How Cordless Power Tool Battery Packs Work: Voltage, Cells, and Capacity

Every cordless power tool runs on a battery pack assembled from small lithium-ion cells, and the two numbers printed on the pack label, voltage and amp-hours, describe how those cells are wired. Voltage sets how much power the tool can draw, while amp-hours set how long it can run. Both figures come from the same cells arranged in different patterns, which means a pack with a higher voltage number is not automatically a better pack. This article walks through the cell-level basics, explains how series and parallel wiring change voltage and capacity, and shows how to compare packs before you spend money. It also covers the care side of the equation: modern lithium-ion packs do not suffer from the memory effect that plagued older nickel chemistries, and the truth about cordless power tool battery care is worth reading before you set up a charging routine.

How a Lithium-Ion Cell Works

Almost every modern cordless tool battery is built from cylindrical lithium-ion cells. The most common size in 18V-class packs is the 18650 cell, named for its dimensions: 18 mm in diameter and 65 mm in length. Each cell stores a small amount of energy and delivers a fixed voltage, and packs get their useful ratings by combining many cells together.

A single lithium-ion cell produces about 3.6 volts of nominal voltage. Some manufacturers quote 3.7 V, and a freshly charged cell can read 4.2 V with no load attached, but 3.6 V is the standard working figure used in engineering calculations. When you see a tool advertised as 40V Max, the system is really running at 36 V nominal, in exactly the same way that 20V Max tools operate at 18 V. The practical effects of running tools at higher voltages are spelled out in our breakdown of how the new cordless platform works for contractors, where the differences in power, weight, and runtime are matched to real tool classes.

Cell form factors matter more than you might think

Beyond 18650 cells, newer packs increasingly use 21700 cells, 21 mm by 70 mm, which hold more energy per cell and handle higher discharge currents. A pack built from 21700 cells can reach the same capacity with fewer cells, which changes the pack shape and weight. Cell quality also varies by grade. Premium cells hold their capacity longer over hundreds of charge cycles, which is one reason two packs with the same printed rating can age very differently.

Nominal voltage versus open-circuit voltage

The label on a battery pack usually shows the maximum voltage, which is measured right off the charger before any load is applied. The nominal voltage is a more representative measure of the operating voltage, and it is the number you should use when comparing systems. Under load, pack voltage sags further, which is why a 36 V pack might read closer to 30 V while it is driving a demanding tool.

Series Wiring: How Voltage Adds Up

When cells are connected in series, their voltages add together while capacity stays the same. Five 3.6 V cells in series produce an 18 V nominal pack, and ten cells in series produce a 36 V pack. The charge capacity of the pack equals the capacity of a single cell, because the same current flows through every cell in the chain.

The Max part of any marketing voltage refers to the maximum reading off the charger, before the battery does any work. A 40V Max pack is therefore a 36 V system, and the number is not a lie so much as a measurement taken at the most favorable moment. Nominal voltage is the honest comparison point between systems.

Higher series counts matter most for tools that draw heavy current for sustained periods. Angle grinders are a good example: a 40V Max XGT angle grinder can hold cutting speed through a full disc of work where an 18V model starts to bog down. The same motor draws less current at higher voltage for equal power, which means less heat in the tool and better runtime from the same stored energy.

Parallel Wiring: How Capacity Adds Up

When cells are connected in parallel, capacity adds while voltage stays the same. Two 2.5 Ah cells in parallel act like a single cell with 5.0 Ah of capacity, still at 3.6 V. Parallel wiring gives a pack more runtime and lets it supply more current, because the load is shared across multiple cell paths.

Voltage labels span a much wider range than the packs most people own, from small 12V Max drivers to 108V systems used in industrial equipment. Understanding what 12V Max and 108V battery ratings mean helps put the mid-range packs in context, because the same series and parallel logic applies at every scale.

Reading the amp-hour rating

An amp-hour rating describes how much current a pack can deliver for one hour. A 2.5 Ah pack can theoretically supply 2.5 amps for one hour, or 5 amps for 30 minutes, before the cells are empty. Real runtime is shorter, because tools draw bursts of current far above the continuous rating and because packs shut off before cells are fully drained to protect them.

Series-Parallel Combinations: One Pack, Many Configurations

The same ten cells can be arranged in very different ways. Ten cells in series, written 10S, produce a 36 V pack with the capacity of one cell. Split the same ten cells into two parallel groups of five in series, written 5S2P, and you get an 18 V pack with double the capacity. The cell count stays identical; only the wiring changes.

Manufacturers choose the configuration to fit the tool line. High-voltage packs power the demanding tools, and high-capacity packs power the workhorses. Battery systems evolve over time, and the story of voltage transitions, compatibility, and battery management explains why new platforms appear every few years and how the packs change with them.

Balancing and the battery management system

Every multi-cell pack needs a battery management system to keep the cells balanced. Cells in a series string must stay at similar charge levels; if one cell lags, it can be overcharged or over-discharged while the others look fine. The management system monitors each cell group, balances charge during charging, and cuts power when any cell leaves its safe range.

What Capacity Numbers Mean for Runtime

The most honest way to compare packs is watt-hours, which is voltage times capacity. A 36 V, 2.5 Ah pack stores 90 Wh, exactly the same energy as an 18 V, 5.0 Ah pack. The higher-voltage pack delivers that energy at a higher rate, so it can power a more demanding tool, while the lower-voltage pack lasts longer in a lighter tool.

• A 90 Wh pack will run a tool drawing 900 W for about 6 minutes at full load, because 90 Wh divided by 900 W is 0.1 hours.
• Expect 20 to 30 percent less runtime than the math suggests, since tools rarely run at a steady load and packs protect themselves from deep discharge.
• Doubling capacity does not always double runtime; a heavier tool may simply draw more current when the pack can supply it.

Choosing the Right Battery Pack for the Job

Start with the tool. Match the pack voltage to the tool platform, then choose capacity based on how the tool is used. A drill used in short bursts works fine with a compact 2.0 Ah pack, while a circular saw or reciprocating saw earns its keep with 4.0 Ah or more.

• Check the platform first: the pack must be designed for the tool voltage class and connection.
• Buy the largest capacity you can comfortably swing; weight grows with capacity, and a heavy pack changes the balance of a drill or grinder.
• Keep at least two packs per heavy-use tool so one can charge while the other works.
• Avoid storing packs fully discharged or in extreme heat; the longer view of capacity upgrades and battery management systems covers how to keep packs healthy for years.

Battery technology keeps moving, and the direction of travel is more capacity in the same footprint and more voltage where tools need it. For compact tools, the low end of the spectrum still matters, and what 12V Max systems deliver shows how far even the small packs have come. Whatever you buy, the cell math in this article applies to every pack on the shelf, so you can read any label with confidence.