Li-ion Charger (1 cell)

Charges a single lithium cell safely from 5 V (USB) — running the CC/CV profile every lithium cell requires: constant current while the cell is empty, smoothly handing over to a constant 4.2 V as it fills, then tapering off to done.

• U1 (TP4056-class) — the whole charge brain. It measures the cell at BAT and decides which phase you're in. The cell is safe because this loop never lets it see more than 4.2 V or more than the programmed current.
• R2 — the one resistor that sets the charge current: Icharge ≈ 1200 / R(PROG). The 1.2 kΩ here means 1 A. Swap it to match your cell — never charge faster than 1C (a 500 mAh cell wants ≤ 500 mA → 2.4 kΩ).
• R1 — pull-up for CHRG, the open-drain status output: low while charging, releases when full. Wire it to an LED or a GPIO.
• TEMP is tied to ground here, which disables battery-temperature monitoring — fine for a bench project at room temperature; for anything enclosed or outdoors, wire a 10 k NTC against the cell instead.

It's a linear charger: simple and quiet, but it burns (Vin − Vbat) × Icharge as heat — about 1 W into an empty cell at 1 A. That's why the chip gets warm; it's supposed to (it thermally self-limits).

Exposes: vin (5 V), batt (the cell, 4.2 V max), chrg (status), gnd.

⚠ Protected cells only, or add a protection circuit: this block charges correctly but does NOT protect against over-discharge or shorts on the battery side. Most bare 18650s need that protection; most pouch cells with a small PCB already include it.