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.

Choosing between battery types for a project

With voltage and capacity both in hand, picking a battery for a specific project comes down to a few practical questions. How much current does the circuit actually draw, and for how long does it need to run — a quick indicator LED that gets switched on for a few minutes at a time has very different needs from something meant to run continuously for days. Does the project need to be rechargeable, or is a single-use cell fine for something built once and rarely repeated? And does the physical size matter — a coin cell fits where an AA pack won’t, but only at a small fraction of the capacity. There’s rarely a single “best” battery; there’s a best fit for a specific project’s current draw, runtime target, and size constraints.

Self-discharge is worth factoring in too: every battery slowly loses some charge just sitting unused, even with nothing connected, though the rate varies a lot by chemistry — alkaline cells hold their charge for a long shelf life, while some rechargeable chemistries self-discharge noticeably faster and are worth checking before a project that’s assembled and then left dormant for months.

Runtime estimates across a few real devices

Running the same calculation across a few realistic scenarios shows how much runtime depends on both capacity and draw, not just one or the other:

• Two AA cells (2,500mAh) powering a 20mA LED indicator circuit: 106.25 hours — over four days of continuous runtime, because the draw is so small relative to the capacity.
• The same AA cells powering a 300mA project (several LEDs, a small motor, or a more power-hungry sensor): 7.08 hours — a fifteenfold increase in current draw cuts runtime by roughly the same factor.
• A AAA cell (1,000mAh) at a 50mA draw: 17 hours.
• A 9V battery (600mAh) powering that same 20mA LED circuit: 25.5 hours — noticeably less than the AA pair above despite the higher voltage, because a 9V battery’s six tiny internal cells simply hold less total charge.
• A CR2032 coin cell (225mAh) in a very low-draw application like a real-time clock, drawing a tiny 0.05mA: 3,825 hours, or about 159 days — coin cells are built for exactly this kind of near-idle, years-long duty, not for driving anything current-hungry.

The pattern across all five is the same one from the earlier formula: runtime is capacity divided by draw, scaled down by the derating factor. A tiny coin cell can outlast a much bigger AA pack when its job is idling at a fraction of a milliamp, and even a large-capacity pack drains in hours once the draw climbs into the hundreds of milliamps.

How much to trust the derating factor

The default 85% efficiency figure is a reasonable general-purpose estimate, but it’s not the only honest number. Running the same 2,000mAh/100mA scenario at a few different efficiency values shows the spread: at a perfect, unrealistic 100% efficiency, the math gives 20 hours; at the default 85%, 17 hours; at a more conservative 70%, 14 hours; and at a pessimistic 50% — appropriate for cold temperatures, a high discharge rate, or a battery nearing the end of its shelf life — just 10 hours. None of these numbers is “wrong”; they represent different assumptions about how forgiving the real conditions will be. When a project genuinely needs to last a certain minimum time, planning around the more conservative end of that range avoids an unpleasant surprise.

Battery hazards worth knowing, even at low voltage

Battery-powered projects are low-risk compared to anything connected to mains power, but “it’s only a battery” is itself a belief worth being careful with — batteries store real chemical energy, and a few specific mistakes matter regardless of how small the cell is:

• Coin cells are a serious swallowing hazard for small children. A swallowed coin cell can cause severe internal chemical burns within hours, not days — keep coin-cell projects and spare cells away from young children, use battery holders that aren’t easily pried open, and treat this as a genuine medical emergency requiring immediate care if it’s ever suspected.
• Lithium and lithium-polymer cells can vent, catch fire, or explode if they’re shorted, punctured, physically damaged, over-discharged, or charged incorrectly. Handle rechargeable lithium packs gently, never puncture or crush one, and use a charger designed for the specific cell chemistry.
• Never charge a battery that isn’t designed to be recharged. Standard alkaline cells (the ordinary AA, AAA, C, D, and 9V batteries sold as single-use) are not built to be charged, and attempting it is a real hazard, not just a waste of a charger.
• Shorting a battery pack — even a simple AA pack — heats the wires enough to burn skin. A direct short (the two terminals connected with no resistance in between, such as bare wire touching both ends) draws far more current than any component in the circuit was designed for, and that current turns into heat fast. Double-check wiring for accidental shorts before connecting a battery, and disconnect immediately if anything gets warm unexpectedly.

None of this is a reason to avoid battery-powered projects — it’s the same category of common-sense handling that applies to anything storing energy, and it becomes routine quickly: check polarity, avoid shorts, keep coin cells away from young children, and use each battery chemistry the way it was designed to be used. For a fuller look at realistic runtime planning specifically, see Estimating Battery Life Honestly.

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 using the Battery Life Calculator, and you’ll have a runtime estimate that’s far more useful than trusting the label’s number at face value.