All About Battery Voltages

Power tool batteries have come a long way since the early days of cordless screwdrivers. Today’s lithium-ion batteries are more powerful, compact, and longer-lasting than their predecessors. However, with various voltages and amp-hour ratings available, choosing the right battery for your tools can be confusing.

In this article, we’ll break down the intricacies of power tool batteries, from their internal components to how voltage and amp-hours affect performance. Plus, Ask This Old House master electrician Heath Eastman brings us to the Tool Lab to break down the basics of cordless tool voltages and explain the technical aspects of battery technology in the above video.

What’s Inside a Power Tool Battery?

To understand how power tool batteries work, let’s take a look inside. A typical battery contains individual cells and a circuit board that work together to power your tools.

Battery Cells

At the core of a power tool battery are individual cells resembling AA or C batteries. These cells are arranged in specific ways to achieve the voltage and capacity you need for your tools. The better these cells are organized, the more efficiently the battery delivers power. This, in turn, impacts the tool’s performance.

Circuit Board

Modern lithium-ion batteries have an advanced circuit board that:

• Shows the battery charge level
• Manages power distribution
• Helps the battery use its cells efficiently
• Prevents overheating

This sophisticated circuitry makes today’s batteries much smarter and safer than older models that had fewer protections and didn’t distribute power as efficiently.

Understanding Battery Voltage

Battery voltage plays a large role in how well your tool performs, but what exactly is voltage, and how is it calculated?

How Voltage Is Determined

A battery’s voltage is determined by its cell count. Typically, each lithium-ion cell has a nominal voltage of 3.6 volts. For example:

• An 18V battery has five 3.6V cells (3.6V x 5 = 18V)
• A 12V battery has three 3.6V cells (3.6V x 3 = 10.8V)

Although these voltage designations are standard, they can often cause confusion among users.

The 18V vs. 20V Debate

You may notice manufacturers labeling their batteries as either 18V or 20V. In many cases, these distinctions refer to the same battery capacity. Here’s why:

When fully charged, each 3.6V cell can reach up to 4V. That means an 18V battery (which has five cells) can go up to 20V when fully charged (4V x 5 = 20V). However, under load, the battery drops back to its nominal 18V rating. So, the difference between an 18V and 20V battery often comes down to marketing rather than a noticeable difference in power.

What Are Battery Amp-Hours and Why Do They Matter?

While voltage impacts performance, amp-hours (Ah) tell you how long a battery can last under load.

What Do Amp-Hours Measure?

Amp-hours measure a battery’s ability to deliver current over time. A battery with a higher Ah rating can:

• Deliver more runtime for power-hungry tools
• Handle intense power requirements
• Sustain voltage over extended periods

Essentially, the Ah rating helps you estimate how much work the battery can handle before it runs out.

Matching Amp-Hours to Your Tools

Follow these guidelines for selecting a battery with the correct amp-hour rating for your tool:

• Low-draw tools like drills can work well on batteries with lower Ah ratings.
• High-draw tools, such as circular saws or table saws, need batteries with higher Ah ratings for the best performance.

If you use a battery with too low an amp-hour rating for a high-demand tool, you’ll see a drop in performance and may risk damaging the tool.

Choosing the Right Battery for Your Needs

Here’s what to keep in mind when shopping for suitable batteries for your tools:

Voltage Considerations

Different tasks require different voltage levels:

• 12V systems are ideal for light, compact tools.
• 18V/20V systems are versatile enough for most home projects.
• Higher voltage systems (36V, 40V, or beyond) target heavy-duty or outdoor tools.

Many tool brands design their batteries to work across multiple tools within the same voltage range. So, once you pick a brand, you can often swap batteries between tools to save money and time.

Amp-Hour Requirements

Your battery’s amp-hour rating should match your tool’s needs:

• 2–4 Ah batteries are great for light-duty or occasional use.
• 5–6 Ah batteries suit most home improvement uses.
• 8–12 Ah batteries cater to high-demand tools, best for professional-grade tools or extended sessions.

The Future of Battery-Powered Tools

Battery technology continues to evolve. As Eastman points out, even larger tools are migrating to battery power.

Emerging Trends

As technology improves, we’re seeing:

• Faster charging times
• Higher voltage options for increased power
• Improved energy density and longer runtimes
• Smart batteries offering improved monitoring

New Uses for Battery Power

Battery technology is branching out into previously uncharted territories, including:

• Electric lawnmowers
• Snowblowers
• Table saws
• Various large, stationary equipment

These advancements make cordless tools more of an option for pros and homeowners alike, even for bigger projects.