6.4V / Lithium Ion Charger With 6.4V Battery Pack

The process of charging and discharging batteries is fundamentally a chemical interaction, but Lithium-ion (Li-ion) batteries showcase a unique aspect. Researchers in the field reference energy flow in context with ion movement between the anode and cathode. While this theory holds some truth, if entirely accurate, it would imply that these batteries possess infinite life. They attribute capacity decline to ion entrapment, yet parallel issues such as internal corrosion and various degenerative reactions affecting both the electrolyte and electrodes equally contribute to performance loss. (See BU-808b for insights on Li-ion battery degradation).

Li-ion chargers function similarly to lead-acid systems, acting as voltage-managing devices. One notable difference is the higher voltage per cell and stricter voltage tolerances, with no trickle or float charge once fully charged. While lead-acid systems afford some leeway in voltage cut-off, Li-ion manufacturers enforce strict adherence to recommended settings, as exceeding this threshold can lead to overcharging. Unfortunately, chargers marketed as miraculous in extending battery life through pulses and other techniques do not genuinely exist; Li-ion systems are efficient and utilize only the energy they can safely handle.

Understanding Cobalt-Based Li-ion Charging

With traditional cobalt-bonded Li-ion chemistries, charge levels typically reach 4.20V per cell. Variations exist, with certain nickel-based options reaching 4.10V and high-capacity configurations potentially achieving 4.30V. Increasing charge voltage can enhance capacity, but exceeding specifications can compromise both safety and longevity. Protection mechanisms embedded in battery packs prevent surpassing calibration standards.

Figure 1 illustrates voltage and current signatures during Li-ion charging stages, highlighting transitions between constant current and topping charge cycles. A full charge is attained when the current falls to between 3 and 5 percent of the battery’s amp-hour rating.

Li-ion batteries reach fullness when the current decreases to a designated threshold. Instead of trickle charges, some chargers use topping charges as the voltage declines.

The recommended charge rate for Energy Cells ranges from 0.5C to 1C, with total charging times typically between 2 to 3 hours. Battery manufacturers suggest maintaining charging rates at or below 0.8C to support battery longevity, although many Power Cells thrive under higher rates with minimal impact. Charge efficiency hovers around 99 percent, with cells remaining stable and cool throughout the process.

A rise in temperature of approximately 5°C (9°F) may occur in some Li-ion packs upon reaching full charge, potentially due to built-in protection circuits or higher internal resistance. Should temperatures escalate beyond 10°C (18°F) at moderate charging speeds, it is advisable to cease battery or charger use.

A battery is deemed fully charged once it reaches the voltage threshold and the current dips to 3 percent of the rated level. Conditions leading to elevated self-discharge could also contribute to this scenario.

Increasing charge current doesn’t necessarily expedite the time taken to reach a full charge. While it may quicken the voltage peak, the saturation will take longer, meaning that a high current charge can typically fill around 70 percent of the battery level very rapidly.

Unlike lead-acid alternatives, Li-ion batteries do not require a full charge, and doing so may even be counterproductive. Preferably, the voltage threshold should be set lower or entirely eliminate saturation charging to extend overall battery life, albeit at the cost of runtime. Consumer product chargers generally prioritize maximum capacity, often disregarding long-term service life.

Some inexpensive consumer chargers employ a simplistic ‘charge-and-run’ strategy, achieving battery charges in one hour or less without undertaking Stage 2 saturation charging; the status “ready” signifies attainment of voltage threshold in Stage 1, equating to about 85 percent charge, potentially sufficient for many users.

Certain industrial chargers deliberately set lower charge voltage thresholds to maximize battery longevity. Table 2 depicts estimated capacities achieved at varying voltage thresholds both with and without saturation charge. (Refer to BU-808 for methods to prolong lithium-based battery life)

Charge V/cell

Capacity at cut-off voltage*

Charge time

Capacity with full saturation

3.80

~40%

120 min

~65%

3.90

~60%

135 min

~75%

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4.00

~70%

150 min

~80%

4.10

~80%

165 min

~90%

4.20

~85%

180 min

100%

Incorporating full saturation at the identified voltage can lift capacity by approximately 10 percent while subjecting the battery to elevated stress due to increased voltage.

Upon initiating charging, the voltage rises sharply. Analogous to lifting a weight with a rubber band, this introduces a feeling of delay. Eventually, as the battery achieves near peak charge, the capacity catches up with the voltage, as shown in Figure 3. This behavior is emblematic of all battery types. The greater the charge current, the more pronounced this elastic effect becomes. Low temperatures or charging batteries with high internal resistance further amplify this characteristic.

There exists a delay between capacity and charge voltage, reminiscent of lifting a weight with a rubber band.

Assessing State-of-Charge (SoC) by examining the voltage of a charging battery proves impractical; instead, measuring open circuit voltage (OCV) after a resting period yields a more accurate representation. Similar to other batteries, temperature influences OCV, as does the active material within the Li-ion battery. Smartphones, laptops, and various devices often evaluate SoC through coulomb counting. (See BU-903: Measuring State-of-charge).

Li-ion batteries cannot tolerate overcharging. When charged to capacity, current must cease immediately. Continuous trickle charging can induce plating of metallic lithium, jeopardizing safety. Limiting exposure at peak cut-off is vital for minimizing stress on the battery.

Once charging ceases, battery voltage declines, providing relief from voltage-induced stress. Following a duration, OCV stabilizes between 3.70V and 3.90V per cell. Note that batteries with full saturation charges maintain elevated voltages longer than those without saturation.

For instance, when Li-ion batteries are left connected to chargers for readiness, some chargers will issue short topping charges to counteract minor self-discharge occurring within the battery and its protective circuit. Charging may activate when the OCV positions itself at 4.05V/cell, subsequently shutting off when it drops to 4.20V/cell. Chargers designed for standby scenarios may allow battery voltage to decrease to 4.00V/cell, only replenishing to 4.05V/cell instead of full 4.20V/cell. These measures assist in lowering voltage-related stress, extending battery life.

Numerous portable devices remain in active charge cradles during operation. This leads to a parasitic load condition, distorting the charge cycle. Battery manufacturers generally advise against parasitic loads during charging, as this can result in mini-cycle occurrences. This situation may be unavoidable in cases where laptops connect to AC power. Consequently, the battery may reach 4.20V/cell, only to be subsequently discharged by the device. Here, the battery experiences heightened stress due to engagement in high-voltage parameters alongside elevated temperatures.

Ideally, portable devices should be powered down while charging. This enables the battery to reach the designated voltage threshold and current saturation uninterrupted. If a parasitic load exists, it misleads the charger by lowering battery voltage, impeding the saturation current from reducing sufficiently, thus instigating leakage currents. While the battery might appear fully charged, underlying conditions will inadvertently trigger continual charging, causing strain.

Charging Methods for Non-Cobalt Blended Lithium-Ion

Despite traditional lithium-ion boasting a nominal cell voltage of 3.60V, Li-phosphate (LiFePO) distinguishes itself with a nominal voltage of 3.20V while charging to 3.65V. More recently, Li-titanate (LTO) has emerged with nominal voltage specifications of 2.40V, charging to 2.85V. (Refer to BU-205 for different Lithium-ion types).

It’s crucial to note that chargers for non-cobalt blended Li-ions are incompatible with standard 3.60-volt lithium-ion batteries. A system identification mechanism must be adopted to ensure correct voltage charging. Utilizing a 3.60-volt lithium battery in a charger designated for Li-phosphate may lead to inadequate charging; conversely, using Li-phosphate chargers on standard lithium batteries can prompt overcharging.

Risks of Overcharging Lithium-Ion Batteries

ChargingLithium-ion batteries is generally safe within designated voltage specifications; however, unintended overcharging can destabilize battery chemistry. Prolonged exposure beyond 4.30V for batteries designed for 4.20V/cell will result in metallic lithium plating at the anode. This transition causes the cathode materials to oxidize, consequently generating carbon dioxide (CO2). Pressure within the cell escalates, culminating in the current interrupt device (CID) disengaging at pressures between 1,000 and 1,380 kPa (145 to 200 psi). If pressures exceed these limits, safety membranes within certain Li-ion cells rupture around 3,450 kPa (500 psi), leading to potential venting incidents accompanied by flames. (Refer to BU-304b for enhancing lithium-ion safety).

Flame venting correlates with elevated temperatures. A fully charged battery exhibits lower thermal runaway temperatures and, as a result, has a higher likelihood of venting compared to a partially charged alternative. This inherent trait underscores why regulatory bodies require lithium batteries to be shipped by air at 30% charge instead of full capacity. (See BU-704a for shipping lithium-ion batteries).

Temperature thresholds for Li-cobalt at full charge reside between 130 and 150°C (266 to 302°F); nickel-manganese-cobalt (NMC) operates between 170 and 180°C (338 to 356°F), while Li-manganese sees temperatures around 250°C (482°F). Notably, Li-phosphate presents similar or even enhanced thermal stabilities in comparison to manganese. (Refer to relevant safety concerns outlined in BU-304a and BU-304b).

Safety risks are not exclusive to lithium-ion batteries; lead and nickel variants may likewise face risks such as melting and potential fire hazards if mishandled. Thus, precise charging equipment design is vital across battery systems, with temperature sensing serving as a reliable precautionary measure.

Conclusion

Charging lithium-ion batteries is less complex than involving nickel-based systems. Charge circuits are straightforward, where voltage and current limits are simpler to manage than deciphering complex voltage signatures, which change with battery age. Charging can be intermittent; as Li-ion systems do not require saturation like lead acid, this feature proves advantageous for renewable energy storage, affecting solar and wind turbine systems where powering the battery might not always be optimal. The absence of trickle charge contributes to a more streamlined charger solution. Equalization charging, necessary for lead-acid batteries, is unnecessary for Li-ion variants.

Most consumer and industrial Li-ion chargers ensure batteries reach full capacity. Adjustable end-of-charge settings that could enhance service life by reducing final charge voltages are rare, as device manufacturers often perceive complexity as detrimental. Electric vehicles and satellites represent notable exceptions, favoring incomplete charging processes to extend operational longevity.

Essential Principles for Charging Lithium-Based Batteries

• Ensure the device is turned off or disconnect the load during charging to facilitate unhindered current drop during saturation without parasitic load interference.
• Execute charging in moderate temperature conditions. Avoid charging in freezing environments. (Refer to BU-410 for implications of temperature during charge)
• Complete charges are unnecessary for lithium-ion batteries; partial charges are oftentimes more beneficial.
• Charger signals indicating full charge may not guarantee actual complete charges, as some chargers may not apply full topping charges; always confirm charge levels through other means.
• If temperature surges excessively within the charger or battery, cease usage immediately.
• Add some charge to an empty battery prior to storage (maintaining between 40 to 50 percent SoC is ideal). (Refer to BU-702 for best practices in battery storage.)

References

[1] Courtesy of Cadex

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