Power & Energy
How to charge a lithium-ion battery
A lithium-ion cell is charged in two stages, called CC-CV. First constant current: the charger pushes a steady current, often 0.5 to 1C, and the cell's voltage rises. Then, at 4.2 V, constant voltage: the charger holds the voltage steady while the cell's current dies away. When the current has fallen to about a tenth of its start, the charger switches off.
Constant current first
An empty lithium cell can't just be connected to a 4.2 V supply: it would gulp far too much current. So the charger starts as a current source, pushing a fixed current whatever the cell's voltage. The cell fills at a steady rate and its voltage climbs.
The current is set in C-rate: 1C is the current that would fill the cell in about an hour, 2 A for a 2000 mAh cell. Many cells take 0.5C to 1C; check the datasheet.
Live experiment
Constant current, then constant voltage
A charger pushes 25, 50 or 100 mA into a stand-in cell from 3.0 V. With no limit it sails straight past full at 4.2 V. Put a 3.9 V zener across it and the cell levels off near 4.1 V while its own current dies away: CC, then CV.
“Filling a cell safely” is lesson 2 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Then constant voltage
A current source never gives up. Leave it on and the cell sails past full. Overcharged, a lithium cell plates metal inside, swells, and can catch fire. So at 4.2 V the charger stops increasing the voltage and holds it there. The cell keeps filling, ever more slowly, and its current falls away.
That's why "fast charge to 50% in 30 minutes" is quoted: it's the constant-current stage. The last 20% always takes longer.
What a real charger does
A charger chip, like the TP4056 on many hobby modules, does all of this:
- Pre-charge: a deeply flat cell, below about 3 V, gets a small current until it recovers.
- Constant current, set by one resistor.
- Constant voltage at 4.20 V, held to about 1%.
- Termination when the current falls to about a tenth.
- Temperature checks: no fast charging when the cell is cold or hot. Charging below 0 °C damages lithium cells.
Discharging needs care too: running a lithium cell below about 3.0 V can damage it for good, which is why packs carry a protection circuit with an under-voltage cut-off.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Filling a cell safely” is lesson 2 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Or start from the very first lesson, no account neededWork it out
Practice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: Rises steadily. A steady current fills the cell at a steady rate, so its voltage climbs.
- Why not “Stays at 4.2 V”? The cell only reaches 4.2 V at the end of the constant-current stage. Before that its voltage is climbing.
- Why not “Jumps straight to 4.2 V”? A cell's voltage rises with its charge, so it climbs gradually rather than jumping.
- Why not “Falls”? A steady current into the cell adds charge, so its voltage goes up, not down.
Show the answer
Answer: Falls towards zero. The voltage is held, so the cell fills ever more slowly and its current dies away. The Zener takes the rest.
- Why not “Rises”? With the voltage held, the cell fills ever more slowly, so its current falls.
- Why not “Stays at 50 mA”? Staying at the charging current would push the cell past full. Holding the voltage makes the current fall.
- Why not “Reverses”? The charger is still holding the voltage at full, so current doesn't flow back out of the cell.
Show the answer
Answer: 2.5 A. 1C empties the cell in one hour: 2500 mA, or 2.5 A. Half of that, 0.5C, is 1.25 A.
- Why not “2500 A”? That mixes up milliamps and amps. 2500 mA is 2.5 A, not 2500 A.
- Why not “1 A”? 1 A would be 0.4C for this cell. 1C is the current that empties it in one hour: 2500 mA.
- Why not “25 mA”? 25 mA would take 100 hours to empty a 2500 mAh cell. That's 0.01C.
“Filling a cell safely” is lesson 2 of 6 in Solar Sound, Power & Energy World 3. Free with an account. Lessons unlock in order, so you’ll do the 15 before it first.
Questions people ask
What is CC-CV charging?
Constant current, then constant voltage. The charger first pushes a fixed current, often 0.5–1C, while the cell's voltage rises. At 4.2 V it holds the voltage steady, and the current falls away as the cell fills. Charging stops when the current drops to about a tenth of its start.
What voltage should a lithium-ion cell be charged to?
Most lithium-ion and lithium-polymer cells are full at 4.20 V, held to within about 1%. Going higher shortens their life and can be dangerous. Some chemistries differ: lithium iron phosphate cells stop at about 3.6 V. Always check the datasheet.
Why does the last 20% take so long to charge?
That's the constant-voltage stage. With the voltage held at 4.2 V, the current falls steadily as the cell fills, so each percent takes longer than the last. Fast-charge claims like 50% in 30 minutes describe the constant-current stage.
Can I charge a lithium battery with a normal power supply?
Not safely on its own. A lithium cell needs the current limited, the voltage held precisely, charging stopped at full, and temperature checked. Use a proper charger chip, like the common TP4056 modules, and a cell with a protection circuit.
Related topics
- How to calculate battery life from mAhRuntime = capacity ÷ current: mAh, watt-hours, C-rate, and why real batteries give a little less.
- Batteries in series vs parallelChain cells for more volts, put them side by side for more hours, and never mix a tired one with a fresh one.
- How does a Zener diode work?A diode built to conduct backwards at a set voltage, then hold it: the simplest regulator.
- Reverse polarity protection explainedStop a backwards battery wrecking a circuit: the simple diode, and the MOSFET trick that costs almost nothing.