The cells inside a battery pack decide more than the amp-hour number

The theory of cordless tool battery packs is well covered elsewhere. This is about the version you meet in practice.

What the two printed numbers cover

The voltage on a pack describes cells in series, since each lithium cell contributes a nominal figure of a little under four volts. The amp-hour figure describes how much charge the pack stores, which translates into runtime at a given draw but says nothing about how hard it can push.

Multiplying the two gives watt-hours, the only figure that lets you compare packs across different voltages honestly. Neither number tells you how many physical cells are inside, and that count is what determines behaviour under heavy load. Manufacturers rarely print cell counts, but pack weight and physical size are a reliable proxy, since cells dominate both.

Series and parallel do different jobs

Cells in series add voltage, which raises the speed and power a motor can reach for a given current. Cells in parallel add capacity and, just as importantly, divide the current each individual cell must supply.

A pack of ten cells in a single series string must ask each cell for the full tool current, while a twenty-cell pack in two parallel strings asks each for half. That is why larger packs on the same platform often deliver more power, not merely longer runtime, on demanding tools. Some tool ranges make this explicit by specifying a minimum pack size for high-draw products such as saws and grinders.

Voltage sag is what you feel

Every cell has internal resistance, so terminal voltage drops the moment current is drawn, and drops further as the pack ages or gets cold. A tool stalls when the sagging voltage no longer supports the torque required, which happens well before the pack is empty. Halving the current per cell roughly halves the sag from that resistance, which is the mechanism behind larger packs feeling stronger.

In the small print, sag also generates heat inside the cells, and heat is the main driver of capacity loss over a pack's life. This is why a small pack used on a heavy tool wears out faster than the same pack used on a drill driver.

Cell format and quality

The cylindrical formats used in tool packs differ in diameter and length, and the larger formats generally offer more capacity per cell. Within any format, cells are manufactured in variants optimised for either high capacity or high discharge current, and the two rarely coexist. A pack advertising a very high amp-hour figure in a small physical size has probably used capacity-optimised cells with weaker current delivery.

After the warranty ends, cell matching matters too, because a pack is limited by its weakest cell and an unbalanced pack ages faster than a balanced one.

None of this appears on the label, which is why teardown coverage by independent reviewers is the only practical source for it.

The management electronics are part of the product

A battery management system monitors cell voltages and temperature, and it decides when to cut off, which protects the cells from the user. Better systems balance cells during charging, which keeps a pack usable for far longer than one that simply charges the string as a whole. Protection thresholds differ between makers, and a conservative cut-off feels like less runtime while actually preserving cell life.

Judged against the category, charger design matters as much: a charger that pauses to cool a hot pack before charging it is doing something valuable that a fast charger may skip. Fuel gauges are estimates derived from voltage and current, and their accuracy varies enough that a four-light display is a rough guide only.

Judging a platform rather than a pack

The battery platform is usually the real purchase, because packs, chargers and tools are locked together for years by the connector. Ask whether the maker has changed platform recently, whether old packs run new tools, and whether packs are sold separately at a sane price. Check that the range includes both compact packs for light work and large packs for the tools that need current, since one size cannot do both.

Look for evidence that cells are replaceable or that packs are serviceable, though most consumer packs are deliberately sealed. This site does not test batteries; reviewers who log voltage under load and run packs to destruction produce the numbers that settle these questions.

The takeaway

Buy the platform, then size the pack to the current the tool needs rather than to the runtime you imagine.

The question is rarely which is best. It is which is enough.