A Designer's Guide to Lithium (Li-ion) Battery Charging

 

Lithium ion (Li-ion) batteries’ advantages have cemented their position as the primary power source for portable electronics, despite the one downside where designers have to limit the charging rate to avoid damaging the cell and creating a hazard. Fortunately, today’s Li-ion batteries are more robust and can be charged far more rapidly using “fast charging” techniques.

This article takes a closer look at Li-ion battery developments, the electrochemistry’s optimum charging cycle, and some fast-charging circuitry. The article will also explain the downsides of accelerating charging, allowing engineers to make an informed choice about their next charger design.

The concept behind lithium-ion (Li-ion) batteries is simple but it still took four decades of effort and a lot of research dollars to develop the technology that now reliably powers the majority of today’s portable products.

The earliest cells were fragile and prone to overheating during charging, but development has seen those drawbacks overcome. Nonetheless, recharging still needs to follow a precise regimen that limits charger’ currents to ensure full capacity is reached without overcharging with its associated risk of permanent damage. The good news is that recent developments in materials science and electrochemistry have increased the mobility of the cell’s ions. The greater mobility permits higher charge currents and speeds up the “constant current” part of the charging cycle.

These developments enable smartphones equipped with the latest generation of Li-ion batteries to be charged from around 20% to 70% capacity in 20 to 30 minutes. A brief battery refresh to three-quarter-capacity appeals to time-poor consumers, opening up a market sector for chargers that can safely support quick charging. Chip vendors have responded by offering designers ICs that facilitate various charging rates to accelerate battery replenishment for Li-ion cells. Faster charging is the result, but as always, there is a trade-off to be made.

Portable power enhancements

Li-ion cells are based on intercalation compounds. The compounds are materials with a layered crystalline structure that allow lithium ions to migrate from, or reside between, the layers. During discharge of a Li-ion battery, ions move from the negative electrode through an electrolyte to the positive electrode, causing electrons to move in the opposite direction around the circuit to power the load. Once the ions in the negative electrode are used up, current stops flowing. Charging the battery forces the ions to move back across the electrolyte and embed themselves in the negative electrode ready for the next discharge cycle (Figure 1).

 

 

 

Today’s cells use lithium-based intercalation compounds, such as lithium cobalt oxide (LiCoO₂), for the positive electrode, as it is much more stable than highly reactive pure lithium and so it is a lot safer. For the negative electrode, graphite (carbon) is used.

While these materials are satisfactory, things are not perfect. Each time the ions are shifted, some react with the electrode, become an intrinsic part of the material, and so are lost to the electrochemical reaction. As a result, the supply of free ions is gradually depleted and battery life diminishes. Worse yet, each charging cycle causes volumetric expansion of the electrodes. This stresses the crystalline structure and causes microscopic damage that diminishes the ability of the electrodes to accommodate free ions. This puts a limit on the number of recharge cycles.

Addressing these weaknesses has been the focus of recent Li-ion battery research, with a primary goal of packing more lithium ions into the electrodes to increase the energy density, defined as energy per unit volume or weight. This makes it easier for the ions to move in and out of the electrodes, and eases the passage of the ions through the electrolyte (i.e. enhancing ion mobility).

Charging time (for a given current) is ultimately determined by the battery’s capacity. For example, a 3300 mAhr smartphone battery will take approximately twice as long to charge as a 1600 mAhr battery, when both are charged using a current of 500 mA. To take account of this, engineers define charging rates in terms of “C”, where 1 C equals the maximum current the battery can supply for one hour. For example, in the case of a 2000 mAhr battery, C = 2 A. The same methodology applies to charging. Applying a charge current of 1 A to a 2000 mAhr battery equates to a rate of 0.5 C.

It would seem to follow, then, that increasing the charging current will decrease the recharge time. This is true, but only to a certain degree. Firstly, ions have a finite mobility, so increasing the charging current past a certain threshold doesn’t shift them any quicker. Instead, the energy is actually dissipated as heat, raising the battery’s internal temperature and risking permanent damage. Secondly, unrestricted charging at a high current eventually causes so many ions to embed into the negative electrode that the electrode disintegrates and the battery is ruined.

Recent developments have significantly improved the ion mobility of the latest Li-ion cells, allowing the use of a higher charging current without dangerously raising the internal temperature. But even in the most modern products there is still a risk in overcharging because it is a direct result of the physical make-up of the cell. Consequently, Li-ion battery makers prescribe a strict charging regimen to protect their products from damage.  

Carefully does it

Li-ion battery charging follows a profile designed to ensure safety and long life without compromising performance (Figure 2). If a Li-ion battery is deeply discharged (for example, to below 3 V) a small “pre-conditioning” charge of around 10% of the full-charge current is applied. This prevents the cell from overheating until such a time that it is able to accept the full current of the constant-current phase. In reality, this phase is rarely needed because most modern mobile devices are designed to shut down while there’s still some charge left because deep discharge, like overcharging, can damage the cell.

 

 

Then, the battery is typically charged at a constant current of 0.5 C or less until the battery voltage reaches 4.1 or 4.2 V (depending on the exact electrochemistry). When the battery voltage reaches 4.1 or 4.2 V, the charger switches to a “constant voltage” phase to eliminate overcharging. Superior battery chargers manage the transition from constant current to constant voltage smoothly to ensure maximum capacity is reached without risking damage to the battery.

Maintaining a constant voltage gradually reduces the current until it reaches around 0.1 C, at which point charging is terminated. If the charger is left connected to the battery, a periodic ‘top up’ charge is applied to counteract battery self discharge. The top-up charge is typically initiated when the open-circuit voltage of the battery drops to less than 3.9 to 4 V, and terminates when the full-charge voltage of 4.1 to 4.2 V is again attained.

As mentioned, overcharging severely reduces battery life and is potentially dangerous. Once the ions are no longer moving, most of the electrical energy applied to the battery is converted to thermal energy. This causes overheating, potentially leading to an explosion due to outgassing of the electrolyte. As a result, battery makers advocate precise control and suitable charger safety features.

Undercharging, while not dangerous, can also have a detrimental effect on battery capacity. For example, undercharging by a little as 1% can reduce the battery capacity by around 8% (Figure 3).

 

Figure 3: Undercharging by just fractions of a percent can leave Li-ion battery capacity significantly reduced. For this reason, it is important that the final voltage during charging be precisely measured.

For these reasons, the charger should control the final voltage to within ±50 mV of 4.1 or 4.2 V and be able to detect when the battery is fully charged. Detection methods include determining when the current drops to 0.1 C during the constant-voltage stage and, in more basic chargers, charging for only a predetermined time and assuming the battery is fully charged. Many chargers also include facilities to determine the battery temperature, so that charging can cease if a threshold is exceeded. ^[1]

Accelerating charging

Because the latest generation of batteries feature higher ion mobility, faster charging without the risk of overheating is possible. Chip makers to date have provided a wide range of integrated solutions for Li-ion battery management to simplify the design of chargers. Now they also offer silicon that allows engineers to design products that take advantage of the faster charging during the constant-current phase. (Note that there is no industry-accepted definition of a “fast or quick charge” for a Li-ion battery. Rather the term is qualitatively applied to any charging regimen that accelerates charging compared to a “typical” 0.5 C charge rate.)  

Maxim Integrated, for example, offers its MAX8900, a Li-ion battery charger based on a switch-mode step-down (“buck”) power supply. The device can deliver up to 1.2 A from a 3.6 to 6.3 V supply while allowing the designer to adjust the charge parameters with external components.

For example, the designer can implement a constant-current fast charge once the battery voltage exceeds the pre-conditioning voltage and until the voltage reaches 4.2 V. the maximum fast charge current is determined by the resistor between the SETI pin and ground (See Figure 4).

 

Figure 4: The charging current in the constant-current phase of Li-ion battery charging delivered by the MAX8900 from Maxim Integrated can be set using the R_SETI resistor shown here bottom center of this application circuit. (Diagram drawn using Digi-Key Scheme-it, based on an original image courtesy of Maxim Integrated)

For example, for R_SETI = 2.87 kΩ, the fast charge current is 1.186 A and for R_SETI = 34 kΩ, the current is 0.1 A. Figure 5 illustrates how the charging current varies with R_SETI. Maxim offers a handy development kit for the MAX8900A that allows the designer to experiment with component values to explore their effects on not only the constant-current charging rate but also charging rates in other parts of the charging cycle.

 

Figure 5: Variation in charging current in the constant-current phase of Li-ion battery charging delivered by the MAX8900 with R_SETI resistor value.

There are some safeguards built into the MAX8900 to ensure the battery temperature doesn’t rise dangerously during fast charging. These adhere to the Japan Electronics and Information Technology Industries Association (JEITA) specifications for the safe charging of Li-ion batteries. For Li-ion batteries at a temperature of between 0? and 15?C, the fast-charge current is limited to 50% of its programmed rate, and if the battery temperature rises above 60?C the current is cut altogether until the temperature drops to a safe level. The chip itself is protected by thermal foldback that limits the charge current to 25% of the maximum level if the internal temperature exceeds 85?C.

Maxim is not alone in giving designers flexibility in the choice of fast charge rate. NXP Semiconductors’ MC32BC3770 switch-mode battery charger brings control to the charging regimen by enabling the designer to not only set the operational parameters via an I²C interface, but also set the charge-termination current, battery-regulation voltage, pre-charge current, fast-charge voltage threshold and charge-reduction threshold voltage, in addition to the fast-charge current.

The fast-charge current itself is programmable from 100 to 2000 mA, with a 500 mA default setting. For safety, the fast-charge current is always limited by the input current limit setting. The MC32BC3770 can operate from an input up to 20 V and features a single input for USB and a dual-path output to power up a device if the battery is completely discharged.

Fairchild Semiconductor’s FAN5400 also allows the designer to program the chip’s charging rates and operating modes via an I²C interface. The device is a USB-compliant battery charger based on a switching power supply that runs from a 6 V (max) input and offers up to 1.25 A charging current.

The FAN5400 is designed to minimize charging time while meeting USB compliance. The designer can select both the maximum charge current and the current threshold to terminate charging during the constant-voltage phase via an I²C host. Safety features include a timer which cuts the power should the charging cycle exceed a preset duration and charge current is limited if the chip’s temperature exceeds 120?C. 

For its part, Texas Instruments offers the bq25898, a switch-mode battery charge management device that supports high-input-voltage fast charging. The device can accept up to a 12 V input and produces up to a 4 A output, making it suitable for charging the larger capacity batteries in the latest generation of smartphones and tablets.

Similar to the NXP Semiconductors and Fairchild solutions, the bq25898 is configured via an I²C serial interface which allows the designer to set charge current and minimum system voltage. Safety features include battery temperature monitoring, charging timer and overvoltage protection.  

Fast charging trade-off

The designer should be aware of the trade-off that comes with fast charging: the quicker the charge, the lower the capacity when the battery switches to the relatively slow constant-voltage part of the charging regime. For example, charging at 0.7 C results in a capacity of 50 to 70 percent when 4.1 or 4.2 V is reached, whereas charging at less than 0.2 C can result in a full battery as soon as the voltage reaches 4.1 or 4.2 V. In other words, if the consumer needs a quick refresh from, say, 25 to 50 percent, fast charging is ideal, but if the consumer habitually plugs in for a full recharge it is typically quicker to do this at a modest charge rate of 0.5 C than a 1 C-or-greater fast-charge rate which then necessitates a longer, relatively slow “top up”.

The other trade-off is that the raised internal temperature created by fast charging—even though it might be below the “safe” threshold determined by the manufacturer of a particular Li-ion cell—could cause slight damage, ultimately resulting in reduced capacity and fewer recharge cycles. That said, with improvements in battery technology increasing the robustness of cells, fast-charge rates would have to be extreme to reduce the battery’s life to less than the portable product’s “useful” existence (defined as the time between the consumer buying the product and replacing it with a newer model).

Conclusion

While some novel battery technologies are under development in the lab, the Li-ion cell looks set to be the mainstay energy-storage medium for portable products for some time to come. As such, the technology will continue to be subject to intense development, addressing its drawbacks. Ion mobility is among these weaknesses and is likely to improve even compared to the latest generation of batteries – leading to faster charging under a constant current.

Designers are able to take advantage of faster charging by choosing a battery management chip that allows them flexibility in the choice of charge rates by the selection of one or two external components or programming via an I²C interface. It also pays to consider the safety features built into battery-management devices as although modern Li-ion cells are a lot more robust than their forebears, rapid charging still introduces some potential hazards that designers need to factor into their design.