When a crew moves from 8.0Ah packs to a 10Ah cordless battery, the first question is usually about size. The second is about price. High-capacity cordless batteries changed the economics of cordless construction work by letting tools run longer between charges without forcing a jump to heavier 15-cell packs. Cell count, energy density, and amp-hour pricing work together, and understanding how they interact makes it easier to budget for new batteries and to get real runtime from the packs you already own. That starts with care: many crews still follow outdated advice about fully draining packs before charging, so our cordless battery care guide explains why modern packs prefer partial discharge and regular top-ups.
How Cordless Battery Capacity Is Measured
Battery capacity is usually quoted in amp-hours, a figure that describes how much current a pack can deliver for one hour. A 10Ah pack can supply 10 amps for one hour, 5 amps for two hours, or 20 amps for half an hour, at least in theory. Real tools draw current in bursts, so runtime also depends on the load, the motor efficiency, and the cut-off voltage where the pack stops delivering.
Amp-Hours versus Watt-Hours
Amp-hours only tell part of the story because they ignore voltage. Watt-hours, calculated as volts multiplied by amp-hours, capture the total stored energy. An 18V pack rated at 10Ah holds roughly 180 watt-hours, while an 18V pack rated at 5.5Ah holds about 99 watt-hours. Two tools on different voltages can carry the same amp-hour rating yet store very different amounts of energy, which is one reason the way cordless power tool platforms evolve around stable voltage ratings while capacity keeps climbing.
Counting Cells in a Pack
The cells inside a pack are arranged in series to reach the working voltage and in parallel to add capacity. A nominal 18V pack uses five lithium-ion cells in series, each contributing about 3.6V. A 10-cell pack pairs two of those five-cell strings in parallel, which is why manufacturers describe these units as two-row packs. Adding cells in parallel raises amp-hours without changing voltage, which keeps the pack compatible with every tool and charger on the platform.
A 10-cell pack is currently the practical sweet spot. Five-cell packs stay compact but usually cap out near 4Ah or 5Ah, while 15-cell packs add capacity at the cost of size and weight. The jump from 10 cells to 15 cells typically means a noticeably bulkier battery that changes the balance of the tool it powers.
Voltage labels add confusion. An 18V pack reads about 20V at full charge, and some platforms market the same cells under a 20V Max label, so comparing brands by voltage alone misleads. Watt-hours and cell count give a cleaner basis for comparison.
Cell Count and Form Factor Tradeoffs
Form factor matters as much as capacity because a pack that is too large ruins the ergonomics of a drill or grinder. The common configurations on 18V platforms break down like this:
- 5-cell packs: compact, roughly 4Ah to 5Ah, best for drills, drivers, and light assembly work.
- 10-cell packs: standard footprint, 5.5Ah to 10Ah, the workhorse range for most site tasks.
- 15-cell packs: larger and heavier, 12Ah and up, used where runtime matters more than weight.
Higher capacity used to require a step up to 15-cell sizing, which meant bigger size and greater weight. The arrival of 10Ah in a 10-cell footprint changes that tradeoff, because the pack keeps the same shape and weight class as older 5.5Ah and 8.0Ah units. Chargers, tool bases, and even the battery holder and storage racks you already own keep working, since the connection interface does not change.
Weight is the hidden cost of capacity. A 15-cell pack can add several hundred grams compared with a 10-cell pack, and that weight sits at the back of the tool, where it affects wrist strain during overhead work. On a full shift of drilling or grinding, a lighter pack often produces more consistent work than a heavier one with extra runtime.
Energy Density and Next-Generation Cells
Packing 10Ah into a 10-cell footprint only works if the individual cells store more energy. A 10-cell pack at 10Ah implies 5.0Ah cells, roughly double the per-cell capacity of earlier generations. Higher energy density lets manufacturers either add capacity to an existing footprint or shrink the pack for a given capacity.
- Same-size packs with more runtime.
- Compact 5.0Ah packs in a form factor that previously topped out near 4Ah.
- Room for 15-cell packs that reach 15Ah without growing proportionally.
Cell chemistry and quality control matter as much as cell count. Higher-density cells run hotter under heavy load, which is why modern packs rely on management electronics to balance charge and limit temperature. Voltage transitions and compatibility become a real concern when brands change cell suppliers or chemistries, a topic covered in how cordless battery systems evolve across voltage and chemistry changes.
For buyers, the practical result is that capacity claims need verification. Two packs with the same amp-hour label can deliver different real runtime depending on cell quality, discharge curves, and the tool they are paired with. Field tests, not labels, settle the argument.
Matching Battery Capacity to Real Work
Runtime planning starts with the job, not the battery. A framing crew rotating two batteries through an impact driver has different needs than a metal fabricator running a grinder continuously. In general, battery systems power modern construction work in three tiers: light assembly on compact packs, standard site work on 10-cell packs, and sustained high-draw tasks on the largest packs available.
- List the tools that draw the most current: grinders, circular saws, and rotary hammers.
- Measure how long each tool runs per day in minutes, not how often you reach for it.
- Divide total runtime by a realistic duty factor, roughly 60 to 70 percent of rated amp-hours under load.
- Add one spare pack per high-draw tool so charging can happen without stopping work.
Estimating Runtime from Watt-Hours
A 10Ah pack on an 8500 RPM grinder might last 25 to 40 minutes of continuous cutting, while the same pack on a drill used in short bursts can stretch across a full day. The watt-hour figure gives a fairer basis for comparison: divide the pack’s watt-hours by the tool’s average draw in watts to estimate minutes of runtime. A 180Wh pack feeding a 400W average draw yields about 27 minutes, before the management system tapers output.
Heat is the fastest killer of lithium-ion packs. Leaving batteries in a closed van in direct sun, or charging a hot pack straight off the tool, accelerates cell aging, which is why the measured runtime of a two-year-old pack can drop by a third. Rotating packs and charging them at room temperature keeps the fleet delivering its rated capacity.
What Capacity Costs: Amp-Hour Pricing Math
Price per amp-hour is the cleanest way to compare battery value across capacities. Catalog pricing for one 18V line, excluding VAT, shows how the cost scales:
| Capacity | Catalog Price (ex VAT) | Price per Amp-Hour |
|---|---|---|
| 5.5Ah | £116.66 | £21.21 |
| 8.0Ah | £182.50 | £22.81 |
| 10.0Ah | £224.18 | £22.42 |
The 10Ah pack costs about £224, yet its per-amp-hour figure sits below the 8.0Ah pack and only slightly above the 5.5Ah pack. That near-parity matters, because it means the newest high-capacity pack does not carry a disproportionate premium over older chemistry. The 4.0Ah compact pack, at £88.32, works out to roughly £22 per amp-hour as well, so buyers are not penalized for choosing the largest option.
Pricing math also exposes the real cost of running many small packs. A crew that needs 20 amp-hours per day can buy four 5.5Ah packs for about £467 and manage four charging slots, or two 10Ah packs for about £448 and manage two. The small per-amp-hour premium buys fewer packs to carry, fewer chargers to run, and less time spent swapping batteries.
Building a Battery Strategy for Your Crew
A practical battery strategy treats packs as an investment with a replacement cycle. Higher-density packs usually cost more up front, but they reduce the number of packs and chargers a crew needs to carry. The capacity upgrades and battery management systems now standard in most packs also extend service life through charge balancing and temperature limiting.
- Buy the highest capacity that fits the tool’s balance, not the cheapest pack on the shelf.
- Keep at least one spare pack for every high-draw tool.
- Label packs with purchase dates and rotate usage so the oldest packs get used first.
- Retire packs that swell, overheat, or deliver visibly shorter runtime.
Replacement timing matters as well. A pack that delivers half its rated runtime has lost capacity from cycling, and keeping it in daily rotation costs more in lost productivity than replacing it. Budgeting one battery purchase per tool every two to three years of steady use keeps the fleet honest.
The final decision comes down to the platform you standardize on. Once you commit to a voltage family, your batteries, chargers, and tools must work as one system, which is exactly what tool kit selection guidance covers in detail. Standardizing on one platform, choosing 10-cell packs for daily work, and keeping compact packs for light tasks gives most crews the best balance of runtime, weight, and cost.