Batteries and BMS Basics: Chemistries, Pack Design and Cell Balancing
Batteries now sit at the centre of electric two-wheelers, home inverters, solar storage and telecom towers. A battery is not just a box of cells: a lithium pack needs a battery management system (BMS) to keep every cell inside its safe voltage, current and temperature window. This article covers the essential terms, compares common chemistries, works through a pack design, and explains what a BMS does and how cell balancing works.
Key Terms You Must Know
| Term | Meaning | Example |
|---|---|---|
| Nominal voltage | Average voltage during discharge | 3.2 V for a LiFePO4 cell |
| Capacity (Ah) | Charge delivered from full to empty at a stated rate | 100 Ah |
| Energy (Wh) | Nominal voltage × capacity | 12.8 V × 100 Ah = 1280 Wh |
| C-rate | Current as a multiple of capacity | 0.5C on 100 Ah = 50 A |
| Depth of discharge (DoD) | Fraction of capacity used | 80% DoD = 80 Ah used of 100 Ah |
| State of charge (SoC) | Remaining charge as % of full | 40% SoC |
| Cycle life | Number of cycles before capacity falls to a set level, often 80% | Strongly depends on DoD and temperature |
Energy (Wh) = V_nominal × Ah Current (A) = C-rate × Ah Runtime (h) ≈ Usable Wh / Load W Series: voltages add, Ah stays the same Parallel: Ah adds, voltage stays the same Notation: 4S2P = 4 in series, 2 such strings in parallel
Comparing Common Chemistries
| Chemistry | Nominal cell V | Full charge V | Typical cut-off V | Strengths | Weaknesses |
|---|---|---|---|---|---|
| Lead-acid | 2.0 | about 2.3 to 2.45 during charge | about 1.75 under load | Cheap, recyclable, tolerant | Heavy, short life at deep discharge |
| Li-ion NMC/NCA | 3.6 to 3.7 | 4.2 | about 2.5 to 3.0 | High energy density | Thermal runaway risk, needs strict BMS |
| LiFePO4 (LFP) | 3.2 | 3.65 | about 2.5 | Long cycle life, safer chemistry | Lower energy density, flat voltage makes SoC hard |
| NiMH | 1.2 | about 1.4 to 1.5 | about 1.0 | Robust, no BMS for small packs | Self-discharge, lower density |
Always take the exact limits from the cell datasheet. The table gives typical values only. LiFePO4 has become popular for solar and inverter storage because four cells in series give 12.8 V nominal, close to a 12 V lead-acid battery, and because it tolerates many more deep cycles.
Worked Example: Designing a 48 V LiFePO4 Pack
Design a pack for a 48 V e-rickshaw or solar system needing about 5 kWh, using 3.2 V, 50 Ah prismatic LFP cells.
- Series cells: 48/3.2 = 15, but 16S is standard for 48 V LFP systems, giving 16 × 3.2 = 51.2 V nominal.
- Voltage range: 16 × 3.65 = 58.4 V full, 16 × 2.5 = 40 V absolute empty; most systems use a narrower window.
- Energy of one string: 51.2 × 50 = 2560 Wh.
- Strings needed for 5 kWh: 5000/2560 = 1.95, so use 2 strings: 16S2P.
- Pack: 32 cells, 51.2 V, 100 Ah, 5.12 kWh nominal.
- Usable at 80% DoD: 4.1 kWh.
- At a continuous 0.5C limit (check the datasheet), maximum current = 50 A.
For cell-level monitoring, the two parallel cells in each group behave as one cell, so the BMS sees 16 series groups. Parallel cells should be from the same batch with matched capacity and internal resistance.
What a BMS Does
- Cell voltage monitoring: Measures every series group and disconnects charge above the over-voltage limit and discharge below the under-voltage limit.
- Current protection: Measures pack current with a shunt resistor and disconnects on over-current or short circuit, usually through back-to-back MOSFETs or a contactor.
- Temperature protection: Thermistors on cells stop charging below about 0 °C for most lithium cells and cut off at high temperature.
- Balancing: Brings cells to the same SoC.
- SoC and SoH estimation: Reports remaining charge and battery health.
- Communication: Sends data over CAN, RS485 or UART to an inverter, charger or vehicle controller.
Analog front-end ICs handle the precise measurement. For example, Texas Instruments' BQ769x0 family covers 3 to 5 cells (BQ76920), 6 to 10 cells (BQ76930) and 9 to 15 cells (BQ76940), with a microcontroller handling the protection logic and SoC algorithm.
Cell balancing
Cells differ slightly in capacity and self-discharge, so over many cycles one cell reaches full voltage before the others. The BMS must stop charging when that single cell hits its limit, leaving the pack under-charged. Passive balancing switches a resistor across the highest cells near full charge to bleed energy as heat. Active balancing moves energy from high cells to low cells using inductors, capacitors or transformers, which is more efficient for large packs.
Passive balancing example: Cell at 3.55 V, bleed resistor 33 Ω I_bleed = 3.55 / 33 = 0.108 A (about 108 mA) To remove 1 Ah of mismatch: 1 / 0.108 ≈ 9.3 hours of balancing
Estimating SoC
Coulomb counting integrates current over time and is accurate in the short term but drifts. Open-circuit voltage lookup works well for NMC but poorly for LFP, whose voltage curve is very flat in the middle. Practical BMS firmware combines both, re-calibrating at full charge.
Safety and Common Mistakes
- Mixing old and new cells, or cells of different capacity, in the same series string.
- Choosing a BMS by continuous current alone without checking its peak and short-circuit settings.
- Setting an inverter's battery type to lead-acid while using LFP, giving wrong charge voltages.
- Using thin wire for balance leads on high-current paths.
Summary
Pack voltage comes from series cells and capacity from parallel strings; energy is V × Ah. Each chemistry has specific voltage limits that the BMS enforces cell by cell, along with current and temperature limits. Balancing keeps series cells equal, and SoC is estimated by combining coulomb counting with voltage.