Always closely monitor a suggested battery/algorithm combination for at least three cycles to ensure the batteries are being properly charged. If you are not sure of how to monitor a battery charge, contact Delta-Q Technical Support for advice. For lithium batteries, see the article Choosing an Algo Contact online >>
Always closely monitor a suggested battery/algorithm combination for at least three cycles to ensure the batteries are being properly charged. If you are not sure of how to monitor a battery charge, contact Delta-Q Technical Support for advice. For lithium batteries, see the article Choosing an Algorithm for a Lithium Battery. 1.
Adhering to voltage requirements, temperature considerations, and lithium battery charging profiles are essential for safe and efficient charging of lithium batteries. Lithium-ion battery charging best practices such as monitoring temperature, avoiding overcharging & following manufacturers'' recommendations can help protect batteries and
Unlock the secrets of charging lithium battery packs correctly for optimal performance and longevity. Expert tips and techniques revealed in our comprehensive guide.
Li-ion batteries like Expion360''s have a unique charging algorithm, and most chargers have a minimum two- or three-state charging profile. For example, two-stage utilizes a bulk state and an absorption stage, whereas three-stage utilizes a bulk stage, absorption stage, and float stage.
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.
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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 charge 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).
Figure 1: In a Li-ion battery, lithium ions move from one intercalation compound to another while electrons flow around the circuit to power the load. (Image source: DigiKey)
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.
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.
Figure 2: Li-ion charging profile using constant-current method until battery voltage reaches 4.1 V, followed by ‘top-up’ using constant-voltage technique. (Image source: Texas Instruments)
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.
About Charging profile for lithium batteries
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