Charging and discharging batteries is really a chemical reaction, but 18650 lithium battery is claimed to be the exception. Battery scientists talk about energies flowing in and out of your battery as an element of ion movement between anode and cathode. This claim carries merits but if the scientists were totally right, then a battery would live forever. They blame capacity fade on ions getting trapped, but as with most battery systems, internal corrosion along with other degenerative effects also called parasitic reactions about the electrolyte and electrodes till be involved. (See BU-808b: What can cause Li-ion to die?.)
The Li ion charger is really a voltage-limiting device containing similarities to the lead acid system. The differences with Li-ion lie inside a higher voltage per cell, tighter voltage tolerances and the absence of trickle or float charge at full charge. While lead acid offers some flexibility when it comes to voltage stop, 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 lifespan and gain extra capacity with pulses and also other gimmicks fails to exist. Li-ion is really a “clean” system and simply takes 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 may go to 4.30V/cell and higher. Boosting the voltage increases capacity, but going beyond specification stresses 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 from the stages for constant current and topping charge. Full charge is reached when the current decreases to between 3 and 5 percent of your Ah rating.
The advised charge rate of the Energy Cell is between .5C and 1C; the total charge time is approximately 2-three hours. Manufacturers of those cells recommend charging at .8C or less to extend battery lifespan; however, most Power Cells may take a greater charge C-rate with little stress. Charge efficiency is about 99 percent along with the cell remains cool during charge.
Some Li-ion packs may experience a temperature rise of approximately 5ºC (9ºF) when reaching full charge. This can be because of the protection circuit and elevated internal resistance. Discontinue while using battery or charger in case the temperature rises more than 10ºC (18ºF) under moderate charging speeds.
Full charge happens 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 in case the current levels off and cannot decrease further. Elevated self-discharge may be the cause of this condition.
Improving the charge current is not going to hasten the complete-charge state by much. Even though battery reaches the voltage peak quicker, the saturation charge will require longer accordingly. With higher current, Stage 1 is shorter however the saturation during Stage 2 can take longer. A higher current charge will, however, quickly fill the battery to about 70 %.
Li-ion does not need to be fully charged as is the situation with lead acid, nor could it be desirable to do so. In fact, it is best not to fully charge as a high voltage stresses the battery. Deciding on a lower voltage threshold or eliminating the saturation charge altogether, prolongs battery lifespan but this decreases the runtime. Chargers for consumer products select maximum capacity and can not be adjusted; extended service life is perceived less important.
Some lower-cost consumer chargers might use the simplified “charge-and-run” method that charges a lithium-ion battery in a single hour or less without going to the Stage 2 saturation charge. “Ready” appears if the battery reaches the voltage threshold at Stage 1. State-of-charge (SoC) at this point is around 85 percent, a level which may be sufficient for most users.
Certain industrial chargers set the charge voltage threshold lower on purpose to prolong battery lifespan. Table 2 illustrates the estimated capacities when charged to various voltage thresholds with and without saturation charge. (See also BU-808: The best way to Prolong Lithium-based Batteries.)
If the battery is first placed on charge, the voltage shoots up quickly. This behavior could be in comparison to lifting a weight by using a rubber band, causing a lag. The capacity may ultimately catch up once the battery is virtually fully charged (Figure 3). This charge characteristic is typical of most batteries. The better the charge current is, the greater the rubber-band effect will likely be. Cold temperatures or charging a cell with higher internal resistance amplifies the outcome.
Estimating SoC by reading the voltage of the charging battery is impractical; measuring the open circuit voltage (OCV) following the battery has rested for a couple of hours can be a better indicator. As with most batteries, temperature affects the OCV, so does the active material of Li-ion. SoC of smartphones, laptops and other devices is estimated by coulomb counting. (See BU-903: The way to Measure State-of-charge.)
Li-ion cannot absorb overcharge. When fully charged, the charge current has to be stop. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To reduce stress, retain the lithium-ion battery with the peak cut-off as short as you possibly can.
When the charge is terminated, the battery voltage actually starts to drop. This eases the voltage stress. As time passes, the open circuit voltage will settle to between 3.70V and 3.90V/cell. Remember that energy battery containing received a completely saturated charge can keep the voltage elevated for an extended than a single which has not received a saturation charge.
When lithium-ion batteries has to be left inside the charger for operational readiness, some chargers apply a brief topping charge to compensate to the small self-discharge the battery as well as its protective circuit consume. The charger may kick in once the open circuit voltage drops to 4.05V/cell and switch off again at 4.20V/cell. Chargers made for operational readiness, or standby mode, often enable the battery voltage drop to 4.00V/cell and recharge to simply 4.05V/cell rather than full 4.20V/cell. This reduces voltage-related stress and prolongs battery.
Some portable devices sit in the charge cradle inside the ON position. The existing drawn from the device is called the parasitic load and may distort the charge cycle. Battery manufacturers advise against parasitic loads while charging mainly because they induce mini-cycles. This cannot continually be avoided plus a laptop linked to the AC main is certainly a case. Battery may be charged to 4.20V/cell then discharged by the device. The worries level about the battery is high as the cycles occur with the high-voltage threshold, often also at elevated temperature.
A transportable device should be turned off during charge. This enables battery to arrive at the set voltage threshold and current saturation point unhindered. A parasitic load confuses the charger by depressing battery voltage and preventing the current within the saturation stage to lower low enough by drawing a leakage current. Battery power can be fully charged, however the prevailing conditions will prompt a continued charge, causing stress.
Whilst the traditional lithium-ion has a nominal cell voltage of three.60V, Li-phosphate (LiFePO) makes an exception by using a nominal cell voltage of three.20V and charging to 3.65V. Somewhat new may be the Li-titanate (LTO) using a nominal cell voltage of 2.40V and charging to 2.85V. (See BU-205: Kinds of Lithium-ion.)
Chargers for these non cobalt-blended Li-ions are not appropriate for regular 3.60-volt Li-ion. Provision needs to be designed to identify the systems and offer the proper voltage charging. A 3.60-volt lithium battery in the charger intended for Li-phosphate would not receive sufficient charge; a Li-phosphate within a regular charger would cause overcharge.
Lithium-ion operates safely within the designated operating voltages; however, battery becomes unstable if inadvertently charged into a beyond specified voltage. Prolonged charging above 4.30V over a Li-ion made for 4.20V/cell will plate metallic lithium about the anode. The cathode material becomes an oxidizing agent, loses stability and produces fractional co2 (CO2). The cell pressure rises and in case the charge is able to continue, the actual interrupt device (CID) responsible for cell safety disconnects at 1,000-1,380kPa (145-200psi). When the pressure rise further, the security 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 completely charged battery carries a lower thermal runaway temperature and definately will vent earlier than one that is partially charged. All lithium-based batteries are safer with a lower charge, and this is the 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 will not be the sole battery that poses a safety hazard if overcharged. Lead- and nickel-based batteries are also seen to melt down and cause fire if improperly handled. Properly designed charging equipment is paramount for all battery systems and temperature sensing can be a reliable watchman.
Charging lithium-ion batteries is simpler than nickel-based systems. The charge circuit is straight forward; voltage and current limitations are easier to accommodate than analyzing complex voltage signatures, which change because the battery ages. The charge process can be intermittent, and Li-ion does not need saturation as is the case with lead acid. This provides an important advantage for renewable energy storage for instance a solar cell and wind turbine, which cannot always fully charge the 18500 battery. The absence of trickle charge further simplifies the charger. Equalizing charger, as is also required with lead acid, is not necessary with Li-ion.