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.
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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.
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.
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.
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.
Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30C (86F) 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.
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.
* 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.
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)
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.
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.
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.
Adrian, Table 3 and Figure 6 allow you to make a rough calculation and I would say that cycling between 75-45% (approx. 10% degradation per annum) is better than storing your battery at 100% (20% degradation per annum). At room temperature (20C) you should expect 80-85% capacity after one year stored at 100% (Table 3). Meanwhile, cycling between 75-45%, let's say you would utilise four cycles per day (30% x 4 = 120% total battery capacity), five days per week, 48 weeks per year. This works out to 960 cycles which would leave you with 96% capacity remaining (Figure 6). Furthermore, assuming you store the laptop overnight at 40%, it will lose an additional 5% capacity (Table 3).
Like many people I normally use a laptop in places with a power outlet. To keep to a 75% to 45% use cycle as recommended, I can run a battery monitor that tells me when to disconnect and reconnect the charger. Is there, in existence or technically possible, a way to set the laptop to stop charging the battery at a lower voltage, so that I can leave the charger connected?Also, if the charger effectively disconnects the battery at 100% and the laptop runs direct from the charger, surely when leaving the charger connected all the time will not affect the battery, which will stay cool, though at 100%. NO discharge cycles, but maintained at 100%. Is this worse than cycling between 75% and 45%?
Context: I'm working on a project that uses an 18650 cell as battery backup to power a small processor, where line power is available almost all the time; running off the batteries would be the exception. Request: Since battery backup systems don't follow the typical charge/discharge pattern, it would be great to see an article specifically focused "best practices for battery backup applications".
A device with Lithium batteries (especially Li-ion & Li-Polymer/LiPo) should not be left connected to chargers for >1 month unattended. Some cheaper chargers are less safe eg. ebikes, escooter, boards & toys. Some devices/chargers stipulate a maximum time for having the charger connected (ofcourse the charger is powered while connected). Notebooks have better battery chargers but you should check atleast monthly for any warping or overheating once you notice the capacity is
GrxIn figure 6 on this page if you look closely you will see that the discharge depth does not wear out, case 1: 75-65% uses 10% and only provides 90,000 units of power and case 2: 75-25% uses 50% and gives 150,000 power units showing that 75-25% is better than 75-65%, in the old comments Reza says just that.These are other sources that show that discharging the battery to 0% is good -us/articles/360016286793-Re-Modeling-of-Lithium-Ion-Battery-Degradation-for-Cell-Life-Assessment =cycle%20life%20testing%20and%20modeling%20of%20graphitescience%20direct&tbm=isch&hl=pt-BR&tbs=rimg:CdBUKNSIrHK4YYsBXih3_11bRsgIMCgIIABAAOgQIABAA&client=ms-android-samsung-gj-rev1&prmd=niv&sa=X&ved=0CBIQuIIBahcKEwiw2MC-nZf3AhUAAAAAHQAAAAAQBQ&biw=412&bih=806#imgrc=4_37iTEG2y_zXM 2ff7e9595c
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