If you’ve recently purchased or are researching Lithium Iron Phosphate batteries (often referred to as Li-Ion or LiFePO4), you may have noticed that they offer several advantages over Sealed Lead Acid (SLA) batteries, including a higher number of cycles, consistent power output, and lighter weight. Did you also know that they charge up to four times faster than SLA batteries? But how exactly do you charge lithium batteries?
Keheng recommends choosing a charger that is designed specifically for your battery chemistry. This means using a lithium charger, such as the Keheng LiFe charger series, when charging lithium batteries.
Can a lead acid charger charge a lithium battery?
You will notice several similarities in the charging curves of SLA and LiPo batteries. However, care must be taken when using SLA chargers for LiPo batteries as they may damage, undercharge, or reduce the capacity of the LiPo battery over time. The differences between lithium and SLA batteries are significant.
Sealed Lead Acid (SLA) Battery Charging Curve
Let’s review the basics of charging a sealed lead acid battery. The most commonly used charging method is the three-stage charging method: initial charging (constant current), saturation charging (constant voltage), and float charging.
In the first stage, the current is limited to a certain range to prevent damage to the battery. During this stage, the voltage is gradually varied, eventually reaching a steady state near the limit of the fully charged voltage. Constant current/stage one charging is critical before moving on to the next stage. Typically, the first stage of charging is performed at 10-30% (0.1C to 0.3C) or less of the battery’s rated capacity.
The second stage is the constant voltage stage, which begins when the voltage reaches its limit (14.7V for fast-charging SLA batteries, 14.4V for most other batteries). During this phase, the current gradually decreases as the battery receives a top charge. This phase ends when the current drops to less than 5% of the battery’s rated capacity. The final stage is the float charge stage, which is critical to preventing self-discharge and capacity loss.
The third stage is used if the battery is on standby; float charging ensures that the battery remains fully charged when needed. In storage applications, float charging keeps SLA batteries at 100% state of charge (SOC), which is critical to preventing plate sulfation and protecting the plates from damage.
Li-FePO4 batteries utilize the same constant current and constant voltage stages as SLA batteries. Although these two stages are similar and serve the same purpose, Li-FePO4 batteries have the advantage of a significantly higher charging rate, which greatly reduces charging time.
The first stage of lithium battery charging is usually between 30% and 100% of the battery’s rated capacity (0.3C to 1.0C). While the first stage of charging for SLA batteries can take about four hours, lithium batteries can be completed in just one hour, charging four times faster than SLA. Even with a charge rate of only 0.5C, lithium batteries charge almost three times as fast!
Both battery chemistries require a second stage to reach 100% SOC. The SLA battery can take up to six hours to complete the second stage, while the LiPo battery can do it in just 15 minutes. Overall, it takes about 4 hours to fully charge a LiPo battery, while an SLA battery takes 10 hours. In cycling applications, the fast charge time of Li-ion batteries is critical because they can be charged and discharged multiple times per day, whereas lead-acid batteries are typically only fully cycled once per day.
Charging Curve Differences: Stage 3
Lithium batteries do not require float charging like lead-acid batteries. For long-term storage applications, it is recommended that lithium batteries not be stored at 100% State of Charge (SoC). Instead, a full charge/discharge cycle should be performed every 6 to 12 months and then stored charged to only 50% SoC.
In standby applications, Li-Ion batteries have a very low self-discharge rate and can provide nearly full capacity even after 6 to 12 months without charging. For extended periods of use, supplemental charging using a voltage-based charging system is recommended. This is especially important for Bluetooth batteries, as the Bluetooth module draws very little current from the battery even when it is not activated.
Charging Characteristics of Li-ion Battery
Voltage and current settings during charging
The fully charged open-circuit voltage (OCV) of a 12V sealed lead-acid (SLA) battery is about 13.1 V, while the fully charged OCV of a 12V lithium battery is about 13.6 V. The battery will be damaged only when the charging voltage significantly exceeds the fully charged voltage. Therefore, during the second stage of charging, SLA batteries should be kept below 14.7V and LiPo batteries below 15V. Float charging applies only to SLA batteries and is usually recommended at around 13.8 V. The charging voltage range of 13.8 V to 14.7 V is sufficient to safely charge both types of batteries without causing damage.
When selecting a charger for either type of battery, it is important to select a charger with an operating voltage within the above range. The charger should match the capacity of the battery being charged, as the charging current depends on the rated capacity of the battery. Lithium batteries can be charged at up to 1C, while lead-acid batteries should be limited to less than 0.3C. For example, a 10 ampere-hour lithium battery can usually be charged at 10 amps, while a 10 ampere-hour lead-acid battery can only be charged at three amps.
The charging cutoff current is set at 5% of the battery capacity, which means that the cutoff current for both types of batteries is 0.5 A. The terminal current setting is usually determined by the charger.
Universal chargers usually have a feature that selects the battery chemistry. This feature optimizes the charging voltage range and determines when the battery is fully charged. If charging a Li-Ion battery, the charger should automatically shut down. Conversely, if charging an SLA battery, the charger should switch to float mode.
Lithium Batteries Replace Sealed Lead Acid Batteries in Float Charging Applications
In float charging applications, lithium batteries are gradually replacing UPS systems and other applications that previously used SLA batteries. Concerns have arisen about the safety of this type of lithium battery usage. In general, it is feasible to use a standard constant voltage SLA charger to charge LiPo batteries from TechHelix, provided certain conditions are met:
- The charger shall not contain a desulfuration setting.
- The fast/bulk charge voltage should be set to 14.7V.
- The recommended float charging voltage is 13.8V.
In addition, some smart or multi-stage SLA chargers have a function to detect open circuit voltage (OCV). An over-discharged LiPo battery in protection mode has an OCV close to 0 V. Such a charger may assume the battery is depleted and no longer attempts to charge it. Instead, a charger equipped with a Li-Ion battery setting will attempt to recover or “wake up” an over-discharged Li-Ion battery in protection mode.
Long-term storage
When storing batteries for long periods, it is important to consider the different storage requirements of SLA and LiPo batteries. The main reason for these differences is the different chemistry of the batteries.
Determining the optimal state of charge (SOC) is critical to battery life. For Keheng, SLA batteries should be stored as close to 100% SOC as possible. This prevents sulfation, the buildup of sulfate crystals on the plates, which can reduce battery capacity. In contrast, with Kaiheng lithium batteries, the structure of the positive electrode becomes unstable when electrons are depleted over a long period. This instability can lead to permanent capacity loss. Therefore, it is recommended that lithium batteries be stored at close to 50% SOC to ensure that electrons are evenly distributed at both ends. For detailed recommendations on long-term lithium battery storage, please consult the relevant guidelines.
Another factor affecting storage is the self-discharge rate. SLA batteries have a high self-discharge rate and, therefore, require float or trickle charging to keep them at close to 100% SOC to avoid permanent capacity loss. In contrast, Kaiheng lithium batteries have a low self-discharge rate, do not require 100% SOC, and may require minimal maintenance charging.
When selecting a battery charger, it is important to ensure that the charger is capable of delivering the appropriate current and voltage for the type of battery. For example, do not use a 24V charger for a 12V battery. It is also recommended to use a charger that is suitable for the chemistry of the battery unless an SLA charger is to be used for lithium batteries. In this case, make sure the charger does not have a desulfuration or battery failure mode.
If you have any questions about the compatibility of your existing charger with Keheng products, we will be happy to help you with your charging needs.