What is cell balancing
Cell balancing is the process of equalising voltages across individual cells in a battery pack with cells connected in series. Without balancing, manufacturing tolerances, temperature differences and aging variations cause cells to drift apart in voltage, leading to reduced pack capacity (limited by the weakest cell), accelerated degradation and potential safety issues.
The Battery Management System (BMS) implements balancing through two approaches:
Passive balancing: dissipates excess charge from higher-voltage cells through small resistors. Excess energy converted to heat. Simple and cheap but wastes energy.
Active balancing: transfers excess energy from higher cells to lower cells through switched capacitors, transformers or other energy-transfer circuits. Energy-efficient but complex and expensive.
For Indian solar storage, passive balancing dominates due to simplicity and cost-effectiveness. Premium and high-performance applications use active balancing for higher efficiency and faster equalisation, with balancing status typically reported to the inverter over CAN bus for remote monitoring.
Why cell balancing matters
Pack capacity. Imbalanced pack capacity = weakest cell capacity.
Cycle life. Balanced cells age uniformly through their rated cycle life; imbalanced cells degrade faster.
Safety. Imbalance can lead to overcharge of the weakest cell, which raises the risk of thermal runaway in extreme cases.
Warranty. Manufacturer warranties assume balanced operation.
SOC accuracy. Balanced cells support accurate SOC estimation.
Passive vs active balancing
| Aspect | Passive | Active |
|---|---|---|
| Energy efficiency | 0% (dissipated) | 70-95% transferred |
| Complexity | Simple | Complex |
| Cost | Low | Higher |
| Speed | Slow | Faster |
| Heat generation | Yes | Minimal |
| Application | Residential, mainstream | Premium, EV, utility |
Technical deep dive
Passive balancing circuit
| Component | Role |
|---|---|
| Per-cell voltage sensor | Detects cell voltage |
| Per-cell shunt resistor | Discharge path for excess |
| Per-cell MOSFET switch | Activated when balancing needed |
| BMS controller | Decides when to balance |
Typical passive balancing current: 50-200 mA per cell. Balancing happens during charging when individual cells approach upper voltage limit.
Active balancing methods
| Method | Description |
|---|---|
| Switched capacitor | Transfers charge between adjacent cells via capacitor |
| Single transformer | Centralised energy transfer through magnetic coupling |
| Multi-winding transformer | Distributed transfer to multiple cells |
| Buck-boost DC-DC | Bidirectional cell-to-cell or cell-to-pack |
| Synchronous switching | Time-shared switching between cells |
Balancing threshold
| Parameter | Typical value |
|---|---|
| Voltage spread to trigger | 30-100 mV |
| Maximum allowable spread | 200 mV |
| Balancing target | All cells within 10 mV |
| Sensing accuracy | ±5 mV typical, ±2 mV premium |
Cell aging effects on balance
Cell capacity variations, tracked as declining State of Health, grow with aging alongside calendar life effects:
- New pack: 1-2% capacity variation
- 5 years: 3-5% variation
- 10 years: 5-10% variation
- End of life: 10-15%+ variation
Balancing maintains usable capacity through aging.
Real-world applications
All lithium-ion battery packs with cells in series.
Residential and C&I solar storage, where correct hybrid inverter battery sizing assumes the pack stays balanced across its rated DOD.
EVs and electric two-wheelers.
Telecom backup batteries.
UPS systems.
Best practices
Choose packs with quality BMS balancing.
Validate balancing function in datasheet.
Monitor cell voltage spread over time.
Schedule periodic full balancing if BMS allows.
Update BMS firmware for algorithm improvements.
Active balancing benefits
Energy efficient (no heat dissipation).
Faster equalisation.
Better suited for high-power applications.
Supports older cells better.
Indian market context
Indian LFP residential packs, including Pylontech, BYD, Goodwe and Sungrow, use passive balancing standard.
Premium packs and EV packs increasingly use active balancing.
Balancing function is verified in the pack’s own BMS test report and the safety standard the exact model is tested to, such as IEC 62619; ask the supplier for it.
Major Indian BMS manufacturers (Inverted Energy, Battrixx, Numeros) support both methods.
Standards and certifications
| Standard | Scope |
|---|---|
| IEC 62619 | Battery safety includes BMS function |
| IS 16893 (Part 2) | Lithium-ion cells for electric road vehicle propulsion; not a stationary-storage standard |
| Manufacturer BMS specifications | Balancing algorithm details |
Key takeaways
Cell balancing equalises voltages across battery pack cells, critical for capacity utilisation and longevity. Passive balancing dissipates excess charge in resistors; active balancing efficiently transfers energy between cells. Passive dominates Indian residential storage; active in premium applications. Modern BMS includes balancing as standard. Cell voltage spread should be monitored to detect imbalance issues.
Further reading
For how Cell Balancing plays out in real projects, these guides go deeper:
- LiFePO4 vs NMC Battery for Solar in India
- Best Solar Battery Brands in India 2026
- Solar Battery Price Guide India 2026
- Best Hybrid Solar Inverter India 2026, Reviewed
Sources
- IEC 62619, Industrial lithium safety.
- IS 16893 (Part 2), BIS (secondary lithium-ion cells for electric road vehicle propulsion; not a stationary-storage standard).
- Plett G. L., Battery Management Systems, Artech House.
- IEEE Power Electronics Society publications on BMS.
- NREL BMS Studies.
- Manufacturer BMS documentation.
- BloombergNEF Battery Storage Outlook.
- IEA Energy Storage Outlook 2024.
- IRENA Battery Storage Publications.
Frequently Asked Questions
What is cell balancing in simple words?
Why is balancing needed?
Passive vs active balancing?
Which is used in residential storage?
Is balancing necessary?
How does passive balancing work?
How does active balancing work?
Is balancing in BMS?
Does balancing waste energy?
How often does balancing happen?
Are cells naturally imbalanced?
Can balancing recover capacity?
What is OCV-based balancing?
Is balancing standard in LFP packs?
What is the typical balancing current?
From the Blog
Co-founder & CEO driving Qbits Energy's product strategy and growth across India.