Cordless tool buyers compare amp-hour ratings the way they compare engine sizes, but amp-hours only tell half the story. A battery’s capacity is the product of its voltage and its amp-hours, which is why an 8Ah pack on a 40V platform holds roughly a third more energy than a 12Ah pack on an 18V platform. Those numbers matter on site, where runtime, charging time, and weight decide whether a crew finishes a shift with one set of batteries or three. The old battery memory myth still confuses people about when to recharge modern lithium-ion packs.
Amp-Hours vs Watt-Hours: What Each Number Tells You
Amp-hours (Ah) measure charge, and watt-hours (Wh) measure energy. Charge tells you how long a battery can deliver a given current; energy tells you how much total work the battery can do. Because tools draw different currents at different voltages, watt-hours is the number that lets you compare packs across platforms.
The math is simple: watt-hours equals nominal voltage times amp-hours. An 8Ah pack on a 36V/40V system stores 288 to 320 Wh, depending on which nominal voltage the manufacturer quotes. An 18V/20V Max 12.0Ah pack stores 216 to 240 Wh. In energy terms, the 40V 8Ah pack is the equivalent of an 18V/20V 16.0Ah pack. The practical effects of that difference show up in how 40V and 80V platforms are built and used.
Runtime also depends on the tool, not just the pack. A compact drill draws roughly 300 W under load, while a circular saw can draw 1000 W or more. The same 288 Wh pack runs the drill for close to an hour of continuous work but only a fraction of that in a saw. Convert capacity to watt-hours first, then divide by the tool’s typical draw to estimate runtime.
The Formula: Watt-Hours = Voltage x Amp-Hours
Run the numbers before buying so capacity claims translate into runtime expectations:
- 18V x 4.0Ah = 72 Wh, a typical compact pack.
- 18V x 12.0Ah = 216 Wh, a large pack for heavy tools.
- 36V x 8.0Ah = 288 Wh, a high-voltage pack in a similar footprint.
| Platform | Nominal voltage | Capacity | Energy | Approx. weight |
|---|---|---|---|---|
| Compact 18V pack | 18 V | 4.0 Ah | 72 Wh | 0.6 kg |
| Large 18V pack | 18 V | 12.0 Ah | 216-240 Wh | 1.4 kg |
| High-voltage 40V pack | 36 V | 8.0 Ah | 288-320 Wh | 1.9 kg |
| 80V pair | 72 V | 2 x 8.0 Ah | 576-640 Wh | 3.8 kg |
Why the Same Number Means Different Things
Two 8Ah packs are not equal if one runs at 18V and the other at 40V. The 40V pack holds about a third more energy at the same amp-hour count. Always convert to watt-hours when comparing batteries from different voltage classes.
How Voltage Systems Shape Capacity
Voltage is the push behind the current, and higher voltage lets a tool draw more power without pulling more current. That is why 40V platforms exist: they deliver more power to circular saws, grinders, and chainsaws while keeping current and heat manageable. Higher voltage does not automatically mean longer runtime, because a more powerful tool tends to burn the extra energy faster.
Battery-powered chainsaw tests show how much sustained cutting a high-voltage pack can support, which is a useful reference for any high-draw tool.
The trade-off cuts the other way for light work. A hammer drill or impact driver rarely needs a high-voltage platform, and staying on an 18V system keeps packs smaller, cheaper, and interchangeable with the rest of the crew’s kit.
Series Wiring: Two Packs on One Tool
Some tools take two packs at once, wired in series to double the voltage. Two 40V packs on an 80V tool deliver roughly double the voltage, which doubles the power available to the motor. The tool also carries the weight of both packs, so the ergonomics change even when the runtime does not.
Voltage Ratings Explained: 20V Max vs 18V
The same 18V pack is often labeled 20V Max in North America, because the marketing number is the peak voltage rather than the nominal voltage. The physical pack is identical. When comparing platforms, use the nominal voltage for energy math and ignore the Max label.
What High-Capacity Packs Cost in Weight and Handling
Capacity and weight grow together, and the biggest packs stop being practical for handheld tools. An 8Ah 40V pack weighs about 1.9 kg (4.2 lb) on its own, and a pair of them for an 80V tool weighs 3.8 kg (8.4 lb). Attaching that to a compact drill or impact driver ruins the balance and tires the wrist long before the battery runs out.
Understanding how battery systems evolve across voltage transitions explains why a brand introduces a new platform instead of simply making bigger 18V packs.
A sensible rule is to buy the smallest pack that completes the typical task and keep one large pack for the heavy tools. That keeps the weight down where it matters and concentrates the cost where it pays.
Choosing Capacity by Tool Type
- Compact drills and impact drivers: small packs that keep the tool balanced.
- Circular saws and grinders: mid-size packs that balance runtime and weight.
- High-draw tools such as chainsaws and demolition hammers: the largest packs the tool accepts.
- Bench or stationary tools: oversized packs, since weight does not matter.
Weight Distribution on Site
A heavy pack changes how a tool hangs on a ladder, how it feels in a drill guide, and how fast an operator fatigues on overhead work. When a tool is used at chest height all day, a 0.5 kg difference in battery weight is more noticeable than a 30-minute difference in runtime.
Charging Times and Battery Management
Bigger packs take longer to charge, and charge time is a real productivity cost. A high-capacity 40V pack can take more than an hour to refill, so a crew with one charger needs either a second charger or a second pack. Charging happens in the background of the workday, and the math belongs in the plan.
Voltage and capacity upgrades have pushed battery management systems to do more, monitoring temperature, voltage, and current on every charge cycle.
Chargers also matter. A fast charger can refill a big pack in about an hour, while a standard charger takes longer. The charger’s output rating, printed in watts, tells you which one you own, and matching it to the pack size keeps charge times predictable.
Charging Habits That Extend Pack Life
- Let packs cool before charging after heavy use.
- Avoid storing packs fully charged for months.
- Use the charger that came with the platform, not a generic unit.
- Keep contacts clean and packs out of direct sun.
Cell Technology: Cylindrical vs Pouch Cells
Battery packs are built from individual cells, and the cell format shapes the pack’s size, weight, and cooling. Cylindrical cells are the classic 18650-style design: proven, inexpensive, and packed in large numbers to reach high capacities. Pouch cells are flat and flexible, which lets manufacturers build slimmer packs with different thermal behavior.
High-capacity battery packs in the 15Ah class show the same trade-off: more cells, more weight, more runtime, and a bigger footprint on the tool.
The cell choice also affects how a pack ages. Cylindrical cells degrade cell by cell, and a weak cell drags the whole pack down, which is why battery management systems balance the pack during charging. Balancing keeps every cell at the same voltage so the weakest one does not limit the strongest.
Why Cell Format Matters
- Cylindrical cells: proven, cheap, and easy to cool between cells.
- Pouch cells: slimmer packs with different cooling requirements.
- Cell count drives pack size more than capacity alone.
Form Factor and Cooling
Cooling matters because heat shortens cell life. Cylindrical packs have air gaps between cells that help heat escape, while pouch packs rely on the case to shed heat. A pack that runs hot during a long charging cycle ages faster regardless of its rated capacity.
Planning Around a Battery Platform
The battery platform decision comes before the tool decision, because batteries cost as much as the tools they power. A platform with a wide tool lineup spreads the battery investment across many tools; a narrow lineup leaves expensive packs idle. Check the tool catalog before committing to a voltage class.
The one battery, many tools model is the whole point of platform buying, and it works best when the platform covers the jobs on your schedule.
Replacement cost matters too. When a pack finally wears out, the price of a genuine replacement belongs in the platform decision, because a cheap pack that dies in a season costs more than a quality pack that lasts years.
Capacity is the easiest specification to compare and the easiest to misread. Convert amp-hours to watt-hours, weigh the pack, count the charge time, and check the tool lineup behind the platform. A battery that cannot be used comfortably is not capacity at all.