18650 Battery Pack Calculator Guide – Design, Formulas & Practical Examples
Calculate voltage (V), capacity (Ah), energy (Wh), current (A), and power (W) for custom 18650 battery packs using clear series/parallel (S/P) logic.
18650 Cell Fundamentals
- Nominal Voltage: 3.6–3.7 V per cell
- Full Charge Voltage: 4.2 V per cell
- Cut‑off Voltage: 2.5–3.0 V (device/BMS dependent)
- Typical Capacity: 2.5–3.5 Ah per cell
- Discharge Rate: cell‑specific (check datasheet)
Match cells by voltage, capacity, and internal resistance for best pack balance and longevity.
Series (S) vs Parallel (P)
- Series (S): increases voltage; capacity stays the same.
- Parallel (P): increases capacity; voltage stays the same.
- Notation: xS yP → x cells in series, y in parallel.
Example: 10S4P means 10 cells in series and 4 parallel groups → 40 cells total.
Core Formulas for Fast Calculation
Worked Example: 36 V 10 Ah 18650 Pack
- Target voltage: 36 V → with 3.6 V cells ⇒ Ns ≈ 10 → 10S.
- Target capacity: 10 Ah using 3.0 Ah cells ⇒ 10 ÷ 3.0 = 3.33 → round up ⇒ 4P.
- Total cells: 10 × 4 = 40 cells.
- Energy: 36 V × 10 Ah = 360 Wh.
Always size for continuous current and thermal limits, not just nominal figures.
Mermaid Diagram – Series/Parallel Topology
Rendered with Mermaid. If the diagram does not appear in your CMS, ensure Mermaid is allowed or export as an SVG beforehand.
graph TD
subgraph Parallel_Group_1 [Parallel Group 1]
A1[Cell]-->S1[Series String]
A2[Cell]-->S1
A3[Cell]-->S1
A4[Cell]-->S1
end
subgraph Parallel_Group_2 [Parallel Group 2]
B1[Cell]-->S2[Series String]
B2[Cell]-->S2
B3[Cell]-->S2
B4[Cell]-->S2
end
S1-->|+| POUT[(Pack +)]
S2-->|–| NOUT[(Pack –)]
POUT --- NOUT
Application‑Driven Configurations
| Application |
Nominal Voltage |
Typical Config |
Example Cell Count |
| Power Banks |
3.7 V |
1S1P–1S4P |
1–4 cells |
| E‑Bikes |
36 V / 48 V |
10S–13S × P |
30–65 cells |
| Electric Scooters |
24–60 V |
7S–16S × P |
20–80+ cells |
| Solar Storage |
12 / 24 / 48 V |
3S, 7S, 13S |
20–100+ cells |
Safety & Protection
- BMS: over/under‑voltage, over‑current, temp, and short‑circuit protection.
- Thermals: design airflow or heat‑spreading for sustained current.
- Assembly: prefer spot‑welding; avoid prolonged solder heat on cells.
Cell Matching & QA
- Match by capacity, voltage, and internal resistance.
- Age and cycle history should be uniform within a pack.
- Log measurements; reject outliers before grouping.
Edit by paco
Last Update:2025-09-06 11:21:22
