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.

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.

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).

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Battery Voltages | Tool Lab | Ask This Old House

[swolf]

I'm a physics professor, and Amp-hours are definitely a unit of charge. (So, basically, it is a rating of how many electrons the battery can deliver.) Volts are (potential) energy per unit charge. I wish they rated the battery with Watts (units of power)--at least that would make more sense to me. My students constantly mix up the ideas of electric charge, field, energy, voltage, etc. And I sometimes feel that the specs we use on tools like this muddies the waters.

Either way, if we can deliver 12 Amp-hours of charge at 18V, that means we have 216 Watt-hours of electrical energy (just under 800kJ).

[patmcgroin]

It's not so much the Ah rating of the battery but the fact that there are parallel cells in the battery that will provide a higher inrush current for jobs that require more power.

[franksteffero]

Thx for the history, the look inside the battery, and the demo of what happens if you use the wrong sized battery.

[jybuis]

Argh, folks keep confusing Amps with Amp-hours. Amp is the unit of measurement for current, or effectively how much power is flowing since battery voltage is a given. Amp-hours are a measurement of *capacity*, i.e. how much energy the battery can store. AH rating and current capacity rating are different and measure two unrelated things.

[MyGloriousMess]

What happened to the 12volt dissection and metered explanation?

[Engineer]

I see a pattern on Youtube; Electric topics are the ones which the comments sections go full blown smartass over. Everyone knows stuff more intense than the comment before that.

[thezfunk]

20V is all marketing. Any tool that claims a '20V' tool is just kinda lying to you. It is 18V. I have also found that the 12V lines most tool companies have meet my home use 95% of the time.

[hobbes]

I was in a hurry once and shoved my 12V Ryobi battery into an 18V charger so I could go out for last call and it electrocuted my balls off LOL

[tehSharpn]

Just like an electrician. Pick a random spot and drill a hole in a perfectly finished piece of hardwood

[russellspear]

With the ability to link a device to the maker of the battery you should be concerned with the compatibility of aftermarket power pack.

[TomCee]

I love how even This Old House uses journalists or actors, or maybe even just DIYers to explain technology. Please get at least an electrician, if not a teacher to explain technology.

[kyletrump]

Weird. I can use my little batter in my dewat hole hog.

[dennisvivoda]

i can see Milwaukee M18 High Output 12ah and Dewalt Flexvolt 12 ah and 15 ah. Why do you hide the brand ?

[deefdragon]

a few minor mistakes, but none that make the video useless. If someone knows the difference in the terms used, then this video is already useless for them, while someone who doesent know the terms is now aware of the importance if sizing the right battery.

[ncooty]

So many words, so little information... and not very well explained. It's not that it's wrong; it's just inefficient, a bit misleading, and much less useful than it could be.

[jeffwilson]

This Old Environmental Disaster of the Future

[berbababy]

This is terrible. Power is voltage, not amperes. You're getting your terminologies completely jumbled. Poor video, nothing is accurate