We often hear or see the term “18650” when talking about lithium batteries — but what does that number actually mean? Let’s break it down. The name 18650 is the battery’s model number. The “18” means it’s 18mm in diameter, the “65” means it’s 65mm long, and the “0” indicates that it’s a cylindrical cell.

Common Applications and Voltage Differences

18650 batteries are commonly used in power tools, lawn mowers, vacuum cleaners, electric bikes, and many other products. In the power tool industry, you’ll often see lithium batteries labeled as 18V, 20V, or even 21V. So why the difference? It’s actually because the rated voltage of a single lithium cell can be listed as 3.6V or 4.2V, depending on how the manufacturer chooses to label it.

Here’s what that means:

• 3.6V refers to the nominal voltage, which is the average voltage during normal use.

• 4.2V is the fully charged voltage when the battery is at 100%.

Most rechargeable 18650 lithium cells are rated at 3.6V or 3.7V, and they reach 4.2V when fully charged. Both ratings are correct—it just depends on the manufacturer’s labeling choice. The voltage number doesn’t directly reflect the battery’s capacity.

Series vs. Parallel: Building a Battery Pack

If you’re still unsure how voltage and capacity work, here’s an easy rule of thumb:

Let’s look at an example. Imagine a battery pack made of 10 cells. Each cell is 4V and 2000mAh. The cells are arranged with 5 in series (side by side horizontally) and 2 in parallel (stacked vertically).

• Series connection (5 cells): 5 × 4V = 20V

• Parallel connection (2 cells): 2 × 2000mAh = 4000mAh = 4.0Ah

So, this battery pack would be rated at 20V and 4.0Ah.

Simple, right? Just remember — series for higher voltage, parallel for more capacity.