Batteries: primary, secondary, and how they work
Primary batteries are single-use, secondary batteries recharge. Tour the Leclanché dry cell, the lead-acid car battery, and the lithium-ion chemistry in modern devices.
Quick answer
A battery is a packaged galvanic cell (or cells in series) that converts stored chemical energy into electrical energy; primary batteries are single-use, while secondary batteries can be recharged by driving the reaction backward.
Example: In a lithium-ion cell, charging pushes Li+ ions into the graphite anode; discharging lets them shuttle back to the metal-oxide cathode, with electrons flowing through the external circuit to power the device.
What you will learn
- Distinguish primary (single-use) from secondary (rechargeable) batteries
- Describe the Leclanché dry cell and the lead-acid car battery
- Explain lithium-ion operation as Li+ shuttling between electrodes
- Interpret capacity ratings such as mAh
- State safe handling rules, including never disassembling batteries
What a battery is: primary vs secondary
Every battery is a galvanic cell in a package: an anode, a cathode, and an electrolyte, with the redox reaction split so electrons must travel through your device to get from one electrode to the other.
Batteries come in two families:
- Primary batteries are single-use. Their redox reaction is not practically reversible, so once the reactants are consumed, the battery is spent. Examples: alkaline AA cells, zinc–carbon cells, lithium coin cells.
- Secondary batteries are rechargeable. An external power supply drives the discharge reaction backward (electrolysis), restoring the original reactants. Examples: lead-acid car batteries, nickel-metal hydride cells, lithium-ion cells.
Never try to recharge a primary battery — it was not designed for reverse current and can leak or rupture.
The dry cell: the original household battery
The Leclanché (zinc–carbon) dry cell was the original household battery and the ancestor of the modern alkaline cell. Zinc metal is the anode (oxidized to Zn2+), and manganese dioxide is the cathode material (reduced), with a moist ammonium chloride/zinc chloride paste as the electrolyte.
The 'dry' in dry cell just means the electrolyte is a paste rather than a free liquid — it made batteries portable and spill-proof for the first time. Alkaline cells are its improved descendants: same zinc anode, but a manganese dioxide cathode in a potassium hydroxide (alkaline) electrolyte, giving longer life and steadier voltage.
These are primary cells: the chemistry is effectively one-way, so when the zinc is consumed, the cell is done.
Lead-acid: the rechargeable workhorse
The lead-acid battery in cars is the classic secondary battery, invented in 1859 and still dominant for engine starting. Its electrodes are lead (anode) and lead dioxide (cathode), both immersed in sulfuric acid.
Discharge: both electrodes convert to lead sulfate, and the sulfuric acid is consumed (which is why a hydrometer can test charge state — the acid gets more dilute as the battery discharges).
Pb + PbO2 + 2H2SO4 → 2PbSO4 + 2H2O
Recharge: the car's alternator forces the reaction backward, regenerating lead, lead dioxide, and concentrated acid. Lead-acid cells deliver huge surge currents for starting engines, tolerate abuse, and are among the most recycled products in the world — over 95% of a car battery is recoverable.
Lithium-ion: the shuttling ion
Lithium-ion cells power phones, laptops, and electric vehicles. Their clever trick is that lithium itself never plates as metal during normal operation; instead, Li+ ions shuttle back and forth between the electrodes through the electrolyte:
- Anode: graphite, which stores lithium ions between its carbon layers (intercalation).
- Cathode: a metal oxide (such as lithium cobalt oxide), which releases and re-accepts Li+.
Discharging: Li+ flows from the graphite anode to the metal-oxide cathode inside the cell, while electrons flow through the external circuit — that electron flow is what powers your device. Charging reverses both flows.
Because lithium is the lightest metal and the cell voltage is high (~3.7 V per cell), lithium-ion packs more energy per kilogram than older rechargeables — which is also why damaged cells demand respect: never puncture, crush, disassemble, or short-circuit any battery.
Capacity: what mAh really means
Battery capacity is rated in milliampere-hours (mAh) or ampere-hours (Ah): the charge the battery can deliver. A 3000 mAh phone battery can theoretically supply 3000 mA for one hour, or 300 mA for ten hours.
Capacity is not the whole story. Voltage matters too — total stored energy is roughly voltage × charge (watt-hours). A 3.7 V, 3000 mAh cell stores about 11 watt-hours. That is why electric vehicles quote kilowatt-hours: it captures both voltage and charge in one number.
Capacity fades with age and cycles because side reactions slowly consume active material and grow resistive films on the electrodes. Heat and sitting at full charge accelerate the fade; cool storage at partial charge slows it.
Common mistakes
Batteries store chemical reactants; electricity is generated on demand when the redox reaction runs. A 'charged' battery is full of chemicals ready to react.
Its reactants are merely converted to products. Recharging (electrolysis) converts them back — until side reactions and material breakdown accumulate over many cycles.
The architecture is shared (anode, cathode, electrolyte), but the chemistry differs enormously: zinc–carbon, lead–acid, and lithium-ion involve completely different redox couples.
Real-world applications
Choosing the right battery type for a device or vehicle
Understanding why phone batteries degrade over time
Reading capacity ratings (mAh) on consumer batteries
Explaining why car batteries can start an engine in winter
Recycling and safe handling of spent batteries
Frequently asked questions
Why can't I recharge ordinary alkaline batteries?
Recharging forces current backward through a cell designed for one direction, generating gas and heat that can rupture the casing. Only cells engineered as rechargeable (secondary) should ever be recharged.
Does a higher mAh rating mean a more powerful battery?
No. Capacity (mAh) measures stored charge; a higher number means longer runtime, not more power. Power delivery depends on the cell design and internal resistance.
How can I make my phone battery last longer?
Heat, full charges held for long periods, and deep discharges accelerate degradation. Storing at partial charge in a cool place slows it. All lithium-ion cells still lose capacity gradually with age.
How should I dispose of old batteries?
Take them to a battery recycling drop-off — never put them in household trash. Damaged lithium-ion cells can short-circuit and start fires, so tape the terminals and handle them gently.