I need a circuit that tells me that the load is complete, If it is possible that it does not take up much space since it is to be installed in a small space.

Hello friend, I leave you a circuit, the potentiometer is to regulate the integrated supply voltage, you can replace it with resistance. R4 is critical, changing its value will be the voltage limit to which the last LED will turn on (the green one), that value of 6K9 is what you need, you have to do it with 2 or 3 resistors in series, you also have to try With what value fits you better, the green would have to turn on between 16.2 V and 16.5 V for 4 series batteries of that type. Remember that LiOn batteries should not be charged with common chargers.