Two numbers, two different jobs
Almost every cordless battery is labeled with a voltage (V) and a capacity in amp-hours (Ah). They are easy to confuse, but they answer different questions.
- Volts describe the electrical “pressure” the pack delivers. Tools are designed around a specific voltage, and higher-voltage systems are generally built for more demanding work.
- Amp-hours describe how much charge the pack stores. Think of it as the size of the fuel tank.
A useful picture is water in a hose. Voltage is like water pressure, and amp-hours are like the amount of water in the tank. Pressure influences what the system can push through, and tank size influences how long it can keep going.
Turning the two numbers into energy
Multiplying volts by amp-hours gives watt-hours (Wh), a measure of stored energy. It is the fairest way to compare packs of different voltages.
Here is a hypothetical example, using round numbers and not any specific product:
- An 18 V pack rated at 2 Ah stores about 18 × 2 = 36 Wh.
- An 18 V pack rated at 4 Ah stores about 18 × 4 = 72 Wh.
The second pack holds twice the energy of the first. If a tool pulled a steady 200 W while working (a made-up figure), the 36 Wh pack could supply it for about 0.18 hours, roughly 11 minutes, and the 72 Wh pack for about 0.36 hours, roughly 22 minutes. Real tools rarely draw a steady amount, and the pack, the temperature and the age of the cells all change the result, so treat this only as a way to understand proportions.
The same logic means a 2 Ah pack at 36 V holds about as much energy as a 4 Ah pack at 18 V. The 36 V system would, however, be designed for a different class of tool.
How amp-hours relate to run time
For the same tool and the same kind of work, a higher amp-hour pack of the same voltage usually runs longer before it needs charging. Doubling the capacity roughly doubles the time, other things being equal.
Run time is also affected by:
- The load. Driving long screws into dense wood draws more current than light drilling, so the pack drains faster.
- The motor. Efficient motors waste less energy as heat. Newer brushless motor design is often more efficient than older brushed designs, which can stretch a charge.
- Temperature. Very cold packs often deliver less, and hot packs may shut down to protect themselves.
- Age. Rechargeable packs slowly lose capacity over time and with use.
What higher voltage does and does not mean
A higher voltage does not by itself mean a tool runs longer. It usually means the system is built to deliver more power to the tool, so it appears on larger or harder-working equipment. A small hand drill and a large cordless saw are often on different voltage classes for that reason.
Be aware of labeling differences. Some brands describe a pack by its peak voltage right after charging and others by its average, or nominal, voltage under use. Two packs that look different on the label may be electrically similar. When in doubt, compare the watt-hour figure if it is printed, or check the manufacturer’s notes for how the voltage is stated.
Bigger packs have trade-offs
A high-capacity pack is not automatically the best choice:
- Weight. More cells mean more weight on the end of the tool, which matters for overhead work and long sessions.
- Cost. Capacity usually adds to the price.
- Charge time. Larger packs often take longer to charge, depending on the charger.
- Balance. On a compact tool, a large pack can make the tool feel top- or rear-heavy.
For short jobs and tight spaces, a smaller pack can be more comfortable. For sustained cutting or sanding, a larger pack saves swaps. Many people keep one of each.
Compatibility and safety
Voltage and amp-hours are only part of whether a pack fits a tool. Packs from different makers generally do not interchange, and even within one maker, different systems may not mix. Choosing a battery platform is a long-term decision, because chargers and spare packs add up. Check the manual before using a pack or charger that is not specified for your tool.
Handle lithium-ion packs with care. Do not use a pack that is swollen, cracked or leaking, do not charge it on a flammable surface, and follow the manufacturer’s storage guidance. Ongoing battery chemistry advances keep improving capacity and weight, but the basic rules of reading volts and amp-hours stay the same.
A quick way to compare
- Match the voltage to what the tool requires.
- Multiply volts by amp-hours to get watt-hours if you want to compare packs.
- Think about the work: a light-duty impact driver may be fine on a compact pack, while sustained heavy cutting suits more capacity.
- Weigh the extra weight and cost against the run time you actually need.