Why is a Voltage Monitoring System (VMS) required

Question:  I’ve heard that lead-acid (PbA) battery cells are more “forgiving” than Li-family battery cells if they are overcharged.  Why is a Voltage Monitoring System (VMS) required when charging and discharging Lithium-Iron-Phosphate battery cells, while not required for lead-acid battery cells?

Answer:  Unlike PbA battery cells that can tolerate a certain amount of overcharging when exceeding normal voltage and current thresholds, Li-family battery cells do have strict requirements to not exceed their maximum and minimum thresholds.  The chemistry advantages gained in performance by Li-family battery cells do come at a cost that requires strict compliance to narrower charge/discharge constraints during each battery cell cycle.  LiFePO4 charging systems must never exceed thresholds where voltage and current applied to a Lithium-Iron-Phosphate cell could overcharge it.  When discharging Li-family cells, the applied load should also not discharge the cell completely but leave some residual current available to maintain cell chemistry.  These differences are due to how the electrolyte is used in PbA and LiFePO4 chemistries.

Although frequent overcharging of PbA battery cells will shorten their total life cycle performance over time, PbA batteries can tolerate some overcharging and total discharging under load.  Because a PbA cell’s anode, electrolyte and cathode materials regularly undergo chemical reactions and changes every charge/discharge cycle, PbA battery packs can tolerate more abusive charging and discharging conditions.  A PbA battery pack can be simply charged by just applying voltage and current across the positive and negative ends of the entire pack from a simple AC or DC source to charge up all the battery cells at the same time.  Some individual cells may charge more quickly than others, causing the electrolyte to “boil over” and evaporate.  PbA cells can tolerate this more volatile chemical change after discharge if the electrolyte can be rebalanced and regenerated from the anode and cathode materials to restore some current capacity when recharged. 

The same is true when discharging a PbA battery under a heavy load such as an electric motor.  The PbA cells may stop performing when most of the current has been drained from the battery pack while driving an electric vehicle (EV).  EV drivers have found however, that unlike gasoline, residual current can be coaxed out of the remaining cell capacity after resting the battery cell for several minutes and allowing the PbA electrolyte to slightly regenerate from the anode and cathode materials.  Although this abuse can shorten the overall life cycle of individual cells, it does provide an extra margin of safety to EV drivers if they accidentally run out of “fuel”.  The residual current capacity that can be regenerated in the electrolyte from the anode and cathode by just resting the PbA cell can be enough to drive the electric vehicle a little further to a safe location.

In contrast, overcharging LiFePO4 battery cells just one time will disable them by damaging the ion-transference capability of their electrolyte chemistry.  During normal operation, the LiFePO4 electrolyte material remains relatively unchanged as cations and anions move through the electrolyte between the anode and cathode materials of the cell during charge/discharge cycles.  The electrolyte material can also be damaged when totally discharging the LiFePO4 cell under load.  These two conditions must never occur and strict thresholds must be established when charging and discharging each LiFePO4 cell within the entire battery pack to prevent this. 

Fortunately, modern day electronics technology has enabled the creation of inexpensive “smart” charging and monitoring systems that can effectively control how voltage and current are applied to or discharged from each individual cell inside the LiFePO4 battery pack.