BORON charge led blinks when 5V present in VBUS

I am trying to charge the battery connected directly to BORON’s LiPo connector via 5V source from a connector as shown below.
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I see 5.04V at +5V pin and VBUS shows about 4.6V.

When 5V is present, I see the yellow led on the side of micro USB connector on BORON blinks about once in 500ms.

But when powered by USB connector, the led become constant by which means it is continuously charging.
Anyone can point out why it is just blinking and does not charge?

Do I have to explicitly add below code to enable charging via VBUS line?

  pmic.begin();
  pmic.setInputVoltageLimit(5080);  //  for 6V Solar Panels   
  pmic.setChargeVoltage(4208);      //  Set Li-Po charge termination voltage to 4.21V,
  pmic.enableDPDM();  
  pmic.enableBuck();                //  enableBuck required when using EN Pin Shutdown ??.
  pmic.enableCharging(); 

When you say:

This means, to me, you are supplying too high a voltage directly to the Li+ pin (4.2V max). The led flashes yellow showing this issue because you are bypassing the power management IC with 5V. Here is a thread about it:
https://community.particle.io/t/power-supply-to-lipo-plug-instead-of-vbus/47869/16

Thanks for your reply.
As in the schematics, I supply 5V to VBUS only and not to Li+ pin.
When measured Li+ pin, I see voltage around 3.8V
That’s why I am confused.

What is your power source? Power adapter or solar? And what is the capacity (in watts or mA)?

When the input is a small solar panel you can get that behavior because when charging turns on, it defaults to more mA than the panel can supply, which causes the voltage to drop, which turns off charging, but with charging off, the voltage recovers, and its keeps turning on and off forever. Reducing the input current limit usually helps with this.

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