Before you use any lithium batteries you should know some things about them (you use it in almost every device today).
What are lithium batteries?
Lithium batteries are disposable (primary) batteries that have lithium metal or lithium compounds as an anode.
They stand apart from other batteries in their high charge density (long life) and high cost per unit. Depending on the design and chemical compounds used, lithium cells can produce voltages from 1.5 V (comparable to a zinc–carbon or alkaline battery) to about 3.7 V.
By comparison, lithium-ion batteries are rechargeable batteries in which lithium ions move between the anode and the cathode, using an intercalated lithium compound as the electrode material instead of the metallic lithium used in lithium batteries.
Lithium batteries are widely used in products such as portable consumer electronic devices.
What Constitutes a Discharge Cycle?
Most understand a discharge/charge cycle as a function to deliver all stored energy, but this is not always the case. Rather than a 100 percent depth of discharge (DoD), manufacturers prefer rating the batteries at 80 percent DoD, meaning that only 80 percent of the available energy is being delivered and 20 percent remains in reserve. A less-than-full discharge increases service life, and manufacturers argue that this is closer to a field representation because batteries are seldom fully discharged before recharge.
There are no standard definitions to define what constitutes a discharge cycle. A smart battery keeping track of cycle count may require a depth of discharge of 70 percent to qualify for a discharge cycle; anything less may not count as a cycle. A battery in a satellite has a typical DoD of 30–40 percent before the batteries are recharged during the satellite day. An EV battery charges to about 80 percent and discharges to 30 percent when new. The bandwidth gradually widens as the battery fades. This reduces battery stress and provides similar driving distances as part of aging. A hybrid car only uses a fraction of the capacity during acceleration before the battery is being recharged. This protects the battery. The lowest discharge cycle is starting a vehicle; motor cranking draws less than 5 percent of energy from the battery, and yet this is also be called a cycle in the automotive market.
So what causes Li-ion to age and what the battery user can do to prolong its life?
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 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.In lieu of cycle count, some device manufacturers suggest battery replacement on a date-stamp, but this method is not reliable either because it ignores environmental conditions. A battery may fail within the allotted time due to heavy use or unfavorable temperature conditions, but most quality 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 a role but these play a less significant role with modern Li-ion in predicting the end-of-battery-life.
Figure 1 illustrates the capacity drop of 11 Li-polymer batteries that have been cycled at a Cadex laboratory. The 1500mAh pouch cells for mobile phone were first charged at a current of 1500mA (1C) to 4.20V/cell and allowed to saturate to 0.05C (75mA) as part of the full charge procedure. The batteries were then discharged at 1500mA to 3.0V/cell, and the cycle was repeated. The expected capacity loss of Li-ion batteries was uniform over the 250 cycles and the batteries performed as expected.
Eleven new Li-ion are tested on a Cadex C7400 battery analyzer. All packs start at a capacity of 88–94% and decrease to 73–84% after 250 full discharge cycles. The 1500mAh pouch packs are used in mobile phones.
Courtesy of Cadex
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 have contributed to this loss. In addition, manufacturers tend to overrate their batteries; knowing that very few customers would complain. Furthermore, single-cell devices, such as mobile phones and tablet, do not need matched cells as multi-cell packs do. This opens the floodgate for a much broader performance acceptance range. Many lower-performing cells may go unnoticed by the consumer.
Similar to a mechanical device that wears out faster with heavy use, so also does the depth of discharge (DoD) determine the cycle count. The shorter 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.
How to Calibrate Batteries?
Batteries for critical devices are made smart and the preferred protocol is the System Management Bus (SMBus). SMBus batteries provide state-of-charge (SoC) estimations and historic battery data, but much of this information is hidden in the background and can only be made available to the user through a host device or battery analyzer capable of displaying the data.
A “smart” battery should be self-calibrating, but in real life a battery does not always get a full discharge at a steady current followed by a full charge. The discharge may be in form of sharp pulses that are difficult to capture; the pack may then be partially recharged and stored at high temperature, causing elevated self-discharge that cannot be tracked.
To correct the tracking error that occurs, a “smart battery” in use should be calibrated once every three months or after 40 partial discharge cycles. This can be done by a deliberate discharge of the equipment or externally with a battery analyzer. A full discharge sets the discharge flag and the subsequent recharge establishes the charge flag.
Table 2 compares the number of discharge/charge cycles Li-ion can deliver at various DoD levels before the battery capacity drops to 70 percent. All other variables such as charge voltage, temperature and load currents are set to average default setting.
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 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; 4.00V/cell should deliver 1,200–2,000 and 3.90V/cell 2,400–4,000 cycles.
On the negative, a lower peak charge voltage reduces the capacity the battery stores. As a simple guideline, every 70mV reduction in charge voltage will lower 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.
What causes Li-ion to die?
Learn what‘s behind the aging process of Li-ion
Energy density has always been a top criterion of a battery, but after the 2006 callback when Li-ion disassembled in consumer products, the safety of batteries in consumer products gained new attention and batteries became safer. With the advent of the electric vehicle, longevity is moving to the forefront and experts begin exploring what causes batteries to fail.
While a three-year battery life with 500 cycles is acceptable for laptops and mobile phones, the eight-year warranty for an EV appears short when considering that a replacement battery carries the price of a new compact car with internal combustion engine. If the life of the battery could be extended to, say, 20 years, then driving an EV would be justified even if the initial investment is high.
Manufacturers of electric vehicles choose battery systems by picking cells that are optimized for longevity rather than high specific energy. These batteries are normally larger and heavier than those used in consumer goods that are tailored for high watt/hours (Wh) but limited in longevity and load capabilities.
Nissan selected a manganese-based Li-ion for the Leaf EV because of good performance. Batteries go through strenuous life cycle testing and to beat the clock, the test protocol often mandates a rapid charge of 1.5 C (less than one hour) and a discharge of 2.5C (20 minutes) under a temperature of 60°C (140°F). Under these harsh conditions the battery may lose only 10 percent after 500 cycles, which represents 1–2 years of driving. This would emulate driving an EV through the heat of a biblical hell, leaving rubber marks for aggressive driving, and still come out with a battery boasting 90 percent capacity.
In spite of the careful selection and extensive testing, the owners of the Nissan Leaf realized a far larger capacity loss of 27.5 percent after 1–2 years of ownership. Why then would the Leaf under more sheltered conditions drop the capacity by so much? Stress modes involve more than just cycling.
Table 4 summarizes the capacity as function of charge levels. All values are estimated.
Most chargers for mobile phones, laptops, tablets and digital cameras bring the Li-ion battery 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 examples where the importance of longevity surpasses harvesting maximum capacity.
For safety reasons, many lithium-ion cannot exceed 4.20V/cell. (The exception is the high energy-dense NMC that charges to 4.30V/cell.) 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.
Higher charge voltages boost capacity but lowers cycle life and compromises safety.
Source: Choi et al. (2002)
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. When fully charged, remove the battery and allow to voltage to revert to a more natural level. This is like relaxing the muscles after strenuous exercise. Although a properly functioning Li-ion charger will terminate charge when the battery is full, some chargers apply a topping charge if the battery terminal voltage drops to a given level.
So we can say that you can choose to have more charges or better full capacity, its actually your own choice how you use it.
Simple Guidelines for Charging Lithium-based Batteries
A device should be turned off while charging. This allows the battery to reach the threshold voltage unhindered and reach a low saturation current when full. A parasitic load confuses the charger.
Charge at a moderate temperature. Do not charge below freezing.
Lithium-ion does not need to be fully charged; a partial charge is better.
Depending on charger, the battery may not always be fully charged when the “ready” indication appears. Not all apply a toping charge and the runtime will be slightly less.
Discontinue using charger and/or battery if the battery gets excessively warm.
Apply some charge to an empty battery before storage (40 percent SoC is ideal)
The four suspected renegades responsible for capacity loss and the eventual end-of-life of the Li-ion battery are:
1. Mechanical degradation of electrodes or loss of stack pressure in pouch-type cells. Careful cells design and correct electrolyte additives minimize this cause.
2. Growth of the solid electrolyte interface (SEI) on the anode that forms a barrier and obstructs the interaction with graphite, resulting in an increase of internal resistance. SEI is seen as a dominant aging process in most graphite-based Li-ion during storage at high voltage (4.10V/cell and higher). Electrolyte additives can reduce this effect.
3. Formation of the electrolyte oxidation at the cathode that may lead to a sudden capacity loss. Keeping the cells at a high voltage and at an elevated temperature promotes this phenomenon.
4. Lithium-plating on the surface of the anode caused by high charging rates. (The elevated capacity loss at higher C-rates in Figure 4 might be caused by this.)
TESLA has released their POWERWALL so if you think of buying one, you should know some of this at least. Watch his video debut here:
Some trivia for the closure :
Lithium batteries and methamphetamine labs
Unused lithium batteries provide a convenient source of lithium metal for use as a reducing agent in methamphetamine labs. Some jurisdictions have passed laws to restrict lithium battery sales or asked businesses to make voluntary restrictions in an attempt to help curb the creation of illegal meth labs. In 2004 Wal-Mart stores were reported to limit the sale of disposable lithium batteries to three packages in Missouri and four packages in other states.
Hope this helps you at better understanding batteries that include our everyday life.
Source : http://batteryuniversity.com/