Inside the cell
A battery stores energy in chemicals. Cut a lithium-ion cell open and you find three parts: a negative electrode (the anode), a positive electrode (the cathode) and an electrolyte between them. Electrode means a solid that charge enters or leaves.
Connect a lamp and the cell starts to discharge. Each lithium atom in the anode splits into a positive ion and an electron. The ions drift through the electrolyte and the separator into the cathode. An ion is an atom with an electric charge.
The electrolyte blocks electrons, so they must go round the outside. They leave the anode at the − terminal, flow through the wire and the lamp, and enter the cathode at +. That flow is the current that lights the lamp.
Volts and capacity
The push (the voltage) comes from the chemistry, not the size. A lithium-ion cell gives a nominal 3.6 to 3.7 V; an alkaline AA gives 1.5 V. The voltage sags slowly as it empties: about 4.2 V full, about 3.0 V empty.
Capacity says how much charge a cell holds. 3000 mAh (milliamp-hours) means 3000 mA for one hour, or 300 mA for ten hours. Energy also counts the push: 3.7 V × 3.0 Ah ≈ 11.1 Wh (watt-hours).
C-rate compares the current with the capacity. 1C would empty a full cell in about an hour: 3.0 A for this 3000 mAh cell. 0.5C (1.5 A) takes about two hours; 2C (6.0 A) about half an hour. Faster means more heat.
Charging
The charger pushes electrons back into the anode, and the lithium ions follow them home. First it holds the current steady (constant current, here 1.5 A, or 0.5C) while the voltage climbs toward 4.2 V.
At 4.2 V the charger switches: it holds the voltage steady and lets the current fall. When the current drops to a few percent of 1C, the cell is full and the charger stops. Lithium-ion takes no trickle charge.
Safety and packs
Keep pushing past 4.2 V, short the terminals or damage the cell, and unwanted reactions start that make heat. Heat speeds those reactions up, which makes more heat. This loop, called thermal runaway, can end in fire.
So every lithium-ion pack carries a protection circuit. It watches the voltage, the current and often the temperature. If any goes past its limit, it opens a switch in the cell lead (two transistors) and the current stops.
Cells join into packs. In series (+ to −) the voltages add: two give 7.4 V with the same 3000 mAh. In parallel (+ to +) the capacities add: 6000 mAh at 3.7 V. Either way the energy doubles.
In detail
;; step 1/11 · parts Inside one lithium-ion cell anode (−) graphite (carbon) on copper foil separator a porous plastic film: ions pass, the electrodes never touch electrolyte a liquid with a lithium salt: carries ions, blocks electrons cathode (+) a lithium metal oxide (for example cobalt oxide) on aluminium foil Charged: the lithium sits in the anode (yellow dots).
The cell is a teaching model: 3000 mAh, 0.1 Ω inside and an illustrative voltage curve between 3.0 V empty and 4.2 V full. Real cells vary with their chemistry and maker. Lithium-ion nominal voltage is 3.6 V (some makers quote 3.7 V) and full charge is 4.20 V; an alkaline cell is 1.5 V (Battery University BU-303). Charging is constant current, then constant voltage at 4.20 V, ending when the current falls to 3 to 5 percent of the Ah rating; lithium-ion takes no trickle charge (Battery University BU-409). The dots on the wires are electrons, which flow from − to + outside the cell. Conventional current is drawn the other way.