Cell Balancing in EV Battery Packs — Passive Dissipation, Active Transfer, and Why It Matters at 100% SOC

In a perfect world, every cell in an EV battery pack would have exactly the same capacity, exactly the same internal resistance, exactly the same self-discharge rate, and would age at exactly the same rate. The pack would charge and discharge as if it were a single large cell. In the real world, manufacturing produces cell-to-cell variation of 1–3% in capacity and ±10–20% in internal resistance even at the same production lot. As the pack ages, temperature gradients within the pack cause cells in warmer positions to age faster. Self-discharge differs cell to cell by tens of microamps. Over time, these small initial differences accumulate into significant cell-to-cell SOC divergence.

Cell balancing is the BMS function that actively manages this divergence — measuring each cell's state of charge, identifying which cells are ahead and which are behind, and either dissipating or redistributing the excess. Understanding how balancing works, when it operates, and what limits its effectiveness explains why battery packs age the way they do and what distinguishes a well-designed BMS from a cheap one.

Why Cell Imbalance Is a Fundamental Problem

Consider a simplified pack: four LFP cells in series, each nominally 100 Ah. After 3 years of use, their actual capacities have diverged: Cell 1 = 95 Ah, Cell 2 = 92 Ah, Cell 3 = 88 Ah, Cell 4 = 82 Ah.

When charging at constant current, all four cells receive the same current (series connection). Cell 4 (82 Ah) fills up first — it reaches 3.65V (LFP full charge) before the other three. The BMS must stop charging at this point to protect Cell 4 from overvoltage. At this point:

• Cell 4 is at 100% SOC (3.65V)
• Cell 3 is at approximately 93% SOC
• Cell 2 is at approximately 89% SOC
• Cell 1 is at approximately 86% SOC

The pack has "charged" to its capacity ceiling, but Cells 1, 2, and 3 still have 7–14% of their capacity unused. The effective usable capacity of this unbalanced pack is limited not by its total capacity, but by how much the weakest cell constrains the full charge.

Passive Balancing: The Resistor Method

Passive balancing is the simplest and most widely implemented approach. Each cell (or cell group) in the series string has a parallel bypass resistor controlled by a MOSFET switch. When the BMS identifies that a particular cell's SOC is higher than the pack average (or higher than the lagging cell), it closes the MOSFET, connecting the bypass resistor across that cell. Current flows through the resistor, discharging the high-SOC cell until it matches the lower-SOC cells.

Cell (high SOC) ──┬── to next cell in series
                  │
              [MOSFET]
                  │
              [Balancing R]
                  │
              ───┴─── (cell negative)

The balancing circuit is activated in the BMS IC itself — most integrated BMS chips (Texas Instruments BQ series, Analog Devices LTC series) include passive balancing switches with programmable balancing current (via external resistor selection).

When Passive Balancing Operates

Most EV BMS implementations use passive balancing in the top-balancing mode: balancing activates only when the pack is near full charge (typically above 90–95% SOC). The logic:

• At low SOC, small voltage differences are hard to interpret accurately (the OCV-SOC curve is flat for LFP in the 20–80% range)
• At high SOC, the OCV-SOC curve is steep — small SOC differences produce measurable voltage differences
• The critical failure mode (overvoltage) occurs at the top of charge
• Balancing while charging (during the CV phase for NMC, or the tail-end constant current for LFP) allows simultaneous charge delivery and cell equalisation

Active Balancing: Charge Transfer Without Heat

Active balancing circuits do not dissipate the excess charge from high-SOC cells — they transfer it to low-SOC cells. The most conceptually elegant outcome: the energy that would have been wasted as heat in passive balancing is instead delivered to the cells that need it.

The engineering challenge: cells in a series string are at different absolute voltages. Cell 1 at the bottom of the string might be at 3.5V referenced to pack ground. Cell 96 at the top of the string is at 3.5V + 95 × 3.5V ≈ 335V above pack ground. Transferring charge from Cell 96 to Cell 1 requires electrical isolation — the circuit cannot simply connect the two cells without isolation because their ground references differ by 335V.

Flying Capacitor Topology

A capacitor is connected to Cell N (charged from it), then the capacitor connection is switched to Cell N+1 or N-1 (discharged into it). The capacitor flies between cells, redistributing charge. This only works efficiently for adjacent cells — charge transfer between distant cells requires many intermediate steps.

Advantage: Simple switching topology, low component count Disadvantage: Only balances adjacent cells efficiently; slow for large pack-level imbalances; capacitor voltage rating must accommodate full pack voltage in some implementations

Isolated DC/DC Converter Topology

Each cell has a dedicated isolated DC/DC converter (flyback or forward converter with galvanic isolation through a small transformer). The converter can either charge its cell from the pack bus (top-down) or feed back from the cell into the pack bus (bottom-up).

Advantage: Full isolation, can balance any cell to any other cell, high balancing current capability (0.5–2A) Disadvantage: High component count (one converter per cell), complex PCB routing, expensive for high cell-count packs

Why 100% SOC Is the Critical Balancing Point

The voltage behaviour of lithium-ion cells is highly non-linear. For NMC chemistry:

• Between 20% and 80% SOC: voltage changes only 0.1–0.2V — the curve is nearly flat
• Between 80% and 100% SOC: voltage rises steeply from ~3.9V to 4.2V — roughly 0.3V per 20% SOC
• Above 4.2V: electrolyte oxidation begins — accelerating SEI growth and irreversible capacity loss

This steep upper voltage region is why cell imbalance is catastrophically problematic at high SOC. Consider a 2% capacity difference between cells in an NMC pack:

• At 50% SOC: the stronger cell is at 3.68V, the weaker cell is at 3.66V — 20mV difference, negligible
• At 98% SOC: the weaker cell is at 4.18V, the stronger cell is at 4.12V — but the difference is large enough that if charging continues, the weaker cell hits 4.2V before the stronger one does, forcing charge termination and leaving the stronger cell at only ~95% actual SOC

For LFP chemistry, the OCV-SOC curve is extremely flat between 20–90% SOC (less than 50mV total change), making SOC estimation much harder mid-range but making the upper voltage limit (3.65V) sharply defined — imbalance at the top of charge is even more critical because there is little voltage headroom to detect approaching overvoltage before it occurs.

Balancing and SOC Estimation Interact

The BMS estimates each cell's SOC continuously. SOC cannot be measured directly — it is calculated from:

• Voltage at rest (OCV method) — accurate but requires zero current for a settling period
• Coulomb counting (integrating measured current over time) — accurate short-term, accumulates error over time
• Model-based fusion (Kalman filter combining voltage and current) — most accurate, computationally intensive

Balancing decisions depend on SOC accuracy. If the BMS incorrectly estimates Cell 5 as 5% higher SOC than Cell 6 (when in reality they are equal), it will activate balancing for Cell 5 unnecessarily — dissipating usable energy. This is a real-world problem in cheap BMS implementations that rely solely on voltage at rest: if the pack is never fully at rest (vehicle always in use), OCV measurements are contaminated by current-induced voltage offsets.

Practical Implications for Indian EV Owners

Understanding cell balancing translates into a few concrete ownership practices:

Charge to 100% regularly (but not always): Full charge to 100% is when top-balancing occurs. If you only ever charge to 80% (to reduce degradation), the BMS rarely gets to balance cells. Charging to 100% once a week or before a long trip allows the BMS to complete its balancing session and maintain cell equalisation.

Allow post-charge sitting time: Balancing occurs at the end of the charge cycle (CV phase for NMC, top of CC for LFP). If you unplug immediately at 100% and drive away, some balancing sessions may be incomplete. Leaving the car plugged in for 30–60 minutes after reaching 100% allows balancing to complete.

Monitor range loss progression: A pack losing range linearly (consistent 1–2% per year) is aging normally. A pack showing sudden range loss (5%+ in one year, or irregular range behaviour day-to-day) may have a cell with significant imbalance developing. In India, warranty service for range loss is typically triggered when range drops below 70% of rated — but catching the trend early allows OEM service to investigate before it becomes severe.