Charging of Lithium Ion Batteries: How Charging Works, Safety and Battery Life
- How the Charging of Lithium-Ion Batteries Works
- Lithium-Ion Battery State of Charge and Voltage
- Factors That Affect Charging Time and Battery Lifespan
- Charger Output and Battery Charging Limits
- Safe Charging and Everyday Lithium-Ion Battery Care
- Lithium-Ion Charging in Larger Home Battery Systems
- Electrical Compatibility, Capacity and Installation Requirements
- Conclusion
- FAQs
Lithium-ion batteries are found in everything from cell phones, laptops, and electric cars to larger energy-storage systems. They are popular because of their relatively high energy density, rechargability and capability to deliver useful power in compact battery packs. But there is something more involved in charging lithium-ion batteries than providing power until the battery is charged. The charging is accomplished in a controlled sequence to control voltage, current and temperature, and to protect the cells.
To better understand these charging stages may help explain why the speed of charging varies as the battery fills, the importance of temperature, and how charging habits can impact battery health. It also offers some insight into larger lithium-ion storage applications where battery storage management and controlled charging are even more critical.
How the Charging of Lithium-Ion Batteries Works
There are a variety of lithium-ion batteries, but they are all charged in a similar manner. The lithium ions flow between the battery's electrodes, and electrical energy is provided from an external charger. The process is controlled so that the risk of degradation due to too much voltage, current or heat is avoided.
Lithium-Ion Movement During Charging and Discharging
Lithium ions shuttle between the positive and negative electrodes in lithium-ion battery cells during charging and discharging. During charging, the electrical energy provided by a charger will cause the lithium ions to move to the negative electrode, where they are stored chemically.
The process is reversed when discharge occurs. The lithium ions return to the positive electrode, and the electrons flow through the external circuit to provide power to the device it is connected to. It is this reverse reaction that allows lithium-ion batteries to be recharged so many times before they are thrown away.
Charging Characteristics of Lithium-Ion Batteries
The charging characteristics of lithium-ion batteries are different from those of the older rechargeable battery technologies. Lithium-ion cells are not to be charged with an unrestricted current, but with a carefully controlled charging current.
As the battery charges, voltage gradually increases, and a charging system keeps track of the situation so that the battery is not allowed to go outside a safe range. The charging behaviour also varies when the battery is nearing full charge, so the last increment of charge may take longer than the previous increments.
Charging response can be affected by battery chemistry, cell design, battery capacity, temperature and charger capability.
Constant-Current and Constant-Voltage Charging Stages
The charge process of a lithium-ion battery can be divided into two phases: constant current and constant voltage.
The constant-current stage is the period when the charger applies a regulated current, and the voltage of the battery increases. This is typically where much of the energy is recovered in the battery.
When the upper charging voltage of the cell is reached, the charger switches to the constant-voltage stage. As the charging current slowly decreases, the voltage is maintained near the allowable value. This stops the charger from continually putting the same current into a nearly full battery.
When the current drops to a proper level as determined by the charging system, the battery is considered to be sufficiently charged.
Charging Profile of a Lithium-Ion Battery
A charging profile for lithium-ion battery cells is thus not constant from empty to fully charged. Usually, the charging rate will be higher at the beginning of the charging cycle and will decrease the closer the battery gets to the fully charged state.
Due to its controlled profile, it is able to balance charging time and cell protection. It also offers an explanation for the charging specifications that claim to deliver a quick initial charge, but not necessarily the same percentage for the remainder.
The profile can be further adapted depending on the temperature or the condition of the battery. Under certain conditions, such as excessive battery temperature, cooling or excessive heating, or nearing other operating limits, a charging system may be forced to lower current.
Battery Management Systems During Charging
The battery management system, or BMS, is a system that is used to manage the charging of modern lithium-ion battery packs. The BMS can check various parameters during charging of lithium-ion battery packs, like cell voltage, pack current and temperature.
It can also be used to prevent overcharging, over-discharging and/or unsafe temperatures. The management systems may also monitor differences between individual cells in multi-cell battery packs to ensure that the pack is kept within its operating parameters.
This is especially critical when battery packs are scaled up. Regardless of whether the battery is located in a hand-held electronic product or in a large energy-storage system, the ability to charge the battery safely requires maintaining electrical and thermal limits for the cells.
Lithium-Ion Battery State of Charge and Voltage
Once you have an understanding of the charging stages, it's time to find out how much energy you can use in the battery when it is needed. It is usually expressed as a percentage known as state of charge, but the correlation between charge percentage and battery voltage is not linear.
Lithium-Ion Battery State of Charge Explained
The lithium-ion battery state of charge is the percentage of the battery's capacity that is being utilised. The battery's SOC of 100% means that the battery is in a fully charged state, and lower values mean the battery has already discharged some of the energy stored.
The S.O.C is typically not measured directly, but is estimated by the battery-management system. The system can take into consideration voltage, existing current flow, charging history and other data on the battery to determine a sensible percentage for the user.
Lithium-Ion Battery State of Charge Versus Voltage
The relationship between lithium ion battery state of charge vs voltage can be used to give an indication of the amount of energy remaining, though this is not necessarily exact to a percentage.
Battery voltage typically increases with the charging of the cell and decreases with the discharging of the cell. But temperature, battery chemistry, current draw and recent charging activity can all influence the reading. This does not imply that two batteries with similar voltage necessarily have the same SOC.
Reading a Lithium-Ion Battery State of Charge Chart
A lithium-ion battery state of charge chart is usually a comparison of battery voltage and an estimation of how much charge is left in the battery. While these charts are helpful in understanding the general behaviour of these batteries, they should not be assumed to apply to all lithium-ion batteries.
The voltage ranges can vary depending on the cell chemistry and battery-management settings. The system can also determine the state of charge for larger battery packs based on information taken from a number of cells instead of just a single voltage measurement.
Checking the Charge Level of a Lithium-Ion Battery
If you are not sure how to check the charge of lithium-ion battery packs, the safest and most practical way to do so is to check the battery's built-in display, connected device or management interface (where applicable).
Such systems are intended to translate battery information, not to rely on a user's voltage guesses. With larger storage systems, it is even more crucial to monitor the charge level to know when to store or use the energy, or to save it up for later household needs.
Factors That Affect Charging Time and Battery Lifespan
With knowledge of the charging process and the state of charge, it is easier to understand why two lithium-ion batteries may require different charging times, or may age at different rates. The charging of lithium-ion batteries and their long-term condition is influenced by a number of factors, such as battery capacity, charger power, temperature, charging speed and previous use.
Battery Capacity and Charging Power
The capacity of a battery is the amount of energy that the battery can hold, and the charging power is the speed at which the energy can be recharged. Usually, the bigger the battery, the longer it will take to charge if the battery charger output is the same.
Capacity can be given in amp hours or watt hours, depending on the application. For those who wish to learn more about these ratings, please find an explanation of Battery Capacity and Amp Hours.
Increased charging power equals faster charging time (provided the battery accepts it).
Charger Output and Battery Charging Limits
Charger and battery should have compatible electrical specifications. The battery-management system may limit the amount of current the battery cells can take, even if the charger supplies it.
This is why the actual charging speed is determined by the lower value of the charger or battery. A higher charging power does NOT necessarily result in a higher charging speed.
The Battery's health also plays a role. A lower charging level may be used when a battery is getting close to full charge or if the battery management system determines that a lower charging rate is needed.
Temperature and Charging Performance
Temperature has a major influence on the charging characteristics of lithium-ion batteries. Very high temperatures can cause stress to the battery materials; very low temperatures can make charging more challenging and may require the system to slow or even temporarily stop charging.
The heat produced by charging should therefore not be neglected. This is evident in smaller electronics, such as a phone getting hot when it charges.
Built-in temperature monitoring will be present on larger battery packs and will allow the charging system to regulate operation when temperatures fall outside of the desired range.
Fast Charging and Battery Degradation
While fast charging can be convenient, at faster charging rates there is a higher thermal and electrical stress on the cells when they are used frequently or in inappropriate conditions. It is dependent on battery chemistry, temperature control, charger design and battery-management strategy.
New systems are able to self-regulate charging, especially when the battery is nearing full charge. This is what makes fast charging most efficient in the first part of the charging cycle and not from empty to full.
This does not imply that occasional fast charging is the same as continuous high charging rates and excessive heat on a battery.
Battery Age and Changes in Charging Behaviour
Lithium-ion batteries lose a small amount of usable capacity over time. This loss can be due to repeated charge-discharge cycles, high temperature, long duration of operating close to the extreme states of charge and general calendar ageing.
Thus, an older battery will be able to hold less energy than when new and the charging characteristics of the battery may also vary with age. The management system can also use more conservative limits as the battery's state changes.
That is why charging and discharging of lithium-ion battery systems should not be thought of as charging periods, but as a long-term process. Avoidable stress can be minimised, and more consistent battery performance can be maintained during the service life with suitable charging conditions, temperature control and the proper use of chargers.
Safe Charging and Everyday Lithium-Ion Battery Care
The speed of a battery charge is just one aspect of battery performance. However, everyday practices also help ensure the lithium-ion battery's ability to function safely and consistently over time. Good charging practice is based on the use of suitable charging equipment, avoiding unnecessary deep discharge, avoiding excessive heat, and maintaining the battery within a sensible state-of-charge range.
Safe Charging of Lithium-Ion Batteries
The first step in safe charging of lithium-ion batteries is to use a charger that is compatible with the battery or the device. The electrical and thermal stress can be higher due to incorrect voltage, damaged cables and poor-quality charging equipment.
The battery must also be charged in a fairly well-ventilated area, away from direct heat sources. If a device or battery is getting hot, swollen or damaged, don't continue charging it.
While battery-management systems can be built-in for protection, smart charging is still necessary.
Best State of Charge for Lithium-Ion Batteries
How the battery is used will determine its optimum state of charge. Cycling a battery to full charge or allowing it to discharge fully on a regular basis can cause stress on an everyday battery.
Partial charging is usually feasible and can be used for regular operation, as Li-ion batteries do not require full discharge before recharging. Maintaining the battery in a moderate range of convenient operation can minimise time at extreme SOC.
This doesn't imply that users are required to keep an eye on percentages all the time. The goal is not to make it a habit to go all the way to depletion or to max charge.
First Charge of a Lithium-Ion Battery
Unlike older rechargeable battery technologies, the initial charging procedures, which were previously so lengthy, are not normally required for the first charge of lithium-ion battery products. Modern Lithium-ion batteries and charging systems are made to take care of the charging process automatically.
Users should therefore not intentionally leave a new battery on charge for an unduly long time but follow the manufacturer's instructions instead.
The same is true for daily charging: the right equipment and normal charging practices are more important than attempting to "condition" the battery by repeated full charge/discharge cycles.
Avoiding Excessive Heat and Deep Discharge
Some factors that can speed up the ageing of lithium-ion batteries are heat. It can be difficult for heat to escape when charging is done under pillows, in enclosed bags or in direct sunlight.
Where possible, also avoid deep discharge. Letting the battery go into a very low state of charge is not the same as running the battery down completely before recharging it.
Thermal behaviour can also be affected by some charging techniques. If you're in the midst of talking about different charging methods with readers, this discussion about whether wireless charging can impact battery health may be helpful to explain how charging methods relate to heat and long battery life.
Charging Habits That Support Longer Battery Life
Good charging of lithium-ion batteries is typically more a thing of habit than one perfect charging routine. Stress can be minimised by avoiding extreme temperatures, using correct chargers and recharging before the battery is allowed to become deeply discharged for extended periods.
The technology and design of the battery also play a role: the life expectancy may vary from one lithium-ion battery to another. Here are some additional details about battery chemistry, performance and lifespan when comparing LiFePO4 batteries and lithium-ion batteries.
These habits are more critical with larger battery systems, the key being that home energy storage involves a series of cells charging and discharging in a controlled manner, not a small battery.
Lithium-Ion Charging in Larger Home Battery Systems
As battery capacity increases, so do the same principles of controlling smaller lithium-ion batteries. Home storage systems will need to manage charging, discharging, temperature monitoring and household demands from a much larger battery pack. Thus, the electricity stored can be used safely and at useful times instead of the battery being used as a mere electricity storage device, which is to be charged when it runs out.
From Small Device Batteries to Home Energy Storage
A phone battery only provides power for one phone, whereas a home battery can communicate with the solar generation, home appliances and the grid. This implies a more complicated energy management of charging and discharging of Li-ion battery systems in homes.
The importance of capacity also increases because the battery can be required to power multiple loads in the home for extended periods of time. Those interested in storage but not solar can also investigate how home battery storage can function as part of a larger electricity-management plan.
Battery Management at Larger Storage Scale
In larger storage projects, the battery management system (BMS) monitors a number of cells instead of a small battery. It maintains cell voltage equilibrium and regulates charging or discharging if limits are about to be exceeded and/or monitors the temperature.
This is especially crucial for the BMS in home charging of lithium-ion batteries since they are used daily, often to store large amounts of energy
Charging and Discharging Around Household Electricity Demand
Home batteries are most valuable when the charge/discharge is synced with the home energy signature. A battery could charge when the electricity from solar generation is available, and discharge when the electricity from the grid is more expensive to store.
This is not the same as 100% charging of a portable device. It is intended to control the stored energy when the house requires it. If you're thinking about resilience and daily energy management as well, you can also check out the basics of how to build a home battery backup system.
EcoFlow STREAM 5000 for New Home Energy Storage Planning
EcoFlow STREAM 5000 may be an option for households that are considering storage within a larger electricity-management system where the stored energy would be used to power their homes.
It is not meant to modify the fundamental charging properties of a lithium-ion battery but to implement controlled charging, monitoring and energy management at the household level. The correct setup should take into account electricity usage, installation space, and whether or not a solar generation system is being considered.
EcoFlow STREAM AC 5000 for Homes With Existing Solar Panels
If you are looking to install battery storage on an existing solar panel setup, EcoFlow STREAM AC 5000 could be an option to consider. The objective is to capture more of the electricity generated during the day to be used later in the day when demand is greater.
This extends charging behaviour to a broader solar self-consumption approach and not just a battery process.
Electrical Compatibility, Capacity and Installation Requirements
The battery capacity should be based on the energy consumption of the household, not just on the biggest battery that is available. Other considerations include the electrical compatibility, space for installation, and the current solar or home electric system.
Larger home batteries should be considered as professionally designed energy systems. While understanding lithium-ion battery charging is a big component of the technical foundation, safe storage at home requires that the system is properly designed, installed and managed.
Conclusion
Understanding the charging of lithium ion batteries can help explain why charging rate, temperature, state of charge and battery-management systems all impact performance and battery life. Whether you're charging a phone or your laptop, or a larger home battery, a controlled charging process will not only help preserve your cell's life but also ensure the battery is there when you need it.
For larger household storage, the same rules apply, but additionally, electricity needs, solar power generation, size of storage, and professional installation are required. For households with existing solar installations who wish to install a battery for future use, STREAM AC 5000 may be a viable solution. The final installation should ultimately mirror the electricity production, storage and usage in the house.
FAQs
Can a Lithium-Ion Battery Be Recharged Before It Is Empty?
Yes, a lithium-ion battery does not require full discharge before recharging. In fact, most of the time partial charging is appropriate for daily use and can help to minimise the time the battery spends in a very low charge state. The key to good charging of lithium-ion batteries is paying attention to the proper temperature, appropriate charging equipment, and avoiding unnecessary extremes, rather than waiting until the battery is discharged.
Why Does Lithium-Ion Charging Slow Down Near Full Capacity?
Typically, the charging rate is reduced as the battery approaches full charge because it switches from a fast constant current charging phase to a slower, controlled constant voltage phase. In this stage of the charging process of the lithium-ion battery cell, the charging current slowly drops as the battery approaches the upper limit of the voltage. This helps to minimise electrical stress, and charging to its full completion will take place more safely.
Is Partial Charging Better Than Charging to 100% Every Time?
Partial charging may be useful for routine use, since it will minimize the time a battery spends at extremes of charge. While there are devices that are best charged to 100% state of charge, this is not always required and can lead to repeated full charging for long periods for lithium-ion batteries, resulting in reduced battery life. Users are still advised to adhere to the manufacturer's instructions for the battery or device.
Can Lithium-Ion Batteries Be Charged in Very Cold Conditions?
Chemical processes within the lithium-ion cells can be impacted by very cold temperatures and may cause a reduction or temporary halt to charging from the charging system. The safest charging temperature is that recommended by the battery manufacturer. In larger battery systems, temperature sensors and battery-management controls can be used for restricting charging in inappropriate circumstances.
Why Can a Battery State-of-Charge Reading Change After Charging Stops?
Battery state of charge (SOC) indication after charging may differ slightly due to the settling of the battery voltage after removal of charging current. The displayed percentage may also be influenced by temperature and the recent current flow and calculations made by the battery-management system. This is why state-of-charge readings are not "direct" readings of how much energy is left, but rather estimates made from a variety of measurements.