As the State of Charge (SOC) increases, the battery charging current limit decreases in steps. Additionally, we observe that the battery voltage increases linearly with SOC. Here, Open Circuit Voltage (OCV) = V Terminal when no load is connected to the battery. Battery Maximum Voltage Limit = OCV at the 100% SOC (full charge) = 400 V.
However (quoting you): charging at a constant voltage (say 4.2V) so long as the maximum current is limited to a reasonable value for the cell means you will have constant current charger till your cell is at ~95%. Up to this point the voltage across the battery will be less than 4.2V if you measure it.
C/10 and C/30 are common charge termination current limits. When the battery is fully charged, the battery should be disconnected from the charger. Leaving the battery connected to the charger will cause the battery to overcharge and will damage the battery. The 18650 is popular cylindrical lithium cell, with a capacity of 2500 mAh.
You'll lose at least 1.7v from IN to OUT, and another nominal 1.25v from OUT to ADJ, so that's roughly a 3v drop. Your charger will have to be putting out at least 17v to charge the battery up to 14v. A good circuit for battery charging is a constant voltage circuit with current limiting. A few op amps and power transistors can do the whole thing.
The battery cell will have most of its charge when the battery voltage reaches 4.1 V or 4.2 V. At this point, the current going into the battery gradually decreases. When the current drops below a datasheet value, charging should be terminated. C/10 and C/30 are common charge termination current limits.
At this point, the current going into the battery gradually decreases. When the current drops below a datasheet value, charging should be terminated. C/10 and C/30 are common charge termination current limits. When the battery is fully charged, the battery should be disconnected from the charger.