CC and CV mode implementation

[Ravenghost]

I'm working on a project where i need to charge multiple li-ion cells. I'm thinking of buck converter circuit powered by ac-dc 24V 1A wall adapter and controlled by Atmega328P MC PWM. I've covered theory behind li-ion battery charging, buck converters, PWM. I believe constant voltage mode implementation is quite straight forward. A simple voltage divider to get feedback voltage to ADC and Rsense resistor with current sense amp to ADC adjusting PWM duty cycle to keep stable voltage while checking current drop to 10% to cut off power(correct me if i'm wrong about this). But i'm unsure how to approach constant current mode.

[Audioguru]

Multiple lithium battery cells? In series or parallel makes a charger very complicated. You must not charge a cell that has been discharged too low so your charger must detect a low voltage then refuse to charge.

A constant 4.20V charger will explode a lithium battery cell because its current is not limited. The cell will have a recommended charging current and a maximum do not exceed charging current, your charger must limit the current. www.batteryuniversity.com recommends sensing a charging current drop to 3% to 5% of its mAh rating for a full charge then the charger must turn off.

[ion]

Multiple lithium cells in parallel will appear as one larger cell to the charger, so no more complicated to charge than a single cell.  That said, make sure they are within a few mV of each other before connecting them for the first time, and ideally they should be the same type of cell.

Cells in series is where it gets complicated because you really should use balance charging if you want the pack to be reliable and safe.

The constant current mode basically involves limiting the charging current to the cells until the voltage rises to 4.2V (or a bit less to reduce wear on the cells if you don't need every last bit of the capacity).  The current limit would mostly depend on the capacity of the cells you are using.

Then you just keep supplying that voltage until the current drops to some value below 10% of the limit (determined by how full you want the cell vs. how long you want to wait for the charging to end), then you cut off the power - trickle charging is bad for lithium cells.

As for the actual implementation - I would personally look for a li-ion charging chip that meets your requirements first.  It would make the charger design simpler and possibly safer.  As Audioguru mentioned, cells that have been discharged too low can be dangerous, and at least some of these chips are built to handle these situations.