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Last week we shared with you our top 5 tips on how to charge your lithium-ion batteries to extend their lifespan.
In this article, we will focus on how to care you for your Lithium-ion battery while in use to extend their lifespan. Our rechargeable batteries will have no more secrets for you!
But before going any further, let&#;s sum-up the factors influencing the capacity of a Li-ion battery.
At high-rate discharge, eg 1.5 C, the extraction of lithium ions from one electrode and intercalation to the other is too strong to be efficient. This damages the electrodes&#; elasticity. Think about breathing hard and fast all the time, you will lose your breath, without benefitting from the air nor gaining energy.
Strong rates increase the battery&#;s internal resistance. The battery will have to strive to deliver high current and use more power to keep the same voltage level, which will therefore make it age faster.
On new &#;fresh&#; batteries, a 1.5C only impacts the capacity of the battery (ie. its autonomy (see chart below)). For batteries that have endured many cycles though, because of the increase of the internal resistance, not only the capacity will be impacted but also the cycling that is involved. Consequently, only a fraction of the capacity will be usable. At a slow pace (eg: C/8) there&#;s less dissipation, the internal resistance doesn&#;t build up as much and the cell&#;s capacity is extended.
This graph represents the discharge rate capability of a new &#;fresh&#; MP xtd battery.
The voltage level reflects the charge level: 4.2V indicates a full charge, 2.7V indicates that the battery is completely discharged.
As you can see, at a C/8 discharge rate (purple line), the cell offers a 5.8 Ah capacity, at 1.5 C, the cell capacity goes down to 5.5 Ah (green line).
The temperature in which a device operates is the main factor impacting a battery&#;s power consumption. This is true for primary batteries but also for rechargeable batteries.
At extreme temperatures, electrode and electrolyte no longer have the optimal shape to enable efficient lithium-ion exchanges.
At low temperatures, the electrode contracts and as a result, ions cannot extract. The electronic conductivity of the electrolytes decreases too. Ions move more slowly between the 2 electrodes.
Have you ever had the feeling that your mobile discharges faster during the winter? That&#;s because cold temperatures make the battery request more power to maintain the required voltage level to send an SMS or take a picture. Try to keep it warm in your pocket, wait and see!
Under the effect of the heat, the electrode expands, making it easier for the ions to move. The electrolyte is also more conductive. However, extreme warm temperatures damage the electrodes.
Some batteries will behave better than others at high or low temperature depending on the chemistry used by the manufacturer and the construction of the cell.
The graphic below represents the discharge capacity (Ah) of an MP xtd at C/5 at various temperatures.
Saft MP xtd proposes an unrivalled operating temperature range from -40°C to +85°C but at -40°C, you have to make concessions on the capacity and adapt the operating voltage.
The full charging/discharging cycle is called Depth of Discharge (DoD) where 100% is a full cycle.
A 70% DoD means that 70 percent of the available energy is delivered, and 30 percent remains in reserve. One cycle of 100% DoD is approximatively equivalent to 2 cycles at 50% DoD, 10 cycles at 10% DOD and 100 cycles at 1% DOD.
The depth of discharge complements the state of charge (SoC): as the Depth of Discharge increases, the State of Charge decreases.
There is a direct relation between the depth of discharge and the cycle life of the battery. The shallower the DoD, the exponentially higher the number of cycles given by a battery. By restricting the possible DoD in your application, you can dramatically improve the cycle life of your product.
As an example, Saft MP xtd&#;s full charge DoD is 4.2V, the lower charge is 2.7V. A 100% DoD at 25°C, using a C charge and a C/2 discharge rates will allow 4,000 cycles.
A 30% DoD allows 16,000 + cycles, ie 4 times more! A perfect example that fine-tuning DoD, temperatures and C rates can definitely extend your battery's lifetime.
To illustrate the idea, a 100% DoD could be compared to breathing hard, using your full lung capacity, thus exhausting your entire body. But if you are breathing normally, you can keep on moving longer.
Contrarily to some received ideas, Li-ion batteries don&#;t have a memory. They don&#;t need regular full discharge and charge cycles to prolong life. It&#;s actually the contrary: the smaller the discharge (low DoD), the longer the battery will last, the more cycles it will be able to do.
Indeed, a full charging and therefore high currents boost the cell&#;s capacity but cause stress to the electrode which stretches, thickens, and enlarges itself to allow all the ions to penetrate. Conversely, when completely discharged (below 2.7 V), an internal chemical reaction occurs, the electrode oxidizes and retracts, the elasticity changes and the battery ages more quickly.
This graph showcases how lowering the DoD will allow you to increase the numbers of cycles.
A partial charge and discharge will therefore reduce stress and prolong battery life. It is recommended to avoid full cycles and stay between 100% and 50% DoD (0-50% SoC).
When several cells are connected in series in a battery pack, an imbalance might occur. The cells behave unevenly over time: charging/discharging levels, self discharge and impedance (internal resistance) may vary from one cell to another. Cycling the battery when unbalanced worsen the disparities and leads to voltage loss.
This can be limited by requiring that the producer assembles only homogeneous cells in the battery pack, just like we do at Saft.
You will also need to proceed to balancing from time to time so that one cell does not wear out more than the others. Here again a smart embedded BMS takes good care of your battery. It will find the weakest cell and make sure it is charged/discharges at the same level than the others.
Methods used to perform cell balancing can include by-passing some of the cells during charge/discharge to focus on the weakest cells.
We also recommend, for batteries assembled from our MP li-ion range, to proceed with periodic balancing by forcing a full discharge with a low current. Forcing a full cycling helps the battery to recover the available capacity, especially after a long storage time.
The State of Health reflects the general condition of a battery and its ability to deliver energy over time. It gives an indication of how much of the battery&#;s lifetime available energy has been consumed, and how much is left.
It is a very interesting parameter to monitor since it can indicate when the battery is experiencing problems or needs replacement. Indeed, although it may slightly vary from one cell manufacturer to the other, it is generally considered that the electrochemistry is at the end of its life when the battery reaches approximately 70% of capacity (in Ah). For Saft xlr range, the end-of-life SoH is 70%, and 60% for the xtd range. Then the capacity loss accelerates, and the autonomy goes down. The State of Health can be displayed on the Battery Management System as an option. A key maintenance indicator to follow-up!
Let&#;s summarize our 10 top tips on how to care for your industrial-grade lithium-ion batteries during charge and while in operation to optimize their lifespan:
How to charge your industrial-grade lithium-ion batteries to optimize their lifespan:
How to discharge your industrial-grade lithium-ion batteries to optimize their lifespan:
Now, you know everything there is to know about how to optimally charge or to discharge your Li-ion battery to extend its lifetime!
For more information on Saft lithium-ion rechargeable range, visit the product page: https://www.saftbatteries.com/products-solutions/products/mp-small-vl
And if you&#;d like to read more about how our batteries operate, check out our case studies:
Fuji Tecom is preventing water leakage and offering more efficient operation thanks to an innovative water leakage detector.
Kongsberg Seatex AS : An autonomous Saft battery solution to monitor the seas despite extreme cold in the Svalbard archipelago
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 , 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 C battery analyzer. All packs started at a capacity of 88&#;94% and decreased to 73&#;84% after 250 full discharge cycles. The mAh 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.
Want more information on Lithium Battery? Feel free to contact us.
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 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. ()
[3] B. Xu, A. Oudalov, A. Ulbig, G. Andersson and D. Kirschen, "Modeling of Lithium-Ion Battery Degradation for Cell Life Assessment," June . [Online]. Available: https://www.researchgate.net/publication/_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.
For more information, please visit Rack Mount Lithium Battery.