Understanding Series‑Parallel Battery Pack Design: Voltage, Capacity, and Safety

Introduction

The session walked through the step‑by‑step design of lithium‑ion battery packs using series and parallel cell combinations. Participants counted cells, calculated pack voltages and capacities, identified common design errors, and discussed protection strategies such as Battery Management Systems (BMS) and charger selection.

1. Basic Voltage Definitions

• Cut‑off voltage (0 % SOC) – the minimum safe voltage for a cell (≈ 3.0 V for NMC, 2.5 V for LFP).
• Nominal voltage (mid‑range SOC) – the voltage used for most calculations (≈ 3.7 V for NMC, 3.2 V for LFP).
• Full‑charge voltage (100 % SOC) – the maximum safe voltage (≈ 4.2 V for NMC, 3.6 V for LFP).

These three parameters are the foundation for any pack‑design calculation.

2. Parallel Pack Example (14 P)

• 14 cells are wired in parallel.
• Each cell: 3.7 V, 2.6 Ah.
• Voltage stays at 3.7 V (parallel does not change voltage).
• Capacity adds: 14 × 2.6 Ah = 36.4 Ah.
• Resulting pack: 3.7 V / 36.4 Ah.

3. Building Larger Packs by Series Connection

• Two identical 14 P packs placed in series:
• Voltage adds: 3.7 V + 3.7 V = 7.4 V.
• Capacity remains: 36.4 Ah.
• Adding a third pack yields 11.1 V, still 36.4 Ah.
• General rule: Series → voltage adds, capacity unchanged; Parallel → capacity adds, voltage unchanged.

4. Targeting a 60 V Pack

• Desired nominal voltage ≈ 60 V → about 16 series groups (16 S) of 3.7 V cells.
• Each series group is a 14‑cell parallel block (14 P).
• Pack specification:
• Nominal: ≈ 60 V (16 × 3.7 V).
• Full‑charge: 16 × 4.2 V = 67.2 V.
• Cut‑off: 16 × 3.0 V = 48 V.
• Capacity: 36.4 Ah (unchanged by series).
• Total cells: 16 × 14 = 224 cells.

5. Design Mistake Highlighted

The instructor showed a faulty diagram where one series block had only 14 cells while the others had 15. This inconsistency leads to: - Unequal voltage distribution. - Imbalance during charge/discharge. - Potential over‑charge of the stronger block and deep‑discharge of the weaker block.

6. Balancing, Over‑charge, and Deep‑discharge

• Over‑charge (e.g., a cell already at 90 % SOC reaches 100 % while the pack is still below full‑charge voltage) causes heat, leakage, and fire risk.
• Deep‑discharge (cell drops below cut‑off) damages chemistry and may render the cell unusable.
• Proper BMS monitors each cell, shuts off charging at the pack’s full‑charge voltage, and prevents discharge below cut‑off.

7. Role of the Battery Management System (BMS)

• Measures individual cell voltages and currents.
• Balances cells during charge to keep them at the same SOC.
• Provides over‑voltage, under‑voltage, over‑current, and temperature protection.
• Essential for any series‑parallel pack, especially high‑voltage (48 V, 60 V, 72 V) configurations.

8. Charger Selection

• Charger voltage must match the full‑charge voltage of the pack.
• Example: a 60 V NMC pack (16 S) needs a charger that can output ≈ 67.2 V.
• Using a higher‑voltage charger (e.g., 70 V) will over‑charge the pack.
• For LFP chemistry (3.2 V nominal, 3.6 V full), a 60 V pack translates to ≈ 19 S, requiring a charger of ≈ 68.4 V.

9. Practical Calculation Exercises

• 13 S 2 P pack with 5 Ah cells → 48 V nominal, 10 Ah capacity, 39 V cut‑off, 54.6 V full‑charge.
• 60 V 30 Ah pack (NMC) → 16 S, 6 P → 96 cells total.
• 48 V 50 Ah pack → 13 S, 17 P → ~221 cells.

These exercises reinforce the method of deriving series (S) and parallel (P) counts from voltage and capacity specifications.

10. Summary of Key Concepts

• Identify the chemistry (NMC, LFP, lead‑acid) to know its nominal, full‑charge, and cut‑off voltages.
• Use V_nominal = S × V_cell_nominal to find the number of series groups.
• Use Ah_total = P × Ah_cell for capacity.
• Verify that every series block contains the same number of parallel cells to avoid imbalance.
• Always pair the pack with a correctly rated charger and a BMS for safety.

The session concluded with a reminder to upload a video diary containing the calculations and to prepare for the next class, which will dive deeper into BMS design and cell‑balancing algorithms.

Accurate series‑parallel design, proper voltage/capacity calculations, and a reliable BMS are essential to build safe, high‑performance battery packs; mastering these fundamentals lets you size chargers, avoid imbalance, and protect the pack from over‑charge and deep‑discharge.