How Do Lithium-Ion Batteries Charge?

Detailed Explanation

Lithium-ion batteries cannot be charged like a lead-acid or NiMH battery. They require a precisely controlled two-phase charge profile — too much voltage damages the cell irreversibly; too little and you leave capacity on the table. A dedicated charger IC is almost always the right solution.

Phase 1 — Constant Current (CC)

The charger supplies a fixed current — typically 0.5C to 1C — while the cell absorbs charge. At 1C for a 2000 mAh cell, that is 2A. The cell voltage rises steadily during this phase, from its starting voltage (perhaps 3.0–3.5V if partially discharged) up to the charge termination voltage.

Pre-conditioning (trickle charge): if the cell starts below approximately 3.0V — indicating a deeply discharged cell — most charger ICs will apply a reduced trickle current (typically 0.1C) first. This prevents stress on a cell that has been over-discharged. If the cell voltage doesn't recover to the trickle threshold within a timeout period, the charger flags a fault — the cell is likely damaged and should not be charged further.

Phase 2 — Constant Voltage (CV)

Once the cell voltage reaches 4.2V (for standard NMC/LCO chemistry — note that LFP charges to 3.65V), the charger switches to constant-voltage mode. It holds the voltage steady at 4.2V while the current tapers naturally as the cell approaches full charge.

Charging is considered complete when the current falls below the termination threshold — typically C/10 (10% of the rated charge current). At that point the cell is approximately 100% charged and the charger enters a standby state. It does not restart charging until the cell voltage drops below a recharge threshold (typically around 4.0V), which prevents continuous trickle charging that would degrade the cell.

A typical charge profile for a 1000 mAh cell at 500 mA (0.5C):

At 1C (faster), the CC phase is shorter (< 1 hour) but the CV phase is similar. At 0.5C (gentler), the total time is approximately 2.5–3 hours but the cell is stressed less.

Temperature Monitoring

Temperature is the most critical safety variable in Li-ion charging. Standard safe limits are:

• Charge: 0°C to 45°C (cells must not be charged below 0°C — lithium plating occurs, creating dendrites that can cause internal shorts)
• Discharge: −20°C to 60°C (wider than charging; some cells are rated to −40°C)

Most charger ICs provide a TEMP pin expecting a 10kΩ NTC thermistor mounted close to or on the cell. The IC monitors thermistor voltage and suspends charging outside the safe temperature window, resuming automatically when temperature returns to the valid range. This feature must be correctly implemented — floating the TEMP pin, or using the wrong NTC resistance, can disable the safety feature or prevent charging entirely depending on the IC's default state.

Charger IC vs Discrete Implementation

A dedicated charger IC (examples: MCP73831, TP4056, BQ24040, LTC4056, MAX1811) handles the entire CC/CV profile, termination, temperature monitoring, and status signalling in a single package. For single-cell designs, a SOT-23 or DFN charger IC costs under $1 and is the correct approach.

Discrete CC/CV charging using an LDO and a sense resistor is occasionally seen in very-high-volume or cost-constrained designs, but requires more careful design to guarantee proper termination behaviour across temperature and cell tolerance. For any new design, a dedicated charger IC is strongly preferred.

Key charger IC parameters to check in the datasheet:

• Programmable charge current (usually set by an external resistor)
• Input voltage range (must exceed 4.2V + headroom; USB 5V works for most ICs)
• Maximum continuous input current (important for USB VBUS-limited charging at 500 mA or 900 mA)
• Charge termination accuracy (±1% on good ICs; worse on cheap parts)
• Thermal regulation (IC reduces charge current if it overheats — important for compact designs)

USB Charging Profiles

When charging from USB, the available current depends on the USB specification the host port implements:

A charger IC designed for USB inputs includes a current limiter that guarantees the device will not pull more than the source can supply. Without this, plugging a hungry charger into a USB port can brown out the host.

Design Considerations

• Single-cell vs multi-cell: the CC/CV profile applies to each cell independently. Multi-cell series packs require a battery management system (BMS) with per-cell balancing to prevent one cell charging faster than others — an unbalanced series pack is a reliability and safety problem.
• Power path management: in products that must operate while charging (not just charge then use), a "power path" circuit decouples the battery from the system bus. Without it, the system draws from the battery being charged, which interferes with the charger's CC/CV regulation. ICs like the BQ24075 or similar handle this with a separate system output pin.
• Charge current vs thermal: in compact, sealed enclosures, the charger IC's thermal regulation will reduce charge current to stay within its thermal limit. If charge time matters, ensure adequate PCB copper area or a thermal pad connection for the IC's exposed pad. Alternatively, reduce the programmed charge current to stay below the thermal trip point.
• The protection circuit is separate from the charger: the charger IC ensures proper CC/CV charging, but it does not prevent overdischarge during operation, overcurrent from a short circuit, or faults in the battery's own wiring. A protection circuit (or BMS) is a separate component that handles those failure modes.
• Production power management design: combining charger, power path, fuel gauge, and protection into a reliable production design involves multiple ICs with carefully verified interactions. Zeus Design's hardware engineering team designs complete battery power management subsystems for commercial products.

Common Mistakes

• Using a voltage-limited power supply instead of a charger IC: a regulated 4.2V power supply is not a charger — it skips the CC phase, immediately subjects the cell to 4.2V, and has no termination. It will appear to work briefly then degrade the cell rapidly.
• Leaving the TEMP pin floating or incorrectly terminated: charger ICs' default behaviour on an open TEMP pin varies — some inhibit charging (safe but confusing), others skip temperature monitoring (unsafe). Always connect the TEMP pin correctly per the datasheet.
• Not accounting for the headroom between input voltage and cell voltage: a charger IC needs its input to be higher than 4.2V plus the IC's own dropout. At 5V input, most linear-mode charger ICs work fine; at 3.3V input (from a low-voltage system bus), the IC may not have enough headroom to reach 4.2V charge voltage.
• Charging a discharged LFP cell with an NMC charger profile: LFP cells charge to 3.65V, not 4.2V. Applying 4.2V to an LFP cell causes overcharge, gassing, and damage. If a design might ever be used with LFP cells, verify the charger IC's voltage setpoint before committing to the circuit.