Li-ion CC/CV charger from an LM317
Li-ion cells must be charged at a limited current (CC) until they reach 4.2 V, then held at that voltage (CV) while the current tapers off. An LM317 sets the voltage; a sense resistor and one transistor steal its adjust current when the charge current tries to rise above 0.5 A.
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How it works
CV: V_OUT = 1.25 × (1 + R2 ÷ R1) = 1.25 × (1 + 560 ÷ 240) = 4.17 V.
CC: Q1 starts to conduct when the drop across R_sense reaches its V_BE (≈ 0.6 V), so I_max ≈ V_BE ÷ R_sense. Q1 only has to divert the few µA that keep ADJ from rising, so its V_BE is low, about 0.70 V: I_max ≈ 585 mA.
Unlike a TP4056 there is no cut-off: in CV the current tapers but never stops. Real chargers end the charge below C/10.
Try it on the bench
- Watch the cell voltage climb while the current holds steady.
- Change R_sense to 2.2 Ω: about 0.27 A, a gentler charge.
Parts on the bench
- Bench power supply
- LM317 / LM338 regulator
- 4 × Resistor
- 18650 Li-ion cell
- NPN transistor
In the course
Electronics Technician, module “Batteries, chargers and USB”.