connecting a CC/CV module for a battery pack

[Adhith]

hello friends,
I was planning to charge a 4S1P Li ion battery pack with a CC/CV buck module through a BMS board. but as soon as the connection to the input of the module is done and connecting the +ve output of the module to the P+ of the BMS and the -ve output is not connected to the P-, the one of light (green light indicating the full charge status) on the board is turned on without even powering the input. I strongly think that its the reverse current form the battery. or esle what what would be the problem?? I'm attaching the pic of the module below. Thank you in advance

[Buriedcode]

.. the one of light (green light indicating the full charge status) on the board is turned on without even powering the input

Well, what makes you think that light is an indicator for "full charge" ?  I believe that module is indeed a CC/CV power supply, but not necessarily a battery charger - to qualify for that it would have to have a timer, and to detect reduction in current drawn whilst on constant voltage (often when I <C/10 for lipo batteries). 

I suspect the LED's are for "power", "CC" (constant current), and CV (constant voltage).  Perhaps the green LED is simply "power" and is connected to the converters output, which means when you connect a battery across the output, it lights the LED.  It has no way of knowing when a battery is "fully charged".   You could trace out a schematic to see whats happening.

So in a way, yes you are right, it is "reverse current from the battery" - the battery is supplying power to the LED.

A word of safety.  Often these modules don't have a particularly accurate regulation, which is would need for charging lithium batteries (<1% accuracy is often quoted as required).  So, make sure you use a decent voltmeter/multimeter to set the voltage limit/constant voltage value to your cells max voltage (4.2V for lithium polymer).

[Adhith]

Thank you very much for your reply. Its a CC/CV buck module which also can be used as a battery charger since its written in the specification and uses in the website from which I brought it. The green light indicated fully charge status. when not fully charge the blue and red light lights up. The charging will be cut off when the current draw is less than the 1/10 of the charging current. However I connect a schottky diode at the positive output to prevent the backward flow and it works fine and charging is proper.

[Buriedcode]

Then it is working correctly?

As I said, I suspect some of the circuitry is probably powered by the output, which means without any input, the LED will draw its power from the battery connections.

Adding a schottky to the output of the module will ruin the voltage regulation, since its feedback is taking before the diode.  This means the output voltage will be a function of current, as well as temperature (diodes forward voltage is dependent on these two) which makes it somewhat dangerous for charging lithium batteries.  I would leave the diode out, as I doubt it draws that much current if left connected to the battery.

I would trace out a schematic from the module, shouldn't be too difficult, and you can see how it works, and possibly modify the board to obtain the functionality you required.

Also, just because a website claims it is specifically for battery charging, doesn't always mean that is the case.  Sure, most websites are fairly accurate in their description, but from ebay/aliexpress/amazon often descriptions are just copied/pasted from elsewhere by those who dont' know much about electronics.  It certainly looks like that module uses one opamp for constant current, and one for current cut-off (fully charged) but without tracing out a schem,  I dont' know.

I believe I have a found a schem, which does indeed show the low current cut-off indicator:
https://www.aliexpress.com/item/DC-DC-LM2596-Step-down-Adjustable-Power-Supply-Module-CC-CV-LED-Driver/32667762137.html

If you follow the paths to the LED's I believe the rightmost LED could possibly be powered by the output, but its hard to tell.