Power & protection

Battery

Turns stored chemical energy into a steady voltage. Inside, a chemical reaction pushes ions through the electrolyte; outside, the same amount of charge flows round your circuit as electrons.

Think of it as: A water pump powered by a slowly emptying fuel tank: steady pressure until the fuel runs low.

Pins

+ (positive): : The long thin plate in the symbol. The nub on an AA cell, the flat text side of a coin cell.
− (negative): : The short thick plate. Usually your circuit’s ground.

One "cell" gives 1.2 to 1.5 V (alkaline, NiMH), 3 V (lithium coin) or 3.7 V (lithium-ion). A "battery" is strictly several cells in series, like the 9 V block.

What to look for inside

A lithium-ion cell powering a load, which switches off now and then. Over the loop it slowly runs down.
Electrons (gold) leave the − anode, flow through the wire and the load, and return at the + cathode.
At the same time, inside, lithium ions (+) travel through the electrolyte from anode to cathode, keeping the charge balanced.
When the anode has no ions left to give, the cell is flat. Charging pushes them back.

The numbers that matter

Spec	Typical
Nominal voltage The typical voltage. It is higher when full and lower when nearly flat.	1.5 V · 3 V · 3.7 V · 9 V
Capacity How long it lasts: 2000 mAh is about 2000 mA for 1 hour, or 20 mA for 100 hours.	mAh
Internal resistance Sets how much the voltage sags under load. Coin cells are high: they cannot deliver bursts.	0.05 Ω to 20 Ω
Max discharge current Drawing more heats the cell and wastes capacity.	mA (coin) to 20 A (Li-ion)

Where you’ll use it

Powering anything portable.
Coin cells keeping a clock or memory alive when the main power is off.

Common mistakes

Short-circuiting a lithium cell. The current can be tens of amps, and it can catch fire.
Expecting a 9 V block or a coin cell to drive a motor. Their internal resistance is far too high.
Charging lithium cells without a proper charger chip.

Lessons that use it

Go deeper

U_{term} = U_{oc} - I\,R_{int}

Terminal voltage sags with current.

t \approx \tfrac{\text{capacity (mAh)}}{I\,(\text{mA})}\ \text{hours}

Rough runtime. Real capacity falls at high currents and in the cold.

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