How to designing a battery charging indicator circuit for 18650 battery pack

The concept is simple enough; a low value resistor in series with the charge controller, then sense the voltage across that with a transistor base-emitter junction (and base limiting resistor) or some form of voltage comparator.

That then gives you the "full charge" point, when the voltage drop across the resistor reduces and the the transistor turns off.

Use the transistor output to run the flasher circuit and intermittently turn off the LED (that runs from the charger input).

The complexity is that it must be before the charge regulator, or designed in to it.
(Otherwise the voltage drop will mess up the charging voltage).

Also from your other post, note that a BMS board is NOT a charge controller or limiter.
The charge controller needs both precise voltage control and current limiting.

Plus, for any setup whatsoever that uses series connected lithium cells, you MUST have a balance controller of some sort to prevent one overcharging if the charge state is not exactly equal.

Any error whatsoever in charging lithium cells can cause them to burst or catch fire; sometimes immediately, sometimes hours later.

Analog Devices (Linear Technology, Maxim, and ADI) and TI make lotsa LI charger controllers. I strongly recommend that you use one of these devices for the actual charging. Sony is still paying insurance claims for their batch of bad chargers.

ak

What is charging the battery that you are monitoring?