The importance of using the correct charger for lithium batteries becomes apparent when comparing standard 12V lead acid batteries used for deep cycle applications with their lithium counterparts. Although the nominal voltages may seem similar, with lead acid ranging from 12.5V to 12.9V and lithium exceeding 13V, it is essential to understand the differences in charging behavior and requirements.
Lead acid chargers typically allow a longer duration for bulk charging, around 20 hours, while lithium batteries require a much shorter time, usually 4 to 5 hours. This discrepancy arises because the charger's capacity should be within 20-25% of the lithium battery's fully rated capacity. Rushing the charging process for lithium batteries can cause overheating, potential faults, and failures in the Battery Management System (BMS).
The subsequent charging stage, known as the Absorption stage, differs significantly between lead acid and lithium batteries. Lead acid batteries exhibit increased internal resistance as they approach their maximum capacity, absorbing every last bit of energy. This process, accounting for 40% of the total charged capacity, brings the battery to around 80% state of charge (SOC). However, lithium batteries do not require this absorption phase. Instead, the charger algorithm typically transitions straight to a float voltage as the lithium battery is already at 80-90% SOC. The purpose of the float voltage is to slowly balance the remaining capacity without overloading or overheating the BMS. During this absorption charge, the balancing system may activate, and the BMS may initiate a shutdown to allow the unit to cool down and the cell voltages to return to nominal levels. These additional stressors on the BMS can compromise its functionality and longevity.
It is important to note that other charging issues, such as thermal runaway, can occur when using the incorrect charger. These issues further strain the cells and contribute to the premature degradation of the battery. Lithium battery lifecycles are typically measured in years rather than cycles. While the claim that "it works fine" may initially seem valid within the first year, a lithium battery is often sold with the expectation of a 10-year lifespan and 6000 cycles. Evaluating its usable power and performance at the five-year mark will reveal whether the battery is still functioning as originally advertised.
In summary, using the correct charger for lithium batteries is crucial due to the significant differences in charging behavior, absorption stages, and BMS requirements. Neglecting these considerations can lead to safety risks, compromised performance, and a shortened battery lifespan.