Cordless Batteries Explained

1. The Short Answer

Volts describe how hard a tool can work; amp-hours describe how long it can work. Eighteen volts, marketed by some brands as 20V MAX, is the mainstream standard and the right default for a main platform. Two batteries beat one large one, and the brand you pick will still be in your workshop when the drill has been replaced twice.

2. Volts, and the 18V versus 20V Myth

Voltage is electrical pressure. Higher voltage lets a motor draw more power without drawing impractical current, which is why heavy tools such as circular saws and grinders live at 18 volts and above while small drivers manage happily at 12.

The most persistent confusion in the whole category is the 18V versus 20V question, and the answer is that there is no difference whatsoever. A lithium cell has a nominal voltage of about 3.6 volts and a fully charged resting voltage of about 4.0. Five cells in series therefore give 18 volts nominal and 20 volts at peak charge. Some brands print the nominal figure and some print the peak; the pack is identical.

The same arithmetic explains the rest of the numbers. Three cells give 10.8V nominal, marketed as 12V MAX. Ten cells give 36V nominal, marketed as 40V MAX. When you see a battery described as 40V MAX and another as 36V, you are looking at the same thing labelled by different marketing departments.

Higher-voltage platforms — 36V, 56V, 80V — exist mainly for outdoor equipment, where mowers, chainsaws and large blowers need sustained power that 18V struggles to supply. Some brands achieve this with packs that switch internally between 18V and 36V depending on the tool, which is a genuinely clever way to keep one platform serving both.

3. Amp-Hours, and the Number You Should Really Compare

Amp-hours measure how much charge a pack holds, and therefore how long a tool runs before it stops. A 4.0Ah battery holds twice the charge of a 2.0Ah battery of the same voltage and runs roughly twice as long doing the same work. Straightforward so far.

The complication is that amp-hours are only comparable within one voltage. A 2.0Ah pack at 18V holds far more energy than a 2.0Ah pack at 12V, because energy is voltage multiplied by charge. The figure that lets you compare anything with anything is watt-hours: volts times amp-hours.

Note how quickly weight climbs. A 9.0Ah pack turns a comfortable drill into something you notice after twenty minutes overhead, which is why professionals carry several mid-size packs rather than one enormous one. The right answer for most people is two 4.0 or 5.0Ah batteries and a fast charger.

4. What Is Inside the Pack

Every tool battery is a group of cylindrical lithium cells, a small circuit board and a plastic case. The cells are usually one of two standard sizes, and which one a pack uses explains a great deal about how it behaves.

The 18650 cell, eighteen millimetres across and sixty-five long, has been the industry workhorse for years and typically holds 2.0 to 3.0Ah each. The newer 21700 cell is physically larger, holds more charge, and — more importantly — can deliver higher current without sagging or overheating. Most high-capacity modern packs use them.

That current capability is why higher-capacity batteries sometimes make a tool feel stronger. More cells in parallel share the load, so voltage sags less under heavy draw and the tool holds its speed. On a drill driving small screws you will not notice. On a circular saw ripping through a joist, the difference between a small pack and a large one is obvious and immediate.

The circuit board matters too. It monitors cell voltages, balances them, watches temperature and cuts off at both extremes to protect the pack. This is precisely what cheap third-party batteries economise on, and it is the reason a bargain replacement pack can quietly destroy itself or, in bad cases, worse. Buying the manufacturer's own batteries is one of the few places in tools where paying more is straightforwardly correct.

5. Which Size for Which Tool

The work-light row is a genuinely useful habit rather than a joke. A pack that has lost a third of its capacity is still perfectly good for a light, a radio or an inflator, which extends its useful life by years and keeps your good batteries free for tools that need them.

The broader principle is that runtime is only one variable and weight is the other. A drill with a 9.0Ah pack runs a long time and is unpleasant to use for any of it. Two mid-size batteries and a charger that refills one while you use the other is a better system than a single enormous pack in nearly every case.

6. The Platform Decision

No manufacturer's batteries fit another's tools. The footprints, latches and communication protocols are all proprietary, and that incompatibility is entirely deliberate — it is the mechanism by which a first drill purchase becomes a decade of brand loyalty.

So choose the platform rather than the tool. The questions that matter are: how wide is the range of tools on this battery, is it sold where you shop, what do bare tools cost, and will it still be supported in ten years? A slightly better drill on a platform you will abandon is a worse purchase than an adequate drill on one you will build out.

Broadly, the professional platforms offer the deepest ranges, the best performance and the highest prices, and are the right choice if you use tools for a living or intend to accumulate many of them. The homeowner-focused platforms trade some performance for a much lower cost per tool and enormous breadth, which suits someone adding a hedge trimmer, an inflator and a glue gun over the years far better than a professional line would.

Our comparison of Milwaukee and DeWalt covers the two dominant professional systems, and our guides to Ryobi ONE+ and Ryobi tools generally cover the homeowner side of that trade. For the tools themselves, our cordless drill reviews compare across platforms.

7. Making Batteries Last

Tool batteries typically give three to five years of ordinary service, and the difference between the short end and the long end is almost entirely down to heat.

• Let a hot pack cool before charging. A battery straight off a hard cut is already warm, and charging adds more heat. Twenty minutes on the bench first is the single best habit here.
• Do not store them in a vehicle. A van in summer reaches temperatures that age lithium cells several times faster than a workshop shelf, and packs left there over a season lose real capacity.
• Store at around half charge for long periods. Cells age fastest sitting at full charge. If a pack is going into a drawer for the winter, take it off the charger at roughly fifty per cent.
• Do not run packs completely flat repeatedly. The protection circuit prevents true over-discharge, but habitually working to shutdown is harder on cells than swapping at the first sign of fade.
• Keep the terminals clean and the pack dry. Sawdust and grit in the rails cause poor contact and intermittent power that gets blamed on the tool.

The cold rule is worth knowing separately because it looks like a fault. Charging lithium below freezing damages cells permanently, so chargers refuse until the pack warms up. A battery that will not charge in a cold garage in January is protecting itself; bring it indoors and it will be fine.

When a pack does finally fade, recycle it properly rather than binning it. Lithium cells in household waste start fires in collection lorries and sorting facilities, and most tool retailers now take old packs back free of charge.

8. The Case for 12V

Twelve-volt tools are widely dismissed as toys, and that reputation is a decade out of date. Modern brushless 12V drills and drivers handle the overwhelming majority of household and cabinet work, and they do it in a package that is dramatically smaller and lighter.

The advantages are physical. A 12V drill is often two inches shorter nose to tail, which is the difference between reaching inside a cabinet and not. It weighs perhaps half what an 18V tool does, which matters enormously when working overhead or on a ladder for any length of time. And it is far less tiring in a full day of repetitive assembly.

The limits are real too. A 12V platform will not run a circular saw, a grinder or a large reciprocating saw satisfactorily, and heavy masonry drilling is beyond it. Nobody should build their only system around 12V unless their work is genuinely light.

The arrangement many experienced users land on is an 18V platform as the backbone plus a 12V drill and driver for the fiddly work — a second ecosystem, but a cheap one, and the tool that gets picked up most often. If you are choosing a first platform, start at 18V; the 12V question is one to revisit later.