Battery Charger Design Guide
CC/CV Profiles, Charger IC Topologies & Thermal Management
1. Introduction — Charging Is Not “Apply Power”
A lithium-ion cell stores energy electrochemically; how you push charge into it decides safety, cycle life, and whether the pack survives. A naive “constant power until full” approach overcharges and can lead to thermal runaway. Proper charging is a state machine: trickle, constant current, constant voltage, and termination — each with its own current/voltage/thermal constraints. This guide covers the standard Li-ion charge profile, the two dominant charger IC topologies (linear vs. switch-mode buck), how to pick charge current, how to manage heat with JEITA and thermal derating, NTC temperature control, and how the charger cooperates with the protection circuit.
2. The Li-ion Charge Profile (CC/CV)
1. Precharge / trickle (CC, low current): cell below ~2.8–3.0 V → charge at 0.05–0.1 C until above the precharge threshold.
2. Constant Current (CC): charge at the programmed Ichg (e.g., 0.5–1 C) while voltage rises toward 4.2 V.
3. Constant Voltage (CV): hold 4.2 V (±1%), current decays exponentially as SOC → 100%.
4. Termination: stop (or drop to maintenance) when I falls to ~0.1 C (e.g., 50–100 mA for a 1 Ah cell), or after a timeout.
Time split: at 1 C, CC phase delivers ~70–80% capacity; the CV tail can take 20–30% of total charge time for the last few percent. Termination matters: charging indefinitely at 4.2 V float reduces cycle life; most chargers terminate and periodically top-up.
| Phase | Cell condition | Control | Typical current | Purpose |
|---|---|---|---|---|
| Trickle (precharge) | V<2.8–3.0 V | CC at low I | 0.05–0.1 C | Safe recovery from deep discharge |
| CC | 3.0–4.2 V rising | Current regulation | 0.2–1 C | Bulk energy delivery |
| CV | At 4.2 V | Voltage regulation | I decays to ~0.1 C | Top-off, avoid overvoltage |
| Terminate | Full | I threshold / timeout | Stop charge | Protect cycle life |
2.1 Why the CC/CV shape?
In CC the cell absorbs charge fastest within its safe current limit; as voltage nears the ceiling the cell’s internal resistance grows and current must fall to avoid plating lithium or driving the cell above its maximum voltage. CV holds the ceiling while the cell equilibrates. The crossover current for termination (typically C/10) is a compromise: too high leaves the cell only ~95% full, too low extends charging and risks overcharge drift.
3. Charger IC Topologies — Linear vs. Switch-Mode
| Property | Linear charger | Switch-mode (buck) charger |
|---|---|---|
| Efficiency | Low: (Vchg/Vin) — 60–85% typical | High: 85–95% |
| Heat | Ploss = (Vin−Vbat)·I — direct drop | Low — only conduction/switching losses |
| BOM / area | Minimal (a few caps, no inductor) | Inductor + larger caps, EMI care |
| Input range | Vin slightly above Vbat (USB 5 V ok for 1S) | Wide (5–24 V+ for 1–4S) |
| Best for | Low current (<500 mA), small devices, cost | Fast charge (>1 A), high input voltage |
Pdiss = (Vin − Vbat) · Ichg
USB 5 V, 1 A into a 3.7 V nominal cell: Pdiss = (5 − 3.7)·1 = 1.3 W. In a 4 mm × 4 mm QFN with θJA ≈ 40 °C/W, junction rise = 52 °C → Tj ≈ 100 °C at 48 °C ambient. Fix: reduce charge current, improve PCB copper, or move to a buck charger (Ploss ≈ (1−η)·Pout ≈ 0.1·3.7 ≈ 0.37 W).
4. Choosing Charge Current
- Cell rating: standard Li-ion allows 0.5–1 C; high-drain cells allow 2 C+; always check the cell datasheet’s maximum charge current and temperature window.
- Connector/board limit: USB 2.0 = 500 mA, USB BC1.2 = 1.5 A, USB-C PD = up to 3 A/5 A; the charge current must not exceed the negotiated input capability or connector ratings.
- Thermal budget: linear chargers are thermally limited; compute Pdiss at the worst-case Vin−Vbat and keep Tj < 120 °C (derate at high ambient).
- Fast charge trade-off: 2 C halves charge time but adds heat, reduces cycle life, and raises the demand on the protection/battery pack design.
- System load sharing: if the system draws current while charging, the charger’s current sense sees input current, not cell current — most ICs offer a system-load path (PATH/USB) so the charge current is properly budgeted.
5. Thermal Management (JEITA and Derating)
5.1 JEITA guidelines
JEITA (Japan Electronics and Information Technology Industries Association) defines reduced charge limits at temperature extremes:
| Temperature | Charge voltage limit | Charge current limit |
|---|---|---|
| 0–5 °C | 4.1 V | 0.2 C |
| 5–10 °C | 4.1 V | 0.5 C |
| 10–45 °C | 4.2 V | 1.0 C (full) |
| 45–60 °C | 4.1 V | 0.5 C |
| Below 0 °C | Charging prohibited (Li plating risk) | 0 |
Charging a cold lithium cell at high current causes lithium plating on the anode — permanent capacity loss and a safety hazard. Charge ICs with JEITA support (via NTC and register settings) automatically reduce current/voltage by temperature band.
5.2 Thermal derating
Above a die temperature threshold (e.g., Tj = 110 °C), the charger linearly reduces charge current to protect itself and the system. When the PCB copper is inadequate, derating engages early and “fast charge” silently becomes slow charge — the classic “it charges in 3 hours instead of 1” complaint. Design the board for the thermal load first: wide low-resistance traces, thermal vias to an inner/back copper plane, and keep the charger away from hot sources (MCU, radio PA).
6. The Charger State Machine
// State machine typical of a Li-ion charger with NTC + JEITA
enum CHG_STATE { PRECHARGE, CC, CV, DONE, FAULT } state = PRECHARGE;
while (1) {
float v_bat = read_cell_voltage();
float i_bat = read_charge_current();
float t_cell = read_ntc(); // °C via NTC on TS pin
if (t_cell 60) { halt_charge(); state = FAULT; continue; }
switch (state) {
case PRECHARGE:
set_current(0.05 * C_RATE);
if (v_bat >= 3.0) state = CC;
else if (precharge_timeout()) state = FAULT; // cell damaged
break;
case CC:
set_current(pick_jeita_current(t_cell)); // derate by temp band
if (v_bat >= 4.2) state = CV;
break;
case CV:
set_voltage(4.2);
if (i_bat <= TERM_I) state = DONE; // ~C/10 termination
break;
case DONE: /* stop, optional top-up timer */ break;
case FAULT: /* wait for input removal */ break;
}
}
Robust firmware also implements: safety timers per phase, NTC open/short detection (a bad thermistor must fail the charge, not pass it), input over-voltage detection, and re-charge thresholds (typically restart CC at ~4.0–4.05 V to avoid oscillation between DONE and CC).
7. NTC Temperature Control
- Sense location: the thermistor must be in thermal contact with the cell (inside the pack or taped to it), not on the charger IC die.
- TS pin network: a resistor divider from the NTC sets the window thresholds (cold/hot); choose RNTC value so the divider triggers at the JEITA band edges.
- Hardware fail-safe: many ICs treat an NTC short-to-GND (very hot) or open (broken wire) as a fault and stop charging — verify the fail direction is safe (never “keep charging” on fault).
- Multiple cells: monitor the hottest cell; parallel strings need care — charge current divides unevenly if cells are imbalanced, which is why single-cell is preferred for simple designs.
8. Working with the Protection Circuit
The charger and the battery protection IC (e.g., DW01 + MOSFET pair) form a two-layer safety system:
- Overvoltage (OV) / overcurrent (OC) / short: the protection circuit is the hard backstop; the charger should never rely on it for normal operation.
- Undervoltage lockout (UVLO): below ~2.8 V the protection FETs open (protection mode) — the charger’s precharge phase must first push the cell above the recovery threshold, and the protection IC must support “charge mode” reactivation.
- Voltage mismatch at first contact: if the cell is in protection mode (0 V at the pack terminals) the charger sees an open, not a short — a “recovery” charging path may be required to exit protection mode before normal CC can begin.
- Current sense sharing: some designs put the charge current sense across the protection FET’s RDS(on); calibrate it, as RDS(on) drifts with temperature.
9. Common Mistakes
- No precharge path — a deeply discharged cell is charged at full current, risking heat and plating; always trickle below the precharge threshold.
- No termination — floating at 4.2 V forever shortens cycle life; implement C/10 (or 0.1 C) termination plus a safety timeout.
- Undersized thermal pad — linear chargers thermally derate and never reach the advertised current; design copper for Pdiss.
- Charging in the forbidden band — <0 °C charging without JEITA derating causes irreversible lithium plating.
- NTC connected to the wrong location — thermistor on the board rather than the cell reports board temp and disables real protection.
- Charger output permanently connected to a running system — without load-sharing, the charger sees combined current and may overcharge the cell; use the system-load path.
- Ignoring input inrush — hot-plugging USB into a discharged charger causes inrush; add input capacitance and, where needed, inrush limiting.
10. Frequently Asked Questions
Q1. Why does my charger spend so long in CV?
The CV tail is exponential; the last 10–20% of capacity takes as long as the bulk 80%. If you terminate at C/10 you cut this time; charging to exactly 100% always costs time. For “quick” charging use a higher termination current and accept ~95% SOC.
Q2. Can I charge at 2 C safely?
Only if the cell is rated for it (check max charge current and temperature window), the charger IC supports it, and the thermal budget is met. High C-rate charging raises temperature and reduces cycle life; it also demands a proper JEITA temperature window.
Q3. Linear or buck charger for my product?
For a 1S cell from USB 5 V at ≤500 mA, linear is simple and cheap. Above ~1 A or for a higher input voltage (e.g., 12 V input), a buck charger saves heat and enables fast charge — at the cost of an inductor, layout care, and EMI.
Q4. What happens if the NTC wire breaks?
A broken NTC typically reads as a very high resistance; the charger must detect this as a fault (thermistor open) and halt. Check your IC’s fail-safe direction — it must fail to “stop charging,” never “keep charging.”
Q5. Should I terminate charging or keep it at 4.2 V float?
Terminate. Continuous float at 4.2 V accelerates capacity fade. After termination, most designs either do nothing until the cell drops to ~4.0 V, or periodically top up.
Q6. My charger gets hot at 1 A — what’s wrong?
For a linear charger from 5 V, 1 A into 3.7 V means ~1.3 W dissipated. This is expected for the topology. Reduce Ichg, improve PCB thermal path (vias/copper), or switch to a buck charger — the silicon isn’t broken, the thermal budget is.