Lithium battery charging best practices (How to & other tips)

• Introduction - Charging a Lithium Battery
• Open-Loop / Closed-Loop Charging Methods
• Other Differences
  • Charging Efficiency
  • Charge Acceptance
  • Battery Cycles and Depth-of-Discharge (DOD)
  • Cell Balancing
• Comparison of Battery Types
• Summary

Introduction - Charging a Lithium Battery

Charging a Lithium battery is very different from charging a Lead-Acid battery. The most crucial difference is that a Lithium battery charges at a lower voltage than required to charge a Lead-Acid battery. Charging a Lithium battery with a higher Lead-Acid charging voltage will cause the Lithium Battery's Battery Management System (BMS) to self-protect and disconnect the battery from the charging source.

Additionally, determining state-of-charge and charge termination using voltage is more difficult with Lithium than with Lead-Acid. For Lead-Acid batteries, voltage correlates well with SOC and is easier to determine given the steep slope presented by the Lead-Acid SOC vs Voltage curve. With Lithium, the SOC vs Voltage slope has a very flat mid-section making 90% - 10% SOC and charge termination very hard to determine for Lithium without detailed data from the battery.

Open-Loop / Closed-Loop Charging Methods

Open-loop is the term used to describe battery charging, where power conversion equipment works with a pre-configured fixed voltage. To optimize this method of charging, with Lead-Acid batteries, for example, the charging process is often divided into stages (Bulk/Absorption/Float), each with different voltage levels. The Open-Loop method can be used to charge Lithium, but it will be an inherently slower process due to restrictions that must be in place to ensure the safe charging of Lithium.

Closed-loop charging describes the charging method where power conversion equipment receives information directly from the battery and adjusts its charging output accordingly. In this case, the battery BMS shares SOC, voltage, and internal battery temperature to guide the output of the power conversion equipment. Closed-Loop charging is not possible with Lead-Acid because the battery cannot communicate SOC, voltage, or internal temperature.

Other Differences

Various other differences exist between Lithium and lead-acid batteries that affect charging and overall performance. Efficiency, charge acceptance, partial-state-of-charge cycling, depth of discharge (DOD), and cell balancing all present significant differences between Lithium and Lead-Acid batteries.

Charging Efficiency

Lithium batteries charge at 95% to 98% efficiency, which means that if 1000 watts of power is input to the battery, the battery retains 950 to 980 watts. Lithium batteries maintain this efficiency for their useful lifetime. Lead-Acid batteries, best case, charge at 80% efficiency when they are new. However, charging efficiency drops steeply for Lead-Acid batteries as they age, and less than 65% is very common.

Charge Acceptance

The amount of charge current accepted by Lithium batteries varies according to the specifications of the BMS. There are significant differences in BMS specifications, varying from 100% of Capacity (1C) to 20% of Capacity (0.2C), and of course, affect the price of the battery. Lead-Acid batteries have no BMS to impose a fixed restriction and will overheat (leading to a thermal runaway event) if charged with current exceeding their specifications. Most Discover Lithium batteries can charge at a maximum of 1C, whereas Lead-Acid batteries typically charge at C/5 (equivalent to 0.2C), resulting in a much longer elapsed time for 0 to 100% SOC.

Battery Cycles and Depth-of-Discharge (DOD)

A cycle is defined as the amount of energy taken out and then returned to the battery and is used to represent consumed battery life. A cycle is a very different proposition for a Lithium battery than for a Lead-Acid battery. A Lead-Acid battery's lifetime is dramatically affected by the regular Depth-of-Discharge (DoD) and the time between the end of discharge and the start of the charge. Lead-Acid batteries last much longer when discharged, 20-30%, than 50-80%. Lead-Acid batteries do not fair well when left in a partial-state-of charge.

In contrast, Lithium battery lifetime is hardly affected by regular DOD down to 90% or being left in a partial state of charge for more extended periods. Charge cycles are generally a construct used to help define the helpful lifetime of Lead-Acid batteries because a precise measure of energy in and out (throughput) is not possible for this technology. With higher-quality Lithium batteries, energy throughput is measured and retained by the BMS and thus becomes a far more accurate gauge of consumed battery life.

Cell Balancing

An advanced Lithium Battery BMS will passively balance the voltage level of each cell within the battery at the end of the charge cycle above approximately 98% SOC. This is essential to maintaining Lithium capacity and lifetime. Unbalanced cells within a Lithium battery will lead to lower capacity and possibly voltage collapse.

The Capacity of a Lead-Acid battery can sometimes be partially restored through a controlled over-voltage charge process known as Equalization. Equalization of a Lead-Acid battery should be monitored and set for a limited period, as it could result in a thermal runaway event if carried on too long. Equalization (controlled over-voltage charge) should never be attempted on a Lithium battery as this is unsafe and will likely cause irreparable damage.

Comparison of Battery Types

Summary

Charging is interwoven with a battery's characteristics. As detailed above, charging a Lithium battery is very different from charging a Lead-Acid battery due to the differences in characteristics between the two different types of batteries.

With a Lead-Acid battery, voltage is used to identify the battery SOC, charge control is based on Open-Loop settings with a charge efficiency of up to 80%, a depth of discharge between 20 and 50% is required to maintain the ability to recharge, and you can expect a battery lifespan of 2 to 5 years with 500 to 800 cycles.

With a Lithium battery, the BMS measures the SOC, charge control is based on Closed-Loop communication with a charge efficiency of up to 98%, a depth of discharge of up to 100% is possible with no adverse effects, and you can expect a battery lifespan of more than 10 years with more than 2500 cycles.

The most important thing to note about the differences between Lead-Acid and Lithium batteries is that each charged Lithium battery can charge faster, run longer, and last for many more years than a Lead-Acid battery.