AA, AAA, 9V: A Beginner's Guide to Battery Types and Capacity

Every project that isn’t plugged into the wall eventually comes down to a battery choice, and the label on that battery is full of numbers that don’t mean much until you know how to read them. Two figures matter most for a hobby project: the voltage and the capacity.

Voltage: what kind of cell is it?

A single alkaline or standard rechargeable cell (AA, AAA, C, D) is rated at 1.5V (alkaline) or about 1.2V (NiMH rechargeable). A 9V battery isn’t a single cell at all — it’s six small 1.5V cells stacked in series inside one case, which is why it reads 9V (6 × 1.5V). Lithium-ion cells, common in rechargeable packs and single-cell “18650” batteries, are typically rated around 3.7V per cell. Knowing the cell chemistry tells you the nominal voltage before you even look at capacity.

Capacity: what mAh actually measures

Capacity is given in milliamp-hours (mAh) — a measure of total charge, not power. A 2000mAh battery can, in theory, supply 2000mA (2A) for one hour, 1000mA for two hours, 200mA for ten hours, or any other combination whose current × time multiplies out to roughly 2000mAh. Dividing the capacity by your circuit’s current draw gives a first estimate of runtime: a 2000mAh battery powering a 100mA circuit should last around 20 hours by this simple math.

Why real runtime is usually shorter

That 20-hour figure is optimistic, for a few reasons. First, the mAh rating on the label is typically measured under specific lab conditions — a particular discharge rate and cutoff voltage — that may not match your actual circuit. Second, most electronics stop working before a battery is truly empty; they need a minimum voltage to function, and a battery’s voltage sags as it discharges, so the practically usable capacity is less than the full rated figure. Third, capacity is not perfectly constant regardless of load — batteries deliver somewhat less total energy when discharged quickly than when discharged slowly, an effect more pronounced in some chemistries than others.

For a realistic estimate, it’s common practice to apply a derating factor to the ideal calculation — using something like 80–85% of the rated capacity rather than the full number — which brings the estimate closer to what you’ll actually observe in a real project.

Comparing common sizes

Typical rated capacities give a rough sense of scale: alkaline AA batteries commonly fall around 1800–2700mAh, AAA cells around 800–1200mAh (smaller physical size, less room for chemistry, lower capacity), and a 9V battery, despite its higher voltage, is often only around 400–600mAh because its six tiny internal cells are much smaller than a single AA cell. This is why a 9V battery often feels like it drains surprisingly fast in a current-hungry project — the higher voltage doesn’t mean more total energy.

Rechargeable vs. single-use

NiMH rechargeable cells typically have somewhat lower capacity than a comparable single-use alkaline cell but make up for it over dozens to hundreds of recharge cycles, and their flatter discharge curve (voltage stays more constant until it drops sharply near the end) can actually be an advantage for circuits sensitive to supply voltage. Lithium-ion and lithium-polymer cells offer higher energy density (more capacity for the same size and weight) but need protection circuitry to charge and discharge safely, which is usually built into the battery pack or a dedicated charging module.

Putting it into practice

Before committing to a battery for a project, estimate your circuit’s actual current draw (a multimeter in series with the circuit gives a direct reading), apply it against the battery’s rated mAh with a realistic derating factor, and you’ll have a runtime estimate that’s far more useful than trusting the label’s number at face value.