A battery pack can reach its charging or discharge limit while several cells still have usable headroom. Cell balancing addresses one reason for that lost opportunity: differences in charge level among the cells connected in series.
It does not make mismatched cells physically identical. Understanding that distinction helps storage users interpret BMS data and helps engineers choose a balancing system without paying for capabilities that cannot solve the actual problem.
The useful starting question is not “Which balancer is strongest?” It is “Are these cells at different states of charge, or do they have different capabilities?”
Charge Imbalance Is Not the Same as a Weak Cell
Cell balancing adjusts the relative charge levels of series-connected cells or monitored cell groups. Passive balancing removes charge from selected higher-charge cells; active balancing transfers energy through a controlled circuit.
In a series string, the same pack current passes through each series position, apart from balancing paths. Equal current does not ensure equal state of charge (SoC), because SoC expresses remaining charge relative to each cell’s own capacity.
Consider two distinctions:
- Charge offset: Otherwise similar cells start at different SoC levels. Correcting their relative charge can recover operating headroom.
- Capability mismatch: One cell has less capacity, greater resistance, or another defect. Balancing may manage some consequences, but cannot rebuild electrodes or remove an abnormal resistance.
Analog Devices identifies capacity, resistance, self-discharge and aging differences even among apparently similar cells in its passive-balancing explanation. Those are separate variables, not interchangeable descriptions of “imbalance.”
Where parallel cells form one series group, the BMS may monitor and balance that group rather than every physical cell individually. Check the pack architecture before interpreting a display labelled “cell voltage.”
Why Cells Drift Apart—and Why One Can Stop the Pack
Small differences can accumulate during storage and use. The main contributors are:
- Cell-to-cell variation: Manufacturing tolerances and initial charge history create different starting conditions.
- Capacity mismatch: The same amp-hours represent a larger percentage of a smaller cell’s capacity.
- Internal resistance differences: A higher-resistance cell can show greater voltage sag during discharge or a greater voltage rise during charging.
- Temperature variation: Cells in different thermal environments respond differently and can age at different rates.
- Self-discharge and unequal parasitic loads: Charge can drift during long idle periods, including through unequal monitoring loads.
- Aging: Different duty histories gradually change capacity and impedance.
Microchip’s cell-balancing design guidance explains how a limiting series cell can cause early charge or discharge termination, and why balancing time must be weighed against power dissipation.
During charging, one cell may reach its allowed upper limit before the others. During discharge, another may reach its lower limit first. A correctly coordinated system restricts or stops the relevant operation instead of continuing simply because total pack voltage looks acceptable.
This creates a usable-capacity constraint: energy remains in other cells, but the pack cannot necessarily deliver it within every cell’s limits. Repeated early cutoffs can disrupt storage dispatch or backup availability. They can also concentrate operating stress on limiting cells. However, no universal cycle-life gain follows from adding a balancer; chemistry, temperature, current, operating window and cell condition still govern aging.
Analog Devices’ mismatch analysis distinguishes capacity mismatch from impedance mismatch and explains their effects on operating voltage and usable capacity. For background on interpreting sag, see the guide to battery internal resistance.
Passive vs Active Cell Balancing: Where the Energy Goes
Passive: remove excess charge
Passive balancing typically switches a resistor-based bleed path across selected cells. Energy removed from those cells becomes heat. During charging, this can let lower-charge cells continue catching up, provided the charging strategy and protection system support it.
It is often attractive when cells are well matched, routine drift is small and sufficient balancing time is available. The limitation is not only wasted energy: heat and duty restrictions can limit the average correction current.
Passive balancing can recover headroom lost to a charge offset. It cannot recover the missing capacity of a permanently degraded cell or transfer stronger cells’ energy into a weaker cell during discharge.
Active: redistribute energy
Active cell balancing uses a controlled transfer path, which may involve capacitors, inductors, transformers or converters. Depending on topology, energy can move between cells, between a cell and a module, or through a pack-level path.
It can make more stored energy accessible where charge mismatch would otherwise cause an early cutoff. But energy transfer is not lossless, and an advertised balancing current does not by itself establish the current delivered to a recipient cell.
For example, Analog Devices specifies up to 92% charge-transfer efficiency for its LTC3300-1 controller. That is a particular controller’s documented capability, not a guaranteed efficiency for every active balancer or complete battery system.
Conceptual illustration: passive balancing dissipates selected energy; active balancing redistributes it. Not a wiring diagram.
| Decision dimension | Passive balancing | Active balancing |
|---|---|---|
| Energy handling | Selected charge is dissipated as heat | Energy is transferred, with conversion losses |
| Hardware and cost | Usually simpler and less expensive | Usually more components, controls and validation |
| Thermal concern | Bleed paths and nearby components must dissipate heat | Transfer losses and power components still need thermal design |
| Useful fit | Small routine drift with enough permitted time | Recoverable mismatch or correction speed justifies added complexity |
| Main limitation | Cannot feed energy into a low cell | Transfer paths, current and operating windows constrain benefit |
| Neither can fix | Lost capacity, damaged cells or unsuitable pack design | Lost capacity, damaged cells or unsuitable pack design |
Mechanisms: Analog Devices passive and active balancing articles. Selection guidance is conditional editorial synthesis.
Top, Bottom, Threshold and Current: Read the Strategy
Top balancing aligns charge state near the upper end of the permitted operating window. Bottom balancing aligns cells near its lower end. These describe an alignment reference, not two different energy-transfer technologies.
If capacities differ, alignment at one endpoint does not guarantee alignment at the other. Top balancing does not remove the need for low-cell discharge protection; bottom balancing does not remove upper-cell charging protection. Neither means driving a cell beyond its manufacturer’s limits.
The Orion BMS 2 operation manual describes top balancing and alternative strategies within that product’s configuration framework. Its settings should not be copied into an unrelated battery.
A balancing threshold may include a minimum cell voltage, a minimum difference between cells, and a separate stop difference. Other permissions may depend on temperature, faults or operating mode. A trigger is not necessarily the same as an overvoltage protection limit.
Balancing current determines how quickly charge can be corrected while the circuit is actually operating. Distinguish instantaneous current from time-averaged current. Scheduling, thermal pauses and measurement interruptions can make the average substantially smaller.
Texas Instruments’ BQ769x2 application note documents passive balancing during charge or rest, configuration-dependent start/stop conditions and interruptions for measurements. Those features demonstrate why “balances automatically” is incomplete without the relevant firmware and parameter documentation.
Some active architectures support transfer during discharge, but this is not a universal feature of every BMS. Ask when balancing is permitted, how it is monitored and what disables it.
LiFePO4 Voltage Can Hide—or Exaggerate—the Difference
A voltage difference is a useful signal, not a direct measurement of capacity difference. The lithium battery voltage chart guide explains why measurement conditions matter before assigning an SoC range.
LiFePO4 has a relatively flat voltage-versus-SoC region through much of its middle operating range. Similar voltages can therefore conceal meaningful charge differences. Under load, resistance and polarization can also create a voltage difference that shrinks after current stops.
Ewert Energy’s SoC drift-point guidance explains why the flat iron-phosphate region is unsuitable for precise voltage-based SoC correction. Its numerical examples are not universal voltage thresholds.
NMC and other lithium designs have different voltage curves, but that does not make every reading an exact SoC estimate. Temperature, recent charging, relaxation and measurement accuracy remain relevant. Use chemistry-specific characterization and current/temperature history; do not translate one instantaneous millivolt difference into a fixed missing-Ah figure.
Worked Example: Does the Balancer Have Enough Time?
Assume a hypothetical 100 Ah cell has a 1 Ah charge offset relative to a matching cell. This is a charge difference—not a claim that the cell has lost 1 Ah of capacity.
For simple screening:
Correction time ≈ charge to correct ÷ average correction current
Suppose a passive circuit removes 0.10 A while enabled, but operates for only 50% of the eligible period:
- Average current = 0.10 A × 0.50 = 0.05 A.
- Estimated correction time = 1 Ah ÷ 0.05 A = 20 hours.
If a different system achieved 1.0 A of effective average correction, the same charge amount would require about one hour. These are invented comparison inputs, not product ratings or recommended settings.
Only permitted balancing time counts. Two eligible hours per operating day would turn the first estimate into roughly ten days, assuming no additional drift. Real results also depend on transfer losses, changing voltage, channel scheduling and charge/discharge activity.
This is why a high-capacity battery does not automatically need active balancing. A well-matched large pack with little ongoing drift may have a small correction task. Conversely, a smaller pack with severe mismatch or a narrow balancing window can be difficult to maintain. Ewert’s pre-balancing note explicitly distinguishes initial gross imbalance from maintaining an already roughly balanced pack.
Interpret the Symptom Before Choosing the Remedy
Use recorded BMS data and the manufacturer’s service process, not improvised cell-level intervention. The following possibilities are prompts for investigation, not remote diagnoses.
| Imbalance symptom | Possible cause | Balancing relevance | Next check |
|---|---|---|---|
| One cell repeatedly ends charging early | Charge offset or reduced capacity | May correct offset; cannot restore capacity | Compare logged charge/discharge behavior and service history |
| One cell sags mainly during heavy load | Resistance, temperature or connection issue | Charge alignment may not solve sag | Qualified assessment of load, thermal and connection data |
| Spread increases after long storage | Self-discharge or unequal parasitic loads | May manage small drift, not ongoing damage | Trend the same cells across comparable idle periods |
| Balance indicator never activates | Operating permissions, thresholds or faults | Not proof that hardware has failed | Review manual, mode, temperatures and fault log |
| Same cell drifts rapidly after correction | Degradation, leakage or sensing problem | Repeated balancing may mask a persistent fault | Manufacturer-directed diagnosis; avoid forcing further cycles |
Interpretive framework informed by the mismatch sources above and Orion’s balancing troubleshooting documentation. Exact fault criteria remain product-specific.
Specify the Need, Not Just “Active” or “Passive”
For solar storage or RV/off-grid batteries, compare routine drift with the time actually spent in a permitted balancing window. For backup systems, ask how balancing behaves during long standby periods. For high-power or high-series-count packs, assess measurement integrity, thermal gradients and coordination across modules—not just balancing amperes.
A useful supplier request names the chemistry and topology, expected mismatch, transfer direction, effective average current, enable/disable conditions, heat losses, diagnostics and validation evidence. Ask whether the documented performance concerns one cell, a module or the entire pack.
Balancing is one maintenance mechanism within a larger design. Cell matching, thermal management, accurate sensing and coordinated protections remain essential; the battery management system guide explains the wider control chain. Swelling, abnormal heating or critical faults require the applicable stop-and-escalate procedure, not a stronger balancer. Never bypass the BMS, force individual cells beyond their limits or open a high-voltage pack to follow an online guide.
The right choice corrects the expected charge drift within the available time while meeting thermal, reliability and cost requirements. Before buying or changing operating strategy, request the exact pack and BMS documentation. A balancing label alone cannot establish usable capacity, service life or suitability.
Sources
- Analog Devices. Passive Battery Cell Balancing, 2016.
- Analog Devices. Active Battery Cell Balancing, 2016.
- Analog Devices. Simplicity Wins—Part 1, accessed October 2026.
- Analog Devices. LTC3300-1 product documentation, accessed October 2026.
- Texas Instruments. Cell Balancing With BQ769x2 Battery Monitors, revision A, February 2022.
- Ewert Energy Systems. Orion BMS 2 Operation Manual, revision 1.0, 2018.
- Ewert Energy Systems. Selecting SoC Drift Points, revision 1.1, 2018; Pre-Balancing Cells, 2019; Balancing troubleshooting, accessed October 2026.
- Microchip Technology. Cell Balancing Design Guidelines, AN231, 2002.