Charging and discharging batteries can be a chemical reaction, but 18650 lithium battery is claimed being the exception. Battery scientists discuss energies flowing inside and outside of your battery as an element of ion movement between anode and cathode. This claim carries merits however if the scientists were totally right, then this battery would live forever. They blame capacity fade on ions getting trapped, but as with all battery systems, internal corrosion as well as other degenerative effects also known as parasitic reactions in the electrolyte and electrodes till are involved. (See BU-808b: What causes Li-ion to die?.)
The Li ion charger is actually a voltage-limiting device that has similarities towards the lead acid system. The differences with Li-ion lie inside a higher voltage per cell, tighter voltage tolerances and the lack of trickle or float charge at full charge. While lead acid offers some flexibility regarding voltage shut down, manufacturers of Li-ion cells are really strict about the correct setting because Li-ion cannot accept overcharge. The so-called miracle charger that offers to prolong battery life and gain extra capacity with pulses and also other gimmicks is not going to exist. Li-ion is a “clean” system and just takes exactly what it can absorb.
Li-ion together with the traditional cathode materials of cobalt, nickel, manganese and aluminum typically charge to 4.20V/cell. The tolerance is /-50mV/cell. Some nickel-based varieties charge to 4.10V/cell; high capacity Li-ion might go to 4.30V/cell and higher. Boosting the voltage increases capacity, but going beyond specification stresses the battery and compromises safety. Protection circuits included in the pack do not allow exceeding the set voltage.
Figure 1 shows the voltage and current signature as lithium-ion passes throughout the stages for constant current and topping charge. Full charge is reached if the current decreases to between 3 and 5 percent in the Ah rating.
The advised charge rate of the Energy Cell is between .5C and 1C; the total charge time is about 2-3 hours. Manufacturers of these cells recommend charging at .8C or less to extend battery lifespan; however, most Power Cells will take a higher charge C-rate with little stress. Charge efficiency is about 99 percent as well as the cell remains cool during charge.
Some Li-ion packs may experience a temperature rise around 5ºC (9ºF) when reaching full charge. This could be as a result of protection circuit and elevated internal resistance. Discontinue using the battery or charger when the temperature rises more than 10ºC (18ºF) under moderate charging speeds.
Full charge takes place when the battery reaches the voltage threshold along with the current drops to 3 percent of your rated current. A battery is also considered fully charged if the current levels off and cannot drop further. Elevated self-discharge may be the reason for this condition.
Increasing the charge current is not going to hasten the complete-charge state by much. Although the battery reaches the voltage peak quicker, the saturation charge will take longer accordingly. With higher current, Stage 1 is shorter but the saturation during Stage 2 will require longer. An increased current charge will, however, quickly fill battery to around 70 %.
Li-ion fails to need to be fully charged as is the case with lead acid, nor will it be desirable to do this. In reality, it is far better to not fully charge as a high voltage stresses battery. Deciding on a lower voltage threshold or eliminating the saturation charge altogether, prolongs battery but this decreases the runtime. Chargers for consumer products select maximum capacity and cannot be adjusted; extended service life is perceived less important.
Some lower-cost consumer chargers could use the simplified “charge-and-run” method that charges a lithium-ion battery in a single hour or less without seeing the Stage 2 saturation charge. “Ready” appears once the battery reaches the voltage threshold at Stage 1. State-of-charge (SoC) at this stage is around 85 percent, a level that may be sufficient for most users.
Certain industrial chargers set the charge voltage threshold lower on purpose to prolong life of the battery. Table 2 illustrates the estimated capacities when charged to several voltage thresholds with and without saturation charge. (See also BU-808: The way to Prolong Lithium-based Batteries.)
As soon as the battery is first place on charge, the voltage shoots up quickly. This behavior might be in comparison to lifting a weight by using a rubber band, creating a lag. The capacity will ultimately get caught up if the battery is practically fully charged (Figure 3). This charge characteristic is typical of all the batteries. The greater the charge current is, the greater the rubber-band effect is going to be. Cold temperatures or charging a cell with good internal resistance amplifies the impact.
Estimating SoC by reading the voltage of the charging battery is impractical; measuring the open circuit voltage (OCV) following the battery has rested for several hours can be a better indicator. As with all batteries, temperature affects the OCV, so does the active material of Li-ion. SoC of smartphones, laptops as well as other devices is estimated by coulomb counting. (See BU-903: How you can Measure State-of-charge.)
Li-ion cannot absorb overcharge. When fully charged, the charge current must be stop. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep the lithium-ion battery with the peak cut-off as short as possible.
Once the charge is terminated, battery voltage actually starts to drop. This eases the voltage stress. Over time, the open circuit voltage will settle to between 3.70V and 3.90V/cell. Be aware that energy battery that has received a completely saturated charge helps keep the voltage elevated for an extended than one which includes not received a saturation charge.
When lithium-ion batteries should be left inside the charger for operational readiness, some chargers apply a brief topping charge to make up for your small self-discharge battery and its particular protective circuit consume. The charger may start working as soon as the open circuit voltage drops to 4.05V/cell and shut off again at 4.20V/cell. Chargers manufactured for operational readiness, or standby mode, often let the battery voltage drop to 4.00V/cell and recharge to only 4.05V/cell rather than full 4.20V/cell. This reduces voltage-related stress and prolongs battery lifespan.
Some portable devices sit in a charge cradle in the ON position. The present drawn through the device is called the parasitic load and might distort the charge cycle. Battery manufacturers advise against parasitic loads while charging since they induce mini-cycles. This cannot continually be avoided as well as a laptop linked to the AC main is unquestionably an instance. The battery might be charged to 4.20V/cell and after that discharged through the device. The anxiety level in the battery is high since the cycles occur with the high-voltage threshold, often also at elevated temperature.
A portable device ought to be turned off during charge. This enables battery to reach the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to lower low enough by drawing a leakage current. A battery might be fully charged, however the prevailing conditions will prompt a continued charge, causing stress.
While the traditional lithium-ion includes a nominal cell voltage of 3.60V, Li-phosphate (LiFePO) makes an exception with a nominal cell voltage of 3.20V and charging to 3.65V. Somewhat new will be the Li-titanate (LTO) by using a nominal cell voltage of 2.40V and charging to 2.85V. (See BU-205: Varieties of Lithium-ion.)
Chargers for such non cobalt-blended Li-ions usually are not appropriate for regular 3.60-volt Li-ion. Provision has to be made to identify the systems and provide the correct voltage charging. A 3.60-volt lithium battery inside a charger designed for Li-phosphate would not receive sufficient charge; a Li-phosphate within a regular charger would cause overcharge.
Lithium-ion operates safely in the designated operating voltages; however, battery becomes unstable if inadvertently charged into a more than specified voltage. Prolonged charging above 4.30V over a Li-ion created for 4.20V/cell will plate metallic lithium in the anode. The cathode material becomes an oxidizing agent, loses stability and produces fractional co2 (CO2). The cell pressure rises and when the charge is capable to continue, the current interrupt device (CID) in charge of cell safety disconnects at 1,000-1,380kPa (145-200psi). When the pressure rise further, the safety membrane on some Li-ion bursts open at about 3,450kPa (500psi) and the cell might eventually vent with flame. (See BU-304b: Making Lithium-ion Safe.)
Venting with flame is linked with elevated temperature. A fully charged battery carries a lower thermal runaway temperature and may vent earlier than one that is partially charged. All lithium-based batteries are safer at a lower charge, and for this reason authorities will mandate air shipment of Li-ion at 30 percent state-of-charge rather dexkpky82 at full charge. (See BU-704a: Shipping Lithium-based Batteries by Air.).
The threshold for Li-cobalt at full charge is 130-150ºC (266-302ºF); nickel-manganese-cobalt (NMC) is 170-180ºC (338-356ºF) and Li-manganese is around 250ºC (482ºF). Li-phosphate enjoys similar and better temperature stabilities than manganese. (See also BU-304a: Safety Concerns with Li-ion and BU-304b: Making Lithium-ion Safe.)
Lithium-ion is not really really the only battery that poses a safety hazard if overcharged. Lead- and nickel-based batteries will also be proven to melt down and cause fire if improperly handled. Properly designed charging tools are paramount for all battery systems and temperature sensing is a reliable watchman.
Charging lithium-ion batteries is simpler than nickel-based systems. The charge circuit is easy; voltage and current limitations are simpler to accommodate than analyzing complex voltage signatures, which change as being the battery ages. The charge process might be intermittent, and Li-ion will not need saturation as is the situation with lead acid. This offers a significant advantage for sustainable energy storage say for example a solar power panel and wind turbine, which cannot always fully charge the 18500 battery. The lack of trickle charge further simplifies the charger. Equalizing charger, as they are required with lead acid, is not necessary with Li-ion.