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I opened a cheap USB single-cell lithium-ion battery charger, but can't figure what the Q1 is. It only says 'CR' on the surface. When charging, I measure about 0.7 V between its pins 1 and 2 (those on the same side), so maybe it plays a role of a diode that drops the voltage from 5 V to 4.3 V, which is approximately the full charge voltage of the 3.7 lithium cell.

I can also measure with a multimeter in diode mode that it behaves similarly to a PNP: it shows about 0.7 V between pins 3-1 and 3-2, hinting that pin 3 is the base. But then it shouldn't create a voltage drop between emitter and collector when active. Or maybe it's just a custom component?

enter image description here

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    \$\begingroup\$ Seems it charges directly through big 3R9 for current limiting. The designer most probably count with battery with protection circuit inside. \$\endgroup\$ Dec 7, 2023 at 21:54

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That Q1 is an NPN transistor.

And that circuit should not be used to charge lithium batteries. It will overcharge them, overcharging will damages the batteries, and damaged batteries might explode or burn and start a fire.

When battery is empty there will be charging current large enough through the larger 3.9 ohm resistor to have more than 0.7V over the transistor VBE so transistor is on and it turns on the LED.

When battery voltage rises up from charging, the charging current drops, and the voltage over the 3.9 ohm resistor drops below 0.7V and the transistot turns off and LED turns off.

But still the battery is being charged, and at that point, there will already be 4.3V on the battery, so overcharging happens.

You might want to stop using such dangerous charger. The batteries will not last long due to the charger causing they to degrade.

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  • \$\begingroup\$ +1 Scary, but unfortunately not surprising. \$\endgroup\$
    – John D
    Dec 7, 2023 at 21:53

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