Every lithium charger uses CC/CV — constant-current followed by constant-voltage — as its core protocol. It sounds simple. It is not. The CC phase determines charge speed and thermal load; the CV phase determines top-of-charge accuracy and cycle life; the termination condition determines how full the cell actually gets. Get any of the three wrong and you are either leaving capacity on the table or shortening the battery's life.
What happens during the CC phase
The charger delivers a fixed current — typically expressed as a multiple of the cell's rated capacity (C-rate). For a 1,000 mAh cell, 1C is 1,000 mA. During CC, the cell voltage rises from its resting level (typically 3.0–3.7 V depending on state of charge) toward the charge cutoff voltage (4.20 V for standard LiPo, 4.35 V or 4.48 V for HV variants).
The rate of voltage rise during CC is not linear — it accelerates as the cell approaches full charge because the thermodynamic activity of the cathode material changes. The charger sees the cell's voltage and switches to CV when it reaches the cutoff. At this moment the cell is approximately 70–80% full, depending on the C-rate and the exact chemistry.
What happens during the CV phase
Once the charger holds voltage constant at the cutoff, the current it delivers drops exponentially as the cell approaches equilibrium. This slow taper is essential: it allows lithium ions to fully intercalate into the cathode at a rate the structure can accommodate without stress. If you terminate charging at the beginning of CV (i.e., the instant the charger transitions), you get a 75–80% full cell. If you let CV run until the current drops to C/10, you get a 95–98% full cell. If you run it to C/20, you get close to 100%.
Termination current: the trade-off
The termination current is the current threshold at which the charger declares "full" during the CV phase. It directly controls:
- Capacity delivered per charge — lower termination = more energy in per cycle
- Charge time — lower termination = longer time in CV tail
- Cycle life — lower termination = more stress on cathode at full SoC = faster fade
| Termination current | Approximate SoC achieved | Additional time in CV vs. C/10 | Cycle-life impact |
|---|---|---|---|
| C/5 (fast, partial) | ~90% | −15 to −20 min saved | Best |
| C/10 (standard) | ~96% | Baseline | Good |
| C/20 (thorough) | ~99% | +15 to +25 min | Slightly worse |
| C/50 (maximum) | ~100% | +40 to +60 min | Notably worse at high temperature |
For applications where cycle life matters (> 500 cycles), we recommend C/10 termination as the default. For applications where runtime per charge is paramount and cycle count is low (< 200 cycles, e.g. single-use medical devices), C/20 is appropriate.
Fast charging and its implications
Fast charging means a higher CC current — 2C, 3C, or beyond. The physics consequences:
- Higher heat generation. Joule heating during CC scales with current squared. A 2C charge produces 4× the resistive heat of a 1C charge. This matters for enclosures with limited thermal mass.
- Increased lithium plating risk. At high C-rates, graphite kinetics can limit lithium intercalation, leading to surface plating. This is most dangerous above 1.5C below 15 °C. Modern fast-charge protocols use a temperature-vs-rate lookup table to cap current at cold temperatures.
- Voltage polarisation error. At high current, internal resistance drops extra voltage across the cell, making it appear to reach the cutoff sooner than it actually does thermodynamically. This means the CV phase starts earlier and the cell is less full at the start of CV. Paradoxically, fast charging often achieves lower actual SoC in less time than a moderate-rate charge.
Four common mistakes in charger design
1. Charge cutoff voltage that drifts with temperature. The charger's voltage reference and the PCM's overvoltage comparator both have temperature coefficients. If the reference voltage climbs 20 mV with a 20 °C temperature rise, the cell is chronically overcharged in warm environments. Use a temperature-compensated reference or an external precision reference.
2. Missing NTC temperature measurement on the cell body, not the PCB. The cell body temperature lags the PCB temperature during a fast charge by 5–10 °C. If the NTC is soldered to the PCB rather than glued to the cell, the BMS sees a cooler temperature than the cell experiences, and the thermal protection cuts in late.
3. CV phase cut by a timer rather than a current threshold. Firmware that terminates charging after a fixed time in CV (e.g., "if CV phase > 30 min, done") will leave different amounts of charge in the cell depending on the starting SoC, temperature, and C-rate. Use a current threshold, not a timer.
4. No minimum cell voltage on pre-charge. A cell that has been over-discharged below 2.5 V should be pre-charged at C/10 until it reaches 3.0 V before applying normal CC rate. Applying full CC to a deeply discharged cell risks lithium plating and in severe cases, copper dissolution and internal short circuit. Cheap single-chip charger ICs often implement this; verify that your chosen IC has pre-charge mode enabled in its register configuration.