Batteries and Power Supplies

Introduction

Every circuit needs a power source. Choosing the right one affects whether your project works, how long it lasts, and whether it is safe. This lesson covers batteries and the supplies that turn mains into safe DC.

Battery basics

A cell turns stored chemical energy into electrical energy. A battery is one or more cells together. Common types:

• Alkaline (AA, AAA, 9 V): cheap, about 1.5 V per cell, not rechargeable.
• Nickel-metal hydride (NiMH): rechargeable, about 1.2 V per cell.
• Lithium-ion and lithium polymer: rechargeable, high energy for their size, about 3.7 V per cell. They need proper chargers and protection circuits, and are a fire risk if damaged, short-circuited or overcharged.
• Coin cells (such as CR2032): about 3 V for tiny, low-current devices. Keep them away from young children, because swallowing them is dangerous.

Capacity and runtime

Capacity is measured in milliamp-hours (mAh) or amp-hours. A 2000 mAh battery could ideally supply 2000 mA for one hour, or 100 mA for 20 hours.

Runtime ≈ capacity ÷ current
2000 mAh ÷ 100 mA = 20 hours

Real runtime is shorter, because capacity falls with high currents, cold and age, and the battery voltage drops as it empties. Allow a safety margin, such as 70 to 80 percent of the ideal figure.

Combining cells

• In series: voltages add, capacity stays the same. Four 1.5 V cells give 6 V with the capacity of one cell.
• In parallel: voltage stays the same, capacity adds. Two 1.5 V, 2000 mAh cells give 1.5 V and 4000 mAh.

Only combine identical cells of the same type, age and charge. Mixing can cause overheating and leaks.

Voltage regulators

Many circuits need a steady voltage, such as 5 V or 3.3 V, even as a battery drains or a supply varies. A voltage regulator provides it.

• Linear regulators are simple and quiet, but waste the extra voltage as heat. Dropping 9 V to 5 V at 100 mA wastes (9 − 5) × 0.1 = 0.4 W as heat.
• Switching regulators (buck, boost) are much more efficient, but more complex and noisier.

Mains power supplies

A wall adaptor or phone charger contains a transformer or switching circuit, rectifier and smoothing that turn mains into low-voltage DC.

• Use approved, correctly rated adaptors. Check the output voltage, current and polarity of the plug.
• The adaptor's current rating is the maximum it can supply, not what is forced into your device.
• Never use damaged adaptors, or open them. The inside carries mains.
• Do not charge lithium batteries without a proper charger, or leave them charging unattended.

Choosing for a project

1. Find the voltage your circuit needs.
2. Estimate the current, and the runtime you want.
3. Pick a source with enough capacity, and a regulator if needed.
4. Add a fuse or protection where appropriate.
5. Plan for safe handling and disposal. Recycle batteries at proper collection points.

Try it yourself

A project takes 80 mA from three AA alkaline cells in series with 2400 mAh capacity. Calculate the supply voltage, the ideal runtime, and a more realistic runtime allowing 70 percent.

Summary

• Choose a battery type by voltage, capacity, rechargeability and safety, especially for lithium cells.
• Runtime is roughly capacity ÷ current, and real life is shorter.
• Series cells add voltage, parallel cells add capacity, and regulators give steady voltage, with linear ones wasting heat.