How to Connect 18650 Batteries in Parallel

Connecting 18650 batteries in parallel can increase capacity and allow higher discharge rates. But there are some important safety considerations to keep in mind. In this detailed guide, as a professional 18650 battery packs manufacturer, I’ll walk you through everything you need to know about wiring 18650 batteries in parallel.

Overview: Connecting 18650s in Parallel

Connecting 18650 batteries in parallel is relatively straightforward: the positive terminal of each battery connects to the positive busbar, while the negative terminals all connect to the negative busbar. This combines the capacities and discharge rates of each battery while maintaining the same voltage.

For example, connecting two 3,400 mAh 18650 batteries in parallel effectively creates one 6,800 mAh battery still at 3.7 volts. The current capabilities also combine – two 10A batteries become one 20A battery in parallel.

While parallel 18650 connections unlock higher capacities and discharge rates, safety should be the number one priority. Uneven batteries, faulty wiring, or lack of appropriate protection circuits can all lead to catastrophic battery failures.

Later in this guide we’ll cover specific best practices around wiring, cell matching, and protection mechanisms when assembling parallel 18650 packs.

But first, let’s look at why you may want to connect 18650 cells in parallel in the first place.

Key Benefits of Wiring 18650 Batteries in Parallel

There are a few key reasons you may want to connect multiple 18650 cells together in parallel:

1. Increasing Total Capacity

Putting 18650s in parallel combines their individual capacities. Two 18650s with 3,400mAh each become one 6,800mAh battery when wired in parallel.

This capacity boost allows the battery to power devices for longer between charges. Connecting cells in parallel is common in power banks and other applications requiring extended runtime.

2. Allowing Higher Discharge Rates

18650 cells also specify a maximum continuous discharge rating, or the maximum current that can be safely pulled from the cell.

For example, the popular Samsung 30Q 18650 battery has a 15A maximum discharge rate. For high-current devices, 15 amps may not be enough.

Fortunately, wiring in parallel also combines the continuous discharge ratings. Two 15A Samsung 30Qs in parallel can deliver 30A safely.

Step-by-Step Guide: Connecting 18650 Batteries in Parallel

Connecting 18650 cells in parallel to create DIY battery packs provides flexibility but also introduces risks if not done properly.

Let’s walk through the key steps to safely assemble parallel 18650 packs:

1. Select a Suitable Battery Holder

A proper battery holder ensures secure electrical connections and provides protection for the cells. Common options include 3D printed holders or laser-cut plywood holders.

I recommend holders with an integrated voltage monitor so you can keep an eye on each parallel cell group. Slots for fuses are also highly recommended for safety purposes.

2. Match Cells in Parallel Groups

Mismatched cells in parallel introduces the risk of cell reversal and damage from uncontrolled current flows between cells.

Before wiring in parallel, create matched “cell groups” – group together cells with the same voltage, capacity, internal resistance, and age. Keep the voltage difference between cells under 50mV for optimal safety.

Use a multimeter to carefully check each cell or group before connecting.

3. Include Protection as Fail-Safes

With Li-ion batteries, protection circuits are absolutely mandatory to prevent issues like over-discharge, short circuits, or voltage spikes.

Each parallel cell group should have its own protection circuit for cutoff voltages, over-current, etc. High-quality BMS boards provide the overcharge and discharge protection required.

Fuses or fast-acting breakers on each parallel group add another layer of safety in case a cell fails or gets shorted. Size your fuses slightly above your desired maximum discharge rate.

4. Ensure Secure Connections

Given the risks around lithium battery failures, all connections must be mechanically strong and electrically conductive.

For connecting groups in parallel, thick nickel strips work well for flexibility while still allowing high discharge rates. Redundant busbars help too.

Double check your wiring before powering everything on. Loose strands or weak connections can cause hotspots which may ignite.

5. Take Safety Precautions During Use

With an assembled parallel pack, continue to monitor cell voltages and temperatures, especially when charging. If charging multiple parallel groups, use a balancer to avoid overcharging.

Avoid fully discharging cells where possible to maximize cycle life. And store packs at around a 40% charge state during periods of non-use for best longevity.

Following these best practices for cell selection, protection mechanisms, and electrically secure construction will allow building safe parallel packs to power your projects. But never neglect battery safety – take precautions during assembly, charging, storage, and operation.

FAQs Around Connecting 18650 Batteries in Parallel

Let’s review some common questions around assembling DIY 18650 parallel packs:

How Many 18650 Batteries Can Be Connected in Parallel?

There’s no strict limit, but after 4-6 parallel groups, managing so many protection circuits becomes unwieldy. Get to dozens of groups in parallel and a costly specialty BMS would be required.

Start with a 2S or 3S pack with 2-3 parallel groups. This simpler 6-12 cell layout provides flexibility for many applications while remaining reasonably easy to construct safely.

Can You Charge 18650 Batteries Wired in Parallel?

Yes, it’s completely fine to charge parallel 18650 groups. Just be sure that each parallel group has its own protection circuit to avoid cell mismatches during charging. Consider an active balancer too.

And as always, use a quality CC/CV lithium-ion charger to avoid applying unsafe charging voltages.

What Happens if 18650 Batteries of Different Voltages Are Connected in Parallel?

If mismatched cells are wired in parallel, the higher voltage cell will try to “charge up” the lower cell. With lithium-ion batteries, this can lead to excessive heat and the risk of explosion or fire since charging safety mechanisms are bypassed.

Always match cells to within 50mV before connecting in parallel as a safety precaution. Never connect batteries with substantially different voltages.

Conclusion

Wiring 18650 batteries in parallel allows combining discharge rates and battery capacities. With some basic precautions like cell matching, protection circuits, and robust connections, parallel packs can power projects safely.

But never neglect safety with high-energy lithium cells – take measures to prevent short circuits, over-discharge, over-current scenarios, and other hazardous situations.

I hope this guide gave you a solid foundation on how to properly connect 18650 cells in parallel. Let me know in the comments if you have any other questions!