How to fix a Baofeng USB charger that doesn't charge.

If your Baofeng USB charger base never goes green to indicate the battery has reached full charge, this may apply to you.

Another indication of the problem is the LED stays solid red with no battery inserted, it should flash alternate red/green.

I recently bought a Baofeng which was supplied with a USB charger which is convenient but even when I forgot to unplug it one night and left it on for 2 days it never showed 'green' and checking with a multimeter showed the battery was still under 8V (*never* leave Li-Ion cells/batteries charging unattended.)

Turns out the USB cable has a built in boost converter to bump up the 5V to something high enough to charge a 2S 7.4V pack.

Unfortunately the boost converter in my USB cable only bumped it up to 9.2V and the reverse polarity protection diode (circled red) in the charger base was wasting 0.8V of that so there's no chance it would ever reach charge termination voltage of 8.4V


 

A quick google showed the mains powered chargers come with a plug in PSU that supplies 10V so obviously there's something wrong with the boost circuit I got and, indeed, plugging in a 12V wall wart made the charger base LED flash alternately red/green but the linear charge controller IC got uncomfortably warm to the touch.

This is the offending circuit:


The bit outlined in Yellow is the boost converter, the bit in red controls the LED in the USB plug and seems to be sensing current through the paralleled 2R resistors, the LEDs in the charging base itself are controlled by the charge IC.

To set the output voltage it uses on a simple resistor divider to present 0.6V to the feedback pin on the tiny SOT23-6 chip.

On mine it's 143K from the output to the FB pin and then a 10K to ground.

We can go through all the maths but changing the 10K resistor (outlined in red) to a 9K resistor (or 2 18K ones in parallel) bumped up the voltage to a more useful 10.2V

Trust me, that's 2 18K resistors stacked in parallel (before cleanup)

 And, after the mod, close enough for jazz

:


One fully charged battery (yes, I did monitor the charge cutoff voltage, the LED changed to green at 8.44V by my meter which is close enough for me to be happy):

Success!


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