Charging Lithium Batteries with a Standard Charger
One of the most significant benefits of lithium batteries is that they charge faster compared to sealed lead acid (SLA) batteries. You may have heard claims that lithium batteries charge quickly, but how much faster are they compared to SLA? How does this affect you as a user? Lastly, can you charge LiPo batteries with a standard SLA charger?
Charge absorption: SLA vs. LiPo batteries
If you’ve ever experienced the long wait to charge an SLA battery, then you know how time-consuming it can be. For example, charging a 12-volt, 20-amp-hour deep cycle SLA battery typically takes 6.5 hours to fully charge (SOC). In comparison, a 12.8-volt, 20-amp-hour golf cart lithium iron phosphate battery only takes 2.5 hours to fully charge.
The charging cycle for SLA batteries consists of three phases: constant current, constant voltage, and float charging. During the constant current phase, the battery receives a large charge. In our SLA example, the battery reaches 80% SOC during this phase, which is just over half of the total charging time. Constant voltage charging or absorption charging is responsible for charging the last 20% of the battery’s charge, but typically takes up about 50% of the total charging time. Float charging is required for most applications to prevent the battery from over-discharging due to self-discharge.
In contrast, the charging process for Li-Ion batteries is very simple, with only two stages: constant current and constant voltage. During the constant current stage, a Li-ion battery can absorb 99% of its capacity (99% SOC) in 96% of the charge time. For the 12.8V 20AH Li-ion battery mentioned earlier, it can be fully charged in less than two hours. The last 1% of the charge is absorbed in the constant voltage stage, which accounts for only 4% of the total charging time. In addition, due to the low self-discharge rate, lithium batteries do not require float charging.
In this comparison, the Li-Ion battery is fully charged before the SLA battery enters the constant voltage stage, and the entire process takes only one-third of the time required by the SLA battery. This is especially important for applications that require frequent cycling, where the unit needs to be recharged quickly during downtime. In larger SLA installations, extended charging times may require the use of additional batteries while partially charging them.
Li-Ion Battery Charging in SLA Profiles
After understanding how Li-Ion batteries and SLA charging profiles work, you may be wondering what happens if you use an SLA charger to charge a Li-Ion battery. Will this damage the Li-Ion battery? Will it be able to handle voltage differences and float charge adequately?
When charging a Li-ion battery with an SLA charger, the Li-ion battery can reach 95% capacity in approximately 90% of the total charging time if it is charged in a high capacity charging mode of 13.8V (the common float voltage for SLA chargers). This indicates that the remaining 5% capacity will be absorbed in the remaining 10% float charging time. If the Li-Ion battery is charged at a bulk charge of 14.6V (a typical constant voltage for AGM batteries, which is often used since Li-Ion batteries can be substituted for AGM types), 99% capacity can be reached in 95% of the charging time, with only the remaining 1% capacity being absorbed in the last 5% of the charging time.
Charging a Li-ion battery using the SLA charging profile extends the charge time from approximately 2.5 hours for charging using the Li-ion battery charging profile to approximately 5 hours for charging using the SLA charging profile. This highlights the importance of using the proper lithium battery charging method to improve efficiency and performance.
As you can see from the chart below, Li-ion batteries charge faster than SLA batteries when charged according to the SLA charging curve. So, is it possible to use Li-Ion batteries with SLA chargers? Yes, provided the charger does not include a desulfation mode or battery drain detector.The desulfation feature on SLA chargers is designed to restore over-discharged SLA batteries by applying low current and high voltage pulses. This desulfation mode may cause the battery management system of the lithium battery to shut down the battery or even cause damage due to high voltage pulses. Additionally, battery depletion detectors may misdiagnose a Li-ion battery in protection mode as depleted, making it difficult to restore the Li-ion battery to operation.
As described in this blog post, SLA charge curves typically operate in the 13.8 to 14.7 volt range. Within this range, you can take advantage of the fast charging offered by lithium batteries. Li-Ion batteries charge much faster than equivalent SLA batteries, especially at higher voltages, because Li-Ion batteries charge efficiently at 14.6 volts. Even at the lower end of the voltage range, Li-Ion batteries reach a 95% state of charge faster than SLA batteries reach an 80% state of charge. At the upper end of the voltage range typically used for most AGM batteries, the LiPo battery will be fully charged.
Can a standard SLA charger cause damage to lithium batteries? In principle, the use of SLA chargers on LiPo batteries will not cause damage, assuming that the charger does not have a desulphation mode. In frequent cycling applications where the battery is cycled daily and may not have a float charge, it is unlikely that there will be a noticeable degradation in performance. However, it may not be possible to achieve the maximum capacity of a lithium battery using an SLA charger (e.g., a 20 amp hour lithium battery may not reach 20 amps on an SLA charger). If the battery is used infrequently and will be on float charge for an extended period of time, it is recommended that the battery be disconnected from the charger before storage. It is worth noting that lithium batteries perform best when the state of charge is about 50%. In conclusion, you can safely use a standard SLA charger (without desulfurization mode) to charge LiPo batteries.
Keheng recommends using a charger that is customized for the specific chemistry of the battery. It is also recommended to check the voltage of all LiPo batteries every 3-4 months and recharge them if necessary. For comprehensive maintenance recommendations for lithium batteries, please refer to our guide.
For further information on the charging characteristics of Li-Ion and SLA batteries, we recommend you read this blog post for more detailed information.