A battery is only as strong as its weakest cell. Keeping cells in step keeps capacity in use.
A battery energy storage system is built from hundreds or thousands of cells connected in series and parallel. They start almost identical and slowly drift apart. Because every cell in a series string must stay inside its limits, the cell that fills first stops the charge and the cell that empties first stops the discharge, so a small drift can lock away a surprising amount of capacity. This guide explains why cells drift, how balancing works, and why watching the spread is one of the best early warnings a battery gives.
In a series string, the same current flows through every cell. When charging, the battery management system has to stop as soon as any one cell reaches its upper voltage limit; when discharging, it has to stop as soon as any one cell reaches its lower limit. A 2024 paper in Scientific Reports puts it simply: if the cells are not properly balanced, the weakest cell will always be the one limiting the usable capacity of the pack.
The drawing below shows the effect. When cells sit at different charge levels, the fullest one ends every charge and the emptiest ends every discharge, and the capacity in between goes unused.
Charging stops at the fullest cell
In a series string, the same current flows through every cell. Once one cell reaches its upper limit, the BMS has to stop the charge for all of them.
Discharging stops at the emptiest cell
The same is true on the way down. Whatever is left in the other cells stays stranded until the cells are brought back into line.
Cells leave the factory with tiny differences in capacity, resistance and self-discharge. A 2025 analysis of battery packs in the journal Energies found that, in series packs, charge inconsistency driven by differences in coulombic efficiency and initial state of charge is the main reason capacity is lost, while differences in resistance mostly limit power.
Temperature turns small differences into large ones. Sandia's energy storage handbook notes that minimising the temperature spread between cells minimises the variation in how they age. Field data bears this out: in a South Korean grid battery studied over three years, variation between cells within each bank grew every year, concentrated in racks with uneven cooling airflow.
Balancing is the battery management system's job of bringing cells back into line. Sandia's handbook describes the two approaches.
The 2025 Energies analysis found that, for packs built from capacity-screened cells, passive and active balancing are about equally effective, and that active balancing outperforms passive mainly when cell capacities differ significantly, which becomes more likely as a battery ages or when new modules are mixed with old. Passive balancing corrects charge-level differences; it cannot make a smaller cell hold more.
LFP's very flat voltage curve, ideal for delivering steady power, makes balancing harder. Sandia notes that flat-voltage chemistries are bad for cell balancing during prolonged operation at partial state of charge, because cells look alike by voltage even when their charge levels differ. Sloping-voltage chemistries such as NMC can be balanced at almost any state of charge.
In practice, LFP batteries balance best near the top of charge, where voltages separate. A battery that never reaches full charge, because it is always cycled in a narrow middle window, may drift without the BMS noticing. That is a good reason for an EMS to schedule occasional full charges on LFP systems, for balancing and for recalibrating state of charge.
Cell spread is more than a capacity problem. A cell that keeps drifting away from its neighbours, needs more balancing than the rest, self-discharges faster or runs warmer is often the first visible sign of a fault developing. Fault-diagnosis research routinely flags cells by comparing them with the pack's typical cell, with one caveat noted in a 2020 review in the journal Algorithms: active balancing can mask the very differences such methods look for, so balancing activity itself is worth tracking.
This is where analytics adds to the BMS. The BMS balances and protects in real time; analytics watches the trend over weeks and months, notices which modules need ever more balancing, and turns that into a maintenance decision before the weak cell starts limiting the rest.
Ingro Brain tracks voltage, temperature and balancing behaviour at the level of detail the battery management system provides, and watches how the spread between cells and modules changes over time. A module that drifts, needs more and more balancing, or runs warmer than its neighbours becomes a planned inspection, not a surprise.
Because Ingro Brain learns across every site Ingro runs, it knows what normal spread looks like for a given chemistry and age, which makes abnormal drift easier to spot. Ingro EMS does its part by scheduling the full charges LFP batteries need for balancing and recalibration.
Battery health analytics and AI forecasting: state of health, early warnings before failure, and dispatch tuned to make the battery last.
3 questions
Cell balancing is how a battery management system keeps the cells in a series string at similar charge levels. Passive balancing bleeds excess charge from the fullest cells as heat; active balancing transfers charge from fuller cells to emptier ones. Without balancing, the weakest cell limits the whole string.
One common reason is cell imbalance: if cells sit at different charge levels, charging stops when the fullest cell is full and discharging stops when the emptiest is empty, leaving capacity unused in the rest. Ageing, temperature differences and a BMS that rarely gets a chance to balance can all contribute.
Not always. Research shows both are about equally effective for packs built from well-matched cells. Active balancing is more useful when cell capacities differ significantly, for example in older batteries or when new modules are added beside old ones.
Ingro Brain watches every cell and Ingro EMS runs the battery with its health in mind, on new projects and on batteries already in service.