Find out how to prolong battery life by using correct charge methods.
Charging and discharging batteries is a chemical reaction, but Li-ion is claimed to be the exception. Battery scientists talk about energies flowing in and out of the battery as part of ion movement between anode and cathode. This claim carries merits but if the scientists were totally right, the battery would live forever. Scientists blame capacity fade on ions getting trapped, but as with all battery systems, internal corrosion still plays a role.
The Li ion charger is a voltage-limiting device that is similar to the lead acid system. The difference lies in a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge at full charge. While lead acid offers some flexibility in terms of voltage cut off, manufacturers of Li ion cells are very strict on the correct setting because Li-ion cannot accept overcharge. The so-called miracle charger that promises to prolong battery life and gain extra capacity does not exist. Li-ion is a “clean” system and only takes what it can absorb.
Li-ion with the tradition cathode materials of cobalt, nickel, manganese and aluminum typically charge to 4.20V/cell. The tolerance is +/–50mV/cell. Some nickel-based variety charge to 4.10V/cell; high capacity Li-ion may go to 4.30V/cell. Higher voltages increase the capacity, but when going beyond specification, the cell deteriorates and delivers a reduced service life. More important is the safety that is being compromised when beyond limit. Figure 1 shows the voltage and current signature as lithium-ion passes through the stages for constant current and topping charge.
Figure 1: Charge stages of lithium-ion. Li-ion is fully charged when the current drops to a set level. In lieu of trickle charge, some chargers apply a topping charge when the voltage drops.
Courtesy of Cadex
The charge rate of an Energy Cell is between 0.5 and 1C; the complete charge time is about 2–3 hours. Manufacturers of these cells recommend charging at 0.8C or less to prolong battery life. Charge efficiency about 99 percent and the cell remains cool during charge. Some Li-ion packs may experience a temperature rise of about 5ºC (9ºF) when reaching full charge. This could be due to the protection circuit and/or elevated internal resistance. Full charge occurs when the battery reaches the voltage threshold and the current drops to three percent of the rated current. A battery is also considered fully charged if the current levels off and cannot go down further. Elevated self-discharge might be the cause of this condition.
Increasing the charge current does not hasten the full-charge state by much. Although the battery will reach the voltage peak quicker, the saturation charge will take longer accordingly. The amount of charge current applied alters the time required for each stage; Stage 1 will be shorter but the saturation Stage 2 will take longer. A high current charge will, however, quickly fill the battery to about 70 percent.
Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because a high voltage stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. To satisfy maximum runtime, most chargers for consumer products go for maximum capacity; extended service life is perceived less important.
Some lower-cost consumer chargers may use the simplified “charge-and-run” method that charges a lithium-ion battery in one hour or less without going to the Stage 2 saturation charge. “Ready” appears when the battery reaches the voltage threshold at Stage 1. State-of-charge (SoC) at this point is about 85 percent, a level that may be sufficient for many users.
Avoiding full charge has benefits, and some manufacturers set the charge threshold lower on purpose to prolong battery life. Table 2 illustrates the estimated capacities when charged to different voltage thresholds with and without saturation charge.
Capacity with full saturation
Table 2: Typical charge characteristics of lithium-ion. Adding full saturation at the set voltage boosts the capacity by about 10 percent but adds stress due to high voltage.
When the battery is first put on charge, the voltage shoots up quickly. This behavior can be compared to lifting a weight with an elastic band causing a lag. The voltage will eventually catch up when the battery is almost fully charged (see Figure 3). This charge characteristic is typical of all batteries.
Figure 3: Capacity as a function of charge voltage on a lithium-ion battery
The capacity trails the charge voltage, like lifting a heavy weight with an elastic band.
Courtesy of Cadex
Relying on closed circuit voltage (CCV) to read the capacity during charge is impractical. However, OCT be used to predict state-of-charge after the battery has rested for a few hours. As with all batteries, temperature affects the OCV. The active material in a Li-ion also plays a role. [BU-903, How to Measure State-of-charge]
Li-ion cannot absorb overcharge, and when fully charged the charge current must be cut off. A continuous trickle charge (maintenance charge) would cause plating of metallic lithium, and this could compromise safety. To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible.
Once the charge is terminated, the battery voltage begins to drop, and this eases the voltage stress. Over time, the open-circuit voltage will settle to between 3.70 and 3.90V/cell. Note that a Li-ion battery that received a fully saturated charge will keep the higher voltage longer than one that did not receive a saturation charge.
If a lithium-ion battery must be left in the charger for operational readiness, some chargers apply a brief topping charge to compensate for the small self-discharge the battery and its protective circuit consume. The charger may kick in when the open-circuit voltage drops to 4.05V/cell and turn off again at 4.20V/cell. Chargers made for operational readiness, or standby mode, often let the battery voltage drop to 4.00V/cell and recharge to only 4.05V/cell instead of the full 4.20V/cell. This reduces voltage-related stress and prolongs battery life.
Some portable devices sit in a charge cradle in the on position. The current drawn through the device is called the parasitic load and can distort the charge cycle. Battery manufacturers advise against parasitic load while charging because it induces mini-cycles, but this cannot always be avoided; a laptop connected to the AC main is such a case. The battery is being charged to 4.20V/cell and then discharged by the device. The stress level on the battery is high because the cycles occur at the 4.20V/cell threshold, often also at elevated temperature.
A portable device should be turned off during charge. This allows the battery to reach the set threshold voltage unhindered and termination on current saturation. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low. A battery may be fully charged, but the prevailing conditions will prompt a continued charge, causing stress.
While the traditional lithium-ion using cobalt, nickel, manganese and aluminum as active cathode material have a nominal cell voltage of 3.60V, Li-phosphate (LiFePO) makes an exception with a nominal cell voltage of 3.20V, charging to 3.65V. Relatively new is the Li-titanate (LTO) with a nominal cell voltage of 2.40V, charging to 2.85V. (See BU-205: Types of Lithium-ion.)
Chargers for these newcomers are not compatible with regular 3.60-volt Li-ion. Provision must be made to identify the correct systems and provide charging with the correct voltage. While a 3.60-volt lithium battery would not charge on a charger designed for Li-phosphate, the reverse would cause an overcharge. Multi-system chargers must have the provision to adjust the voltage.
Lithium-ion operates safely within the designated operating voltages; however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V forms plating of metallic lithium on the anode, while the cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises, and if charging is allowed to continue the current interrupt device (CID) responsible for cell safety disconnects the current at 1,380kPa (200psi).
Should the pressure rise further, a safety membrane bursts open at 3,450kPa (500psi) and the cell might eventually vent with flame. The thermal runaway moves lower when the battery is fully charged; for Li-cobalt this threshold is between 130–150CC (266–302F), nickel-manganese-cobalt (NMC) is 170–180C (338–356F), and manganese is 250C (482F). Li-phosphate enjoys similar and better temperature stabilities than manganese.
Lithium-ion is not the only battery that is a safety hazard if overcharged. Lead- and nickel-based batteries are also known to melt down and cause fire if improperly handled. Properly designed charging equipment is paramount for all battery systems.
Charging lithium-ion batteries is simpler and more straight-forward than with nickel-based systems. The charge circuit is relatively simple; voltage and current limitations is easier to do than analyzing complex signatures with anomalies that may change as the battery ages. Charging can be intermittent and the battery never needs to reach saturation, as is the case with lead acid. This is a major advantage for renewable energy storage such as a solar panel and wind turbine. The absence of trickle charge further helps simplify the charger. Equalizing chargers, as is required from time-to-time with lead acid is not necessary with Li-ion.
Last updated 2015-05-11
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