A battery is not a tank of electricity waiting to be emptied. It is a box holding a chemical reaction that only happens when you connect something to it. Understanding that one idea makes everything else about batteries easier to follow.
Three parts doing one job
Inside almost every battery you will find the same three things: two different materials that want to react with each other, and something between them that lets charged particles move but keeps the materials apart.
The negative side is the anode. The positive side is the cathode. The stuff in the middle is the electrolyte, with a thin separator to stop the two sides touching and short-circuiting.
| Part | What it is | What it does |
|---|---|---|
| Anode (-) | The material that gives up electrons | Releases electrons when the circuit is closed |
| Cathode (+) | The material that accepts electrons | Takes electrons back in, completing the reaction |
| Electrolyte | A liquid, gel or polymer full of ions | Carries charged particles inside the cell |
| Separator | A thin porous sheet | Keeps the two sides apart so it cannot self-short |
Electrons take the long way round
When you connect a battery to a device, electrons leave the anode and travel through your circuit to the cathode. That flow of electrons is the current that powers the device.
The electrolyte will not let electrons through, so the only path is through your device. Meanwhile ions move through the electrolyte to keep the charge balanced. It is a closed loop: electrons outside, ions inside.
A battery with nothing connected is still chemically active, just idle. It will still slowly lose charge to internal reactions over months, which is why shelf life is not infinite.
Disposable versus rechargeable
In a primary battery the reaction runs one way. Once the anode material is used up, the cell is finished. Alkaline AA batteries and coin cells are almost always primary.
In a secondary battery the reaction can be reversed by pushing current back in. Lithium-ion, NiMH and lead-acid car batteries are secondary. Recharging is never perfectly reversible, which is why every rechargeable battery slowly loses capacity over hundreds of cycles.
Why batteries fade
Nothing lasts forever, and the reasons are usually physical rather than mysterious.
- Cycle ageing: each charge and discharge slightly changes the electrode materials.
- Calendar ageing: batteries lose capacity just by existing, faster when warm and fully charged.
- Heat: the single biggest enemy of lithium-ion life, whether from a hot car or a cheap charger.
- Deep discharge: running a lithium cell flat can damage it permanently.
- High current: pulling more than a cell is designed for wastes energy as heat and shortens life.
Voltage is a push, not a tank level
A 12V battery is not twelve times fuller than a 1.5V one. Voltage is the electrical push, set by the chemistry. Capacity is how much charge it can move, and that is a separate number covered in the next guide.
The short version
- A battery stores energy in chemicals, not as electricity.
- Electrons travel through your device, ions travel inside the cell.
- Rechargeable cells reverse the reaction, imperfectly, which is why they age.
- Heat and deep discharge are the two most common killers.