Design your custom 18650 battery pack. Configure S (series) and P (parallel) cells, visualize the layout, and instantly calculate voltage, capacity, energy, weight, internal resistance and BMS requirements.
🔋 What is S×P Configuration?
S (Series) = cells connected end-to-end to increase voltage. P (Parallel) = cells connected side-by-side to increase capacity (Ah). A 7S3P pack has 7 series groups each with 3 parallel cells — 21 cells total giving 25.2V nominal and 3× cell capacity.
⚠ Always Use a BMS
Never use a lithium pack without a Battery Management System (BMS). It balances cells, prevents overcharge above 4.2V, prevents over-discharge below 2.5V, and protects against short circuits and overcurrent.
18650 Battery Pack Designer — Complete DIY Guide
Building your own 18650 battery pack is one of the most rewarding DIY projects in the electronics world. Whether you are building an e-bike battery, a portable power station, a solar storage system or replacing a laptop battery pack, understanding series and parallel configuration is the foundation of every successful build.
This 18650 Pack Designer helps you calculate everything before you buy a single cell — total voltage, capacity, energy density, weight, internal resistance and what BMS you need. The visual layout generator shows you exactly how to arrange cells and connect nickel strips.
Understanding Series (S) and Parallel (P) Connections
When you connect cells in series, you add their voltages together while capacity stays the same. Two 3.7V cells in series = 7.4V. When you connect cells in parallel, voltages stay the same but capacity adds together. Two 3000mAh cells in parallel = 6000mAh. Real battery packs use both — a 7S3P pack means 7 series groups of 3 parallel cells, giving you the voltage of 7 cells and the capacity of 3 cells.
How to Choose the Right 18650 Cell
For maximum runtime: Samsung 50E (5000mAh) or Panasonic NCR18650G (3600mAh)
For high current applications: Molicel P28A (2800mAh, 35A), Samsung 25R (2500mAh, 20A)
For balanced performance: Samsung 30Q (3000mAh, 15A), LG MJ1 (3500mAh, 10A)
Always buy cells from reputable suppliers — fake cells are a serious fire hazard
Match all cells in a pack — same model, same age, similar internal resistance
Building Your Pack — Step by Step
Design your configuration using this tool and confirm voltage and capacity meet your needs
Test all cells with a capacity tester — reject any cell below 90% rated capacity
Arrange cells in the layout shown by the visualizer, alternating polarity in each row
Spot weld pure nickel strips across parallel groups — never solder directly to cells
Connect series groups with appropriately rated nickel or copper bus bars
Install a BMS matched to your S count and maximum current
Add a temperature sensor (NTC thermistor) for thermal protection
Test fully assembled pack with a bench power supply before first charge
❓ Frequently Asked Questions
What does S and P mean in battery packs?+
S = Series connections (increases voltage), P = Parallel connections (increases capacity/Ah). A 3S4P pack has 3 groups of 4 parallel cells, giving 3× cell voltage and 4× cell capacity, with 12 total cells.
How many cells for a 36V e-bike battery?+
36V nominal requires 10S (10 × 3.6V = 36V). Fully charged is 10 × 4.2V = 42V. Choose your P count based on desired range — 10S4P = 40 cells, giving 4× single cell capacity.
How many cells for a 48V battery pack?+
48V nominal needs 13S (13 × 3.7V = 48.1V). Fully charged = 13 × 4.2V = 54.6V. For an e-bike, 13S4P (52 cells) or 13S5P (65 cells) are popular configurations.
What BMS do I need for my pack?+
Your BMS must match your S count exactly (7S, 10S, 13S etc.) and handle your maximum continuous current. For a 20A pack, use a 25A or 30A BMS for headroom. This designer recommends specific BMS specs based on your configuration.
How do I connect cells in a 18650 pack?+
Use a spot welder with pure nickel strips (0.1mm to 0.2mm thick). First connect parallel groups across the flat bottom of cells, then connect series groups between adjacent groups. Never solder directly to cell terminals as heat damages cells.
What is internal resistance in a pack?+
Internal resistance (mΩ) determines voltage sag under load and heat generated during discharge. Lower is better. Series cells add resistance; parallel cells reduce it. This designer calculates pack internal resistance from individual cell specs.
What nickel strip thickness should I use?+
For most 18650 packs: 0.15mm × 8mm nickel strips handle 5-7A per strip. For high current, double-layer or use 0.2mm strips. As a rule, use 2 welds per strip per cell connection, and calculate strips needed based on your parallel group current.
Can I mix different 18650 cell types?+
Never mix different cell types or capacities in a pack. Always use identical cells from the same batch. Mismatched cells create imbalanced groups that stress cells and can cause premature failure or thermal runaway.
How long does an 18650 pack last?+
Quality 18650 cells last 300-500 cycles for Li-ion and 500-1000 cycles for LiFePO4 cylindrical cells. Keeping charge below 4.1V (instead of 4.2V) and discharge above 3.0V significantly extends cycle life.
Is this 18650 pack designer free?+
Yes, 100% free forever. No signup, no watermark, no account needed. ToolsCoops tools are always free for everyone worldwide.
