Charging a Lithium Ion Battery
The charging voltage depends on the Li-ion chemistry. For example, if the cathode is composed of LiFePO4 (LFP), its operating voltage is 3.2 Volts. Thus 4 cells in series gives an operating voltage of 12.8 V. An LFP cell is typically charged up to a voltage of 3.6 V, which corresponds to 14.4 V for 4 cells in series. This is well within the range of conventionally used charging inverters and alternators that were designed to work with 12 V lead acid or AGM batteries. It is for this reason that a 12.8 V LFP battery pack that is equipped with a battery management system can be used as a drop-in replacement for a nominally 12 V lead acid battery.
The conventional 12 V pack charging algorithm is also well-suited to a LFP battery pack. This algorithm begins with the “bulk” phase, in which the charging voltage is ramped up to 14.4 V. How fast the voltage ramps up determines the charging current. It should be noted that an LFP battery pack can be charged at a much larger current than lead acid (for example, a 1C rate is no problem, if the BMS allows such currents). The bulk phase represents the constant current portion of the algorithm.
The next step is called the “absorption” phase, in which the voltage is held at the maximum charging voltage of 14.4 V. During this phase, the charging current drops from that of the bulk phase to some small fraction (typically around 5%) of the bulk phase charging current. At this point, the Li-ion battery pack is fully charged. However, charging algorithms for standard 12 V lead acid or AGM batteries also include a “float” phase in which the voltage is dropped to 13.6 V and held at that voltage for some prescribed length of time. This step is not necessary for a Lithium Ion battery.
Other cathode materials
Cells composed of other cathode materials, including LCO and NMC, operate at a higher voltage – closer to 3.7 V. Thus, 4 cells in series yield a pack whose operating voltage is 14.8 V. The charging voltage for such cells is closer to 4.1 V, or 16.4 V for 4 cells in series. This voltage is outside the range of an inverter tuned for a 12 V lead acid battery. So, despite the higher energy density and lighter weight of such packs, they are not drop-in replacements for 12 V lead acid batteries.