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October 28, 2021

Professional Tool Reviews for Pros


What is a Lithium-ion Battery? A Better Power Source

Milwaukee 9.0 Ah battery pack insides

It’s about 95 percent likely that you used a device with a lithium-ion battery before you got dressed this morning. Maybe it was even more than one device: a watch, a toothbrush, a laptop or tablet, and almost certainly your smartphone. Even your car might use lithium-ion batteries. Lithium-ion technology powers a mind-boggling amount of stuff around us—even a constantly increasing number of power tools. It’s so common and reliable that it’s easy to take it for granted. So, what is a lithium-ion battery exactly and why does it beat other types of rechargeable technology?


What Is a Lithium-ion Battery Made Out Of?

All batteries create a current by releasing electrons through a chemical reaction. Unsurprisingly, batteries take their names from the elements involved in that reaction. Do you remember using NiMH (Nickel-Metal Hydride) batteries? Did you have tools with NiCD (Nickel-Cadmium) packs? Lithium-ion batteries use lithium, giving us some distinct advantages in the process.

What is a Lithium-Ion Battery?

What is the Electrolyte in a Lithium-ion Battery?

For a battery cell to accomplish the required chemical reaction, a cell must be comprised of two electrodes each with its own conductive metal. We have a cathode with aluminum and an anode with copper. You also need an electrolyte medium and a separator. Now, if you can recall your first chemistry class, you’ll remember that an ion is an atom that has a net charge, either positive or negative, due to the gain or loss of at least one electron.

In a Nutshell: Lithium-ion batteries use lithium (usually with iron and phosphate as well) instead of other elements in one side (the anode). It consists of three main parts: the cathode, anode, and a separator material.

How Electricity Flows in a Lithium-ion Cell

Let’s assume that a battery is charged. In a charged state, lithium atoms (that is, with no net charge) are stored in the anode. Now let’s assume the battery becomes part of a closed—or completed—circuit. That means you inserted in your cordless impact driver, for instance, and pulled the trigger.

It immediately begins discharging. An oxidation reaction occurs in the anode between the lithium and electrolyte. Electrons jump ship from the lithium atoms to create lithium ions.

What is a Lithium-Ion Battery?

The electrolyte is a sophisticated solution. It will not allow electrons to pass through the microscopic holes in the separator. However, it will allow lithium ions to pass. 

Electrons will follow the path of least resistance through a completed circuit, powering your impact driver in the process. The lithium ions, however, move through the separator with the help of static electricity. They meet up with their long-lost electrons in the cathode and create a reduction reaction.

In a Nutshell: A chemical reaction kicks electrons off the lithium compound and they move through the circuit. This creates energy for your tool. The lithium ions move through the separator to reunite on the other side.

Charging the Battery Reverses the Process

Charging the battery reverses the process. Applying a current to the battery, an oxidation reaction occurs in the cathode. Then a reduction reaction occurs in the anode. Electrons move outside the cell to the opposite end. The electrolyte conducts the Lithium ions through the separator again. Finally, lithium atoms again come to rest in the anode as potential energy.

In a Nutshell: Electrons and lithium ions flow one way when you use your tool and the opposite way when you charge the battery.

The Strange Reversible Nature of Rechargeable Batteries

Spoiler alert—lithium-ion batteries are rechargeable! That’s one of their benefits that we’ll talk about in a moment. First, we have to make an important technical distinction. Non-rechargeable batteries are primary cells while rechargeable cells are secondary cells.

Before secondary cells, the cathode was always the positive electrode and the anode was always the negative electrode. However, when secondary cells came along, the polarity could change. When a secondary cell is discharging, its polarity is no different than a primary cell: cathode (+), anode (-). But upon charging, a secondary cell’s cathode becomes the negative electrode, and the anode becomes the positive electrode.

In a nutshell: Regular batteries have defined positive and negative sides. Rechargeable batteries switch depending on whether it is giving power or taking it in on a charge.

Why Lithium-ion Batteries Beat Other Technology

Lithium-ion batteries have won the day for several reasons. Foremost, they have a higher energy and power density pound for pound and inch for inch than their competitors. See how well a lead-acid powered cell phone fits in your pocket! And of course, heavy batteries are really counterproductive to hybrid and fully electric cars. But like anything else, you still need to have high-quality lithium oxide and other materials to have the best performance.

Milwaukee RedLithium USB Flashlight

Secondly, lithium-ion batteries are repeatedly rechargeable with minimal memory effect. That is, their maximum energy capacity doesn’t diminish over time like NiCd or NiMH batteries. They also charge faster and can hold the charge for a long time when not in use.

What’s Wrong with Lithium-ion Batteries?

Of course, it wouldn’t be real life if lithium-ion batteries didn’t have a downside. Lithium-ion batteries have several negatives. First, they cost more than other types of battery cells. Second, they can be damaged by high temperatures. Third, you can’t fully discharge them without ruining the cell or pack. Lastly, as you may have read, there’s a vanishingly small change of a lithium-ion cell catching fire.

And what causes that? Remember the separator—the magical boundary between electrodes that allows ions to pass but no electrons? If the separator is damaged, the resulting short excites the ions rather quickly and a small fire or explosion can occur. As the song says: you gotta keep ’em separated!

For this reason, most manufacturers avoid selling individual 18650 or 2170 cells since loose lithium-ion battery cells are considered dangerous.

In a Nutshell: Lithium-ion battery cells give you more energy for the size, don’t have battery memory, charge faster, and hold their charges longer. The trade-off is higher cost and a tiny risk of fire as seen in certain cell phones and hoverboards.

Lithium-ion Cell vs Battery Pack

Up to this point, we’ve been talking about lithium-ion cells. While some lithium-ion battery packs are made up of just one cell, most power tool batteries use multiple cells. Engineers wire battery packs to charge or discharge the entire group of cells at the same time to provide more power and/or runtime than a single cell can do on its own.

What is a Lithium-Ion Battery?

Building a Power Tool Battery Pack

Okay, in a nutshell, what is a lithium-ion battery? It’s like any other rechargeable battery out there. It just uses lithium rather than the nickel-cadmium or nickel-metal hydride of days past. Each chemistry is different and, for now, at least, lithium is king when it comes to storing and delivering energy.

When it comes to building a lithium-ion battery pack for a power tool, manufacturers take the individual cells we’ve been describing so far and wire them with serial connections in groups of 3 (10.8V/12V), 5 (18V/20V), 6 (21.6V/24V), or 10 (36V/40V). Larger batteries have even more. Usually, these are reserved for outdoor power equipment or much larger tools. The more cells you connect in serial, the higher the voltage goes.

Getting More Runtime or Power

When you take groups of cells, like 2 groups of 5, and use a parallel connection, you increase the amp hours. This effectively gives you the option to run tools longer or with greater amounts of power at the same voltage.

Taking those groups of cells, manufacturers then encase them. This helps dissipate heat and protect the cells from dirty and sometimes wet jobsite conditions. They sometimes add electronic chips to protect it during charging and use. Viola! You have a power tool battery pack.


In a nutshell: A power tool battery takes individual lithium-ion cells and puts them together in a pack that also helps cool the battery and monitor its state electronically.

Wrapping It Up

It all sounds like a little chemistry magic à la Walter White’s fulminated Mercury. But it’s really happening all around us constantly. It helps us stay connected, productive…and foment social unrest online—or at least argue with people we used to like a lot more when we knew them only in real life!

We hope that you’re positively charged to be here today. If you see something we missed or have any questions—be sure to leave them in the comments below.

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