The Lithium 18650 / 21700 S/P Battery Pack Balancer & Busbar Sizer designs custom cylindrical cell battery modules for electric bikes, solar storage walls, and mobile robotics.

### Chemistry Parameters & S/P Calculations Connecting lithium cells in series (S) increases total system voltage, while parallel (P) connections multiply ampere-hour capacity and continuous discharge current (CDR): - **Chemistry Standards**: - **Li-ion NMC ($18650/21700$)**: Nominal $3.6\text{ V}$, Peak $4.2\text{ V}$, Cutoff $3.0\text{ V}$. - **LiFePO4 (LFP)**: Nominal $3.2\text{ V}$, Peak $3.65\text{ V}$, Cutoff $2.5\text{ V}$. - **Lithium Titanate (LTO)**: Nominal $2.3\text{ V}$, Peak $2.8\text{ V}$, Cutoff $1.8\text{ V}$. - **Pack Voltage Math**: $$\text{Nominal Voltage} = S \times V_{\text{nom}}, \quad \text{Max Cutoff} = S \times V_{\text{max}}, \quad \text{Min Cutoff} = S \times V_{\text{min}}$$ - **Pack Capacity & Energy**: $$\text{Pack Capacity (Ah)} = P \times \text{Cell Ah}, \quad \text{Pack Energy (Wh)} = \text{Nominal Voltage} \times \text{Pack Ah}$$ - **Discharge Current & Busbar Sizing**: $$\text{Continuous Current (A)} = P \times \text{Cell CDR (A)}, \quad \text{Max Power (W)} = \text{Nominal Voltage} \times \text{Continuous Current}$$ - **Pure Nickel Strips ($0.15\text{ mm} \times 8\text{ mm}$)**: Safely handles approximately $5\text{ A}$ continuous without excessive heating. The tool calculates required parallel strip bridges: $\lceil I_{\text{max}} / 5 \rceil$. - **Copper Busbar Cross-Section**: Calculated at $6\text{ A/mm}^2$: $I_{\text{max}} / 6\text{ mm}^2$.

### Worked Battery Build Example Consider a standard **$13S5P$** e-bike battery built with $3.5\text{ Ah}$ ($3500\text{ mAh}$) NMC cells rated at $10\text{ A}$ continuous discharge: - **Total Cell Count**: $13 \times 5 = 65$ cylindrical cells. - **Voltage Limits**: - Nominal: $13 \times 3.6 = 46.8\text{ V}$ ($48\text{V}$ class) - Full Charge: $13 \times 4.2 = 54.6\text{ V}$ - Low Voltage Cutoff: $13 \times 3.0 = 39.0\text{ V}$ - **Pack Capacity**: $5 \times 3.5 = 17.5\text{ Ah}$ - **Total Energy**: $46.8 \times 17.5 = 819\text{ Wh}$ ($0.819\text{ kWh}$) - **Max Continuous Current**: $5 \times 10 = 50\text{ A}$ ($2340\text{ W}$ continuous output) - **Busbar Requirements**: $\lceil 50 / 5 \rceil = 10$ pure nickel strip layers per bridge (or an $8.33\text{ mm}^2$ solid copper busbar).

### In-Browser Engineering Tool All battery pack metrics and busbar thermal estimations evaluate locally in your web browser. No proprietary pack schematics, cell sourcing records, or DIY specs leave your computer.