100nF

What a LiPo charger does: constant current, then voltage

A Li-ion charger is a current source that becomes a voltage source: 1C to 4.2 V, then 4.2 V held until the current falls to 0.1C, with a precharge below 2.5 V.

By batterieshardware

A lithium-ion cell — and a LiPo pouch is a lithium-ion cell in a soft case — is charged by a rule that fits in one sentence: push a constant current into it until it reaches its full-charge voltage, then hold that voltage and let the current fall, and stop when the current is small. Everything a charger IC does is that rule with limits around it, and everything that goes wrong with a home-made charger is a limit missing. TI’s SLAA287 sets the rule out stage by stage for a 500 mAh cell, and this article follows it; the Li-ion charge calculator gives the currents, an estimate of the time, and what a linear charger dissipates, for any capacity and rate.

C-rate, the unit everything is in

SLAA287 defines it first: “battery capacity (C), expressed in milliamp-hours (mAh), is a measure of battery life between charges. Battery current has the units of C-rate. For example, for a 500 mA-h battery, the current corresponding to 1C is 500 mA and the current corresponding to 0.1C is 50 mA.” A charge rate of 1C would, if the cell took it all the way, fill it in an hour; 0.5C in two. Cell datasheets quote their maximum charge current in the same unit, and it is the number to look up before choosing a charger — most consumer cells accept 0.5C to 1C, and “what current to charge LiPo” has that answer: the cell’s rated C, which is usually 1C or less, never the discharge rating printed large on the pack.

The three stages

Two traces against time over about one and a half hours. Current: a short low step of 50 milliamps, then a flat 500 milliamps for most of the time, then an exponential fall to 50 milliamps where charging stops. Voltage: 2.5 volts rising through 3 volts to 4.2 volts during the constant-current phase, then flat at 4.2 volts.
Fig 1 — SLAA287's three stages for a 500 mAh cell: "precharging stage using a current of 0.1C" (50 mA) while the cell is below 2.5 V, "constant current charging stage using a current of 1C" (500 mA) until 4.2 V, then constant voltage until "the charging current falls below 0.1C". The times are the calculator's model — SSZTA38's "CC is used roughly for the first 67 % of charging" and an exponential taper — 50 min of CC and 25 min of CV, about 1.25 h; the voltage curve during CC is drawn, not computed.

SLAA287: “a Li-ion battery charging process can include three stages”:

Precharge. “During the slow charge stage, the battery is charged with a constant low charge current of 0.1C if the battery voltage is below 2.5 V.” A cell that deep is either damaged or has been over-discharged and is not safe to hit with full current; the trickle brings it back to where the fast charge may start. The note adds that “Li-ion batteries do not suffer from memory effect and do not need to be fully discharged before recharging. The slow charge stage is rarely used” — in normal use the cell is never that low.

Constant current. “Most Li-ion batteries have a fully charged voltage of 4.1 V or 4.2 V. The battery is first charged with a constant current of 1C until the battery voltage reaches 4.1 V or 4.2 V.” This is where the energy goes in; SSZTA38 puts it at “roughly the first 67 % of charging, when most of the energy transfers from the charger to the battery”.

Constant voltage. “When the battery’s voltage reaches 4.1 V or 4.2 V, the charger switches to constant voltage charging mode. The battery is then charged with a constant voltage source at a fixed battery voltage … During this process the charging current starts to fall due to internal cell resistance. When the charging current falls below 0.1C, the charging process must stop.” The tapering current is the cell’s own: as its internal voltage approaches 4.2 V, the difference driving current through its internal resistance shrinks.

The stop matters more than anything else in the profile. “If charging continues when the battery is fully charged, most of the electrical energy is converted to thermal energy. Overcharging batteries can cause overheating or explosion due to outgassing of the electrolyte and can severely reduce battery life. Li-ion batteries are extremely sensitive to overcharging, and it is critical that the final voltage be controlled to within ±50 mV of 4.1 V or 4.2 V.” SLAA287 lists three ways to decide the cell is full — “when the current drops to 0.1C during the constant voltage charging stage”, a temperature measurement “to avoid overheating”, and a safety timer, “if the charging time is longer than a predetermined time” — and a real charger uses all three.

What to charge to

The number is the chemistry’s. SLUAAR1’s table gives the charge voltage as 3.9–4.2 V per cell for Li-ion and 3.5–3.65 V for LiFePO4, against nominal voltages of 3.6 V and 3.2 V, and the same window for both: “charge temp 0 °C to 45 °C”. A 4.2 V charger on a LiFePO4 cell is an overcharge by half a volt; a 3.65 V charger on a Li-ion cell stops at a fraction of its capacity. The ±50 mV is why charger ICs carry a precision reference rather than a resistor divider: SNVA557’s 3-cell example needed “a required precision of better than ±1 % over temperature” on 12.6 V and found that “if 1 % resistors are used in a divider, they will contribute about 1.4 % total error to the set voltage”, before the reference’s own tolerance — hence a trimmer, or a purpose-made controller. And below 4.2 V there is a trade the table hints at with its 3.9–4.2 V range: charging to a lower voltage gives up capacity and buys cycle life.

How long it takes

Not one hour at 1C. The constant-current phase delivers most of the charge, but the cell reaches 4.2 V before it is full, and the taper is slow. SSZTA38’s split — CC “roughly for the first 67 % of charging”, CV “the last 33 % of the remaining charging time” — with the current in the CV phase falling exponentially from 1C to the 0.1C stop, and the total charge equal to the capacity, gives about 50 minutes of constant current and 25 of constant voltage for the 500 mAh cell: 1¼ hours at 1C, 2½ at 0.5C. That is the calculator’s model, stated as one; a cell’s real curve depends on its internal resistance and temperature, and a charger that terminates at a higher current finishes sooner with less in the cell.

What a linear charger burns

Plot of power dissipated in a linear charger against time for the same charge from a 5 volt input. It starts at 1 watt when the cell is at 3 volts and the current 500 milliamps, falls as the cell voltage rises to 4.2 volts, then falls further with the current during constant voltage.
Fig 2 — A linear charger drops the difference between its 5 V input and the cell across its pass element: (V_in − V_cell) × I. At the start of the constant-current phase that is (5 − 3) V × 500 mA = 1.00 W, falling as the cell voltage climbs and again as the current tapers. SLAA287 avoided it with a buck converter; a linear charger IC pays it as heat in its package.

SLAA287 built its charger around a buck converter, and its arithmetic shows why a linear one is hot: the buck’s output during constant current had to be “VO = Vbat + Ibat × Rsense = 4.2 + 500 mA × 0.75 = 4.575 V”. A linear charger from a 5 V USB rail drops the whole difference between the input and the cell across its pass transistor: at the start of the fast charge, with the cell at 3 V, that is 2 V × 500 mA = 1 W, falling to 0.4 W as the cell reaches 4.2 V and then away with the current. A watt in a small package is a junction temperature question, and the LDO thermal calculator is the same sum for a pass element; it is also why single-chip linear chargers throttle their current with die temperature rather than deliver 1C into a cold cell from a hot package.

The limits around the rule

  • Temperature. 0 °C to 45 °C for charging (SLUAAR1); SLAA287’s firmware stops “if the battery temperature exceeds 40 °C”. Charging below 0 °C plates lithium; a thermistor in the pack is the sensor, and the NTC thermistor calculator turns its resistance into a temperature.
  • A timer. SLAA287’s third termination method. A cell that never reaches 0.1C — because it is damaged, or because the load is drawing from it — would otherwise sit at 4.2 V indefinitely.
  • Termination, not float. Unlike lead-acid, a lithium cell is not held at its charge voltage; after termination the charger disconnects and restarts only when the voltage has sagged. SSZTA38 notes that some chargers “pump small currents … during CV to account for discharging currents”, which is a design choice with a cost in cell life.
  • The protection circuit. SLUAAR1’s table: Li-ion is “fragile: requires protection circuit for safe operation”. The charger’s limits and the pack’s protection are two independent lines of defence, and the second one is not optional because the first exists.

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