Master the Basics: Understanding Battery Capacity
Batteries are one of the most common power sources in electronics, but unlike wall outlets, they don’t last forever. Batteries rely on chemical reactions to generate voltage and push electrons through a circuit—but they can only store a limited amount of energy.
Let’s break down how battery capacity works and what it means for your electronic projects.
Battery vs. Wall Power: A Helpful Analogy
Imagine electricity is like water:
A wall outlet is like a faucet connected to the water supply. It can deliver steady pressure (voltage) and unlimited flow (current).
- A battery, on the other hand, is like a water cooler. It holds a limited amount of water (charge) and gravity provides the pressure to push it out.
- When the water cooler is full, pressure is high. As it empties, the pressure drops. This is exactly how battery voltage behaves—it starts strong, then gradually drops as the battery discharges.
Nominal Voltage: What’s That Number on the Label?
The voltage printed on a battery is its nominal voltage—an average value when the battery is about halfway discharged. A brand-new 9V battery will typically read slightly over 9 volts, while an old one may read below that.
What Is Battery Capacity?
Battery capacity is measured in amp-hours (Ah) or milliamp-hours (mAh). This tells you how much total charge the battery can deliver at its rated voltage.
But here’s where many people get confused:
- Amps measure current flow (how fast electrons are moving).
- Amp-hours measure total charge available (how many electrons you have to work with).
So a 1 amp-hour battery can, in theory:
- Provide 1 amp of current for 1 hour
- Or 2 amps for 30 minutes
- Or 0.5 amps for 2 hours
But keep in mind—batteries have physical limits. Trying to pull too much current too quickly can overheat and damage them.
Analogy Time: Toothpaste Tubes
Think of a battery like a tube of toothpaste:
- Squeezing gently = steady flow of current
- Stomping on it = trying to force too much current too fast
- Result? You could burst the tube (or damage your battery)
Real-World Example: AAA vs. D Cell Batteries
Let’s compare some common alkaline battery types. All have the same nominal voltage (1.5V), but their capacities and current capabilities vary:
| Battery Type | Capacity (mAh) | Typical Current Output |
|---|---|---|
| AAA | ~1000 mAh | ~10 mA |
| AA | ~2500 mAh | ~50 mA |
| C | ~8000 mAh | ~150 mA |
| D | ~13,000 mAh | ~200 mA |
Say your circuit draws 10 milliamps and is powered by a AAA battery rated for 1000 mAh.
1000 mAh ÷ 10 mA = 100 hours of run time (in theory).
But if you draw more current than the battery is designed for, you’ll get less runtime and waste energy through inefficiency.
Different Batteries for Different Jobs
There’s a reason we have so many battery types:
- Watch batteries are tiny and optimized for very low power over a long time.
- Car batteries can deliver hundreds of amps, but only for short bursts.
Matching your battery to your circuit’s current needs is key to maximizing performance and battery life.
Key Takeways
Battery capacity isn't just about how long a battery lasts—it’s also about how much current it can safely provide. Understanding the balance between voltage, current, and capacity helps you choose the right power source for any project, big or small.
