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Battery Voltages | Tool Lab | Ask This Old House

In this Tool Lab video, This Old House master electrician Heath Eastman explains everything we’ve wanted to know about today’s power tool batteries.

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Master electrician Heath Eastman brings us to the Tool Lab to explain everything we’ve ever wondered about power tool batteries. With a brief history of the evolution of power tool batteries, Heath shows us what they look like inside and explains how manufacturers calculate battery voltage.

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Modern batteries have come a long way
Today’s power tool batteries evolved quite a bit since the first cordless screwdrivers hit the market. The original tools were small and featured nickel-cadmium and nickel-metal hydride batteries. These batteries served the purpose, but today’s lithium-ion batteries are more powerful, compact, and longer-lasting.

Under the Hood
The inside of each battery contains a group of cells, which look similar to unbranded AA or C batteries. In nickel-cadmium models, manufacturers would arrange the cells and then wire them directly to a positive and negative terminal. In today’s lithium-ion models, the cells are still wired together, however, they’re more compact and attached to a circuit board. This circuit board controls the use of the cells for better efficiency, to prevent overheating, and even display how charged the battery might be.

Quantity Determines Voltage
Every manufacturer has its own battery-powered tool line, and within that line are different voltages: 12V, 18V, 20V, and so on. But what does that mean?

Amp Hours Matter, Too
Voltage is only half the story: amp-hours matter too. Amp-hours are essentially the amount of current that is available in the battery for the manufacturer. These batteries have more cells arranged in such a way that doesn’t increase voltage but allows the battery to sustain that voltage for a longer period or during heavier draw.

Match the Battery’s Capacity to the Tool
It’s important to match the amp-hour rating of the battery to the tool. For example, a drill can use a 4-amp battery and last a very long time. That 4-amp battery won’t last as long in heavy-draw tools like table saws, right-angle drills, miter saws, and circular saws. In fact, the battery might not even provide enough current for the tool to run. For those tools, batteries with higher amp-hour ratings are necessary. For these tools, batteries with 12 amp-hour ratings are more suitable.

The Future is Bright for Battery Tools
Batteries are already lightyears ahead of where they were, but things are still evolving. While 18 and 20-volt batteries are common on job sites, some larger tools are also making the battery switch. Electric lawn mowers and snowblowers often use even larger, higher voltage batteries to power them—a long way from the cordless screwdrivers of old.

Where to find it?
Heath breaks down the basic voltages of cordless tools and explains the technical aspects of battery technology. He takes a battery apart to show the individual cells and circuit board that make up the battery packs. Then Heath explains the difference between 10.8V and 12V as well as 18V and 20V ratings. FInally, Heath talks about Amp Hour ratings, how they relate to voltage ratings—before giving homeowners some guidance on what level voltage tool they will most likely need (and not need).

About Ask This Old House TV: From the makers of This Old House, America’s first and most trusted home improvement show, Ask This Old House answers the steady stream of home improvement questions asked by viewers across the United States. Covering topics from landscaping to electrical to HVAC and plumbing to painting and more. Ask This Old House features the experts from This Old House, including general contractor Tom Silva, plumbing and heating expert Richard Trethewey, landscape contractor Jenn Nawada, master carpenter Norm Abram, and host Kevin O’Connor. ASK This Old House helps you protect and preserve your greatest investment—your home.

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Battery Voltages | Tool Lab | Ask This Old House

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