 demonstrates cycle count as a function of charge voltage. At 4.35V, the cycle count of a regular Li-ion is cut in half.
Besides selecting the best-suited voltage thresholds for a given application, a regular Li-ion should not remain at the high-voltage ceiling of 4.20V/cell for an extended time. The Li-ion charger turns off the charge current and the battery voltage reverts to a more natural level. This is like relaxing the muscles after a strenuous exercise(See BU-409: Charging Lithium-ion)
Figure 6 illustrates dynamic stress tests (DST) reflecting capacity loss when cycling Li-ion at various charge and discharge bandwidths. The largest capacity loss occurs when discharging a fully charged Li-ion to 25 percent SoC (black); the loss would be higher if fully discharged. Cycling between 85 and 25 percent (green) provides a longer service life than charging to 100 percent and discharging to 50 percent (dark blue). The smallest capacity loss is attained by charging Li-ion to 75 percent and discharging to 65 percent. This, however, does not fully utilize the battery. High voltages and exposure to elevated temperature is said to degrade the battery quicker than cycling under normal condition. (Nissan Leaf case)
* Discrepancies exist between Table 2 and Figure 6 on cycle count. No clear explanations are available other than assuming differences in battery quality and test methods. Variances between low-cost consumer and durable industrial grades may also play a role. Capacity retention will decline more rapidly at elevated temperatures than at 20ºC.
Only a full cycle provides the specified energy of a battery. With a modern Energy Cell, this is about 250Wh/kg, but the cycle life will be compromised. All being linear, the life-prolonging mid-range of 85-25 percent reduces the energy to 60 percent and this equates to moderating the specific energy density from 250Wh/kg to 150Wh/kg. Mobile phones are consumer goods that utilize the full energy of a battery. Industrial devices, such as the EV, typically limit the charge to 85% and discharge to 25%, or 60 percent energy usability, to prolong battery life(See Why Mobile Phone Batteries do not last as long as an EV Battery)
Increasing the cycle depth also raises the internal resistance of the Li-ion cell. Figure 7 illustrates a sharp rise at a cycle depth of 61 percent measured with the DC resistance method(See also BU-802a: How does Rising Internal Resistance affect Performance?) The resistance increase is permanent.
Figure 7: Sharp rise in internal resistance by increasing cycle depth of Li-ion [4]Note: DC method delivers different internal resistance readings than with the AC method (green frame). For best results, use the DC method to calculate loading.
Figure 8 extrapolates the data from Figure 6 to expand the predicted cycle life of Li-ion by using an extrapolation program that assumes linear decay of battery capacity with progressive cycling. If this were true, then a Li-ion battery cycled within 75%–25% SoC (blue) would fade to 74% capacity after 14,000 cycles. If this battery were charged to 85% with same depth-of-discharge (green), the capacity would drop to 64% at 14,000 cycles, and with a 100% charge with same DoD (black), the capacity would drop to 48%. For unknown reasons, real-life expectancy tends to be lower than in simulated modeling(See BU-208: Cycling Performance)
Li-ion batteries are charged to three different SoC levels and the cycle life modelled. Limiting the charge range prolongs battery life but decreases energy delivered. This reflects in increased weight and higher initial cost.
Battery manufacturers often specify the cycle life of a battery with an 80 DoD. This is practical because batteries should retain some reserve before charge under normal use(See BU-501: Basics about Discharging, “What Constitutes a Discharge Cycle”) The cycle count on DST (dynamic stress test) differs with battery type, charge time, loading protocol and operating temperature. Lab tests often get numbers that are not attainable in the field.
Environmental conditions, not cycling alone, govern the longevity of lithium-ion batteries. The worst situation is keeping a fully charged battery at elevated temperatures. Battery packs do not die suddenly, but the runtime gradually shortens as the capacity fades.
Lower charge voltages prolong battery life and electric vehicles and satellites take advantage of this. Similar provisions could also be made for consumer devices, but these are seldom offered; planned obsolescence takes care of this.
A laptop battery could be prolonged by lowering the charge voltage when connected to the AC grid. To make this feature user-friendly, a device should feature a “Long Life” mode that keeps the battery at 4.05V/cell and offers a SoC of about 80 percent. One hour before traveling, the user requests the “Full Capacity” mode to bring the charge to 4.20V/cell.
The question is asked, “Should I disconnect my laptop from the power grid when not in use?” Under normal circumstances this should not be necessary because charging stops when the Li-ion battery is full. A topping charge is only applied when the battery voltage drops to a certain level. Most users do not remove the AC power, and this practice is safe.
Modern laptops run cooler than older models and reported fires are fewer. Always keep the airflow unobstructed when running electric devices with air-cooling on a bed or pillow. A cool laptop extends battery life and safeguards the internal components. Energy Cells, which most consumer products have, should be charged at 1C or less. Avoid so-called ultra-fast chargers that claim to fully charge Li-ion in less than one hour.
[1] Courtesy of Cadex
[2] Source: Choi et al. (2002)
[3] B. Xu, A. Oudalov, A. Ulbig, G. Andersson and D. Kirschen, "Modeling of Lithium-Ion Battery Degradation for Cell Life Assessment," June 2016. [Online]. Available: https://www.researchgate.net/publication/303890624_Modeling_of_Lithium-Ion_Battery_Degradation_for_Cell_Life_Assessment.
[4] Source: Technische Universität München (TUM)
[5] With permission to use. Interpolation/extrapolation by OriginLab.
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