Pick up two batteries for the same drill: one says 2.0Ah, one says 4.0Ah, both say 18V (or 20V MAX). They cost different amounts and people assume the bigger number means a more powerful drill. It doesn't. These numbers describe two separate things and one fake thing, and confusing them is how you buy the wrong pack.
Voltage: how hard it can push
Voltage (V) is, loosely, the electrical "push" available to the motor — think of it as the pressure behind the water, not the size of the tank. Higher-voltage platforms (the 18/20V class, the 36/40V class) can support tools that demand more power: bigger drills, circular saws, hammer drills. Lower-voltage platforms (the 12V class) are lighter and more compact, ideal for drills, drivers, and tight-space work. Voltage is mostly a platform decision — it sets the ceiling of what kind of tools the system can run — not a dial you tune job to job.
Amp-hours: how long it runs
Amp-hours (Ah) is the size of the fuel tank — the battery's capacity, how much charge it stores. A higher-Ah pack of the same voltage runs longer between charges; it does not make the tool more powerful. What it does cost you is size and weight: a 4.0Ah pack is physically bigger and heavier than a 2.0Ah pack because it holds more cells. So a big pack means longer runtime and a heavier tool; a small pack means a lighter, more nimble tool you charge more often.
The "20V MAX" that means nothing new
Here's the one to see through. A pack labeled "20V MAX" and a pack labeled "18V" are, in the common 5-cell design, the same battery. Lithium-ion cells sit around 3.6–3.7 volts nominal; five in series is the 18V that engineers quote. But the instant a fresh-charged pack is disconnected and under no load, each cell reads about 4.0 volts — five of those is 20 volts, the "MAX" peak it holds for a moment before you pull the trigger. So "20V MAX" is the no-load peak and "18V" is the working nominal of the identical pack. One marketing department picked the bigger number. Don't pay extra believing 20V MAX is a more powerful class than 18V — compare watt-hours instead.
The real purchase is the platform, not the tool
This is the part the price tag hides. Over a few years you will own more batteries and chargers than tools, and those cost more than the bare tools. Every brand's battery only fits that brand's tools. So choosing a drill is really choosing a battery ecosystem you'll keep buying into — bare "tool-only" units that share your existing packs, the chargers, the future saw and blower. Pick the platform deliberately up front; switching later means re-buying every battery. Buy bare tools once you're committed to a system, and keep at least two packs so one's always charging.
| Spec | What it controls | Trade-off |
|---|---|---|
| Voltage (V) | How hard the tool can push; what tools the platform supports | Higher = more capable but heavier/bulkier system |
| Amp-hours (Ah) | Runtime — the size of the fuel tank | Higher = longer runtime but a heavier pack |
| Watt-hours (V×Ah) | Total stored energy — the honest comparison | The number to actually compare across packs |
| "20V MAX" | Nothing new — no-load peak of an 18V pack | Marketing; don't pay a premium for the label |
The short version
Voltage is the push (and a platform choice), amp-hours is the runtime (and a weight choice), and watt-hours (volts × amp-hours) is the only number that compares two packs honestly. "20V MAX" is the same battery as "18V" wearing its peak voltage as a label. Above all, you're buying into a battery ecosystem more than a single tool — choose the platform on purpose and keep a spare pack. Then decide whether a brushless motor is worth it, and whether you need a drill, an impact driver, or both.
Batteries, chargers, and bare tools on eBay
Compare packs by watt-hours (volts times amp-hours), match them to your existing platform, and keep a spare on the charger. We earn a small commission on qualifying sales and pass a share to the charity you choose; your price is unchanged. Full disclosure.