The 18650 battery is one of the most commonly used lithium-ion battery formats. But determining which side is positive and which is negative can be confusing. As a professional 18650 battery pack manufacturer, I wrote this comprehensive guide to clarify exactly how to identify the positive terminal on a 18650 cell.
Key Takeaways
- Button top 18650 cells have a raised positive terminal, while flat top cells require other checks.
- On protected batteries, the side with the insulating ring is typically negative.
- Printed symbols and voltmeter readings provide clear confirmation of polarity.
- Always double‑check polarity to avoid device damage or dangerous battery failures.
Introduction
Have you ever struggled to figure out the positive and negative terminals when trying to use or charge a 18650 battery? If so, you’re not alone. The similar flat ends can make it tricky to quickly tell which side is which.
However, identifying the correct polarity is critically important. Inserting batteries incorrectly can damage devices or even cause dangerous failures and explosions in some cases.
So let’s walk through several reliable ways to distinguish the positive side from the negative on any 18650 lithium-ion cell.
What is the Positive Side of a 18650 Battery?
Button Top vs Flat Top Design
The most obvious indicator is the physical design of the positive terminal itself. There are two main types:
Button Top
This format has a slightly raised, button-like shape on the positive end. The protrusion makes contact easier in devices and helps differentiate it from the negative terminal.
Flat Top
As the name suggests, flat top cells have a completely flush terminal surface on both ends. There is no physical indicator for polarity.
So if your 18650 battery has a button top, that raised side is the positive. Just note that not all button tops have an insulating ring for protection. Check for the shrink-wrapped section below.
For flat top cells, you’ll need to use the other methods listed next.
Check for a Protection Circuit
Lithium-ion batteries can be volatile if mistreated. Some 18650 cells include an extra protection circuit to prevent issues like short circuits, overcharging, high heat, etc. This circuit cuts power if thresholds are exceeded.
On most protected cells, the circuit installs between the negative terminal and shrink wrap. Look closely and you should see a thin layer wrapped around the sides of the battery.
If your cell has this insulated ring, then the side WITHOUT it is positive. Just be aware that not all 18650s have protection circuits.
Examine the Labels
Many 18650 batteries feature small logos, text, or diagrams printed around the shell. In most cases, these symbols indicate the positive terminal.
For instance, a battery might show the “+” symbol next to one end. That visually marks the positive side. Other versions display a tiny arrow or diagrams of flowing current.
So inspect all sides for indicators printed on the cell. It takes just a moment but can quickly reveal the correct polarity.
Use a Voltmeter
When physical, visual cues aren’t clear, a voltmeter gives a definitive way to identify positive and negative.
Simply touch the red lead to one flat end of the battery and the black lead to the opposite side. A positive voltage reading indicates the red lead is contacting the positive terminal.
If you see a negative voltage, then flip the leads. The side that shows positive is the polarity you want.
While this method requires an additional tool, a basic voltmeter provides cheap insurance against incorrectly inserting lithium-ion batteries.
Find the Coded ID Digits
This technique won’t help if batteries are already unwrapped. But new, untouched 18650 cells contain an obscured, multi-digit code printed underneath the outer wrapper.
Manufacturers position this ID to align with the negative terminal. So if the code reads right-side-up when you look at one end, the opposite end is positive.
Clearly seeing the digit sequence requires removing covering labels. But with pristine cells, it’s another way to double check polarity before use.
In Summary
As you can see, 18650 lithium-ion batteries have several visual cues and handling methods for safely determining positive from negative terminals.
In review, watch for:
- Raised button tops signify positive.
- Insulating protection rings reveal negative.
- Printed symbols often note positive.
- Voltmeter readings with polarity.
- ID codes facing negative terminals.
Getting in the habit of double checking with two methods is wise. Accidentally inserting batteries backward can ruin devices or even cause dangerous failures.
But armed with these tips for accurately judging positive vs negative, you can wield 18650 lithium-ions batteries with confidence!
