Beginner’s Guide to Building an E-Bike Battery Pack (18650 & 21700 Cells)

Written by: Nelvick Berrios – DIY500AMP.COM

If you’re looking to build your own e-bike battery using 18650 or 21700 lithium-ion cells, this guide will walk you through everything step by step — from wiring to BMS to spot welding. Whether you're upgrading your ride or learning from scratch, this guide is for you.

1. What Is an E-Bike Battery Pack?

An e-bike battery pack is a group of rechargeable lithium-ion cells connected together to store energy and deliver power to your motor. Think of it as the gas tank of your electric bike. The battery determines how far and how fast you can go, and how much power your motor can draw.

2. Understanding Series and Parallel Connections

There are two ways cells are connected: in series and in parallel.

Series (S) increases voltage. Connecting cells end-to-end adds their voltage. For example, seven 3.7V cells in series (7S) = 25.9V.
Parallel (P) increases amp-hour and current. Four 3Ah cells in parallel (4P) = 12Ah.

A 13S4P pack means 13 cells in series and 4 in parallel — 52 cells total.

3. Voltage and Capacity: What Do They Mean?

Voltage (V): Controls speed and power.
Amp-hour (Ah): How much energy you can store. More Ah = more range.
Watt-hour (Wh): V x Ah. A 52V 12Ah pack = 624Wh (watts for one hour).

4. Choosing the Right Cell

Not all lithium cells are the same. Focus on:

Capacity (mAh): Higher = more range.
Continuous Discharge Rating (CDR): How many amps it can safely deliver.
Cycle Life: More cycles = longer lifespan.

Examples:

Samsung 30Q – 3000mAh, 15A
Molicel P42A – 4200mAh, 30A
Samsung 50S – 5000mAh, 25A

Only buy authentic, name-brand cells. Check our 21700 cell catalog here.

5. The BMS: Your Battery’s Guardian

The Battery Management System (BMS) does the following:

Prevents overcharge and over-discharge
Balances voltage across series groups
Monitors cell temps and cutoffs

Choose a BMS that matches your series count and current needs.

6. Charging and Safety Limits

Basic rules:

Never charge above 4.2V per cell
Never discharge below 2.5V per cell
For longer life, charge to 4.1V and discharge no lower than 3.3V

Use smart chargers with CC/CV charging profiles.

7. Pack Layouts and Assembly Tips

Arrange your pack based on your bike’s frame space:

Brick layout: Straight rows. Easier to build.
Honeycomb layout: More compact but harder to build.

Use plastic holders for spacing. Always keep cell orientation consistent.

8. Spot Welding vs Soldering

Spot welding is the safest method. Never solder directly to cells unless you have proper tools and experience. Use:

0.15mm or 0.2mm pure nickel strip
Pulse-controlled spot welder

9. Common Pack Voltages and Uses

Voltage	Series	Common Use
24V	7S	Kids bikes, light scooters
36V	10S	Standard commuter builds
48V	13S	Most popular for power/range
52V	14S	Higher performance setups
72V	20S	Extreme power builds

10. Final Tips Before You Build

Match voltage before connecting in parallel
Never mix different brands or capacities
Test voltage and internal resistance before building
Use gloves, safety glasses, and build in a ventilated area
Charge in a fire-safe area

Need Help or Parts?

Visit DIY500AMP.COM for premium lithium-ion cells, BMS modules, spot welders, and everything you need to build your battery safely and professionally.

Whether you're a hobbyist or launching your first e-bike project, we’re here to help.