This video explains current flow in the system.
Watch it first.
The read the doco I prepared on charging.
It makes sense to me but I wrote it and I'm a bit weird.
How charging works in the context of LFP batteries
LFP = lithium iron phosphate
Voltage characteristics for LFP cells
2.5 volts is dead empty.
This is a hard limit and should be enforced by the BMS
3.2 volts is the nominal voltage for an LFP cell.
This value multiplied by the cells amp hour rating yeilds its watt hour rating.
3.35 volts is the highest "safe" float voltage.
3.65 volts is dead full, this is a hard limit and should be enforced by the BMS
Constant current aka bulk
During this phase the charger controls the charge current by controlling the charge voltage.
Current flow is a product of voltage differential.
In other words there needs to be a difference in voltage between the battery and the charger to make current flow.
As the battery fills up its voltage increases.
The charger increases the charge voltage to maintain the prescribed current flow.
Constant voltage aka absorption
When the charger no longer has to adjust its voltage down from the configured charge voltage to maintain the the prescribed current flow, the charger is in the absorption phase.
During the absoprtion phase the charge current decreases as the battery voltage approaches the charge voltage.
The absorption phase ends when the charge termination criteria is reached.
Charge termination
Exposing an LFP battery to charge voltage above its full resting voltage of aproximately 3.375 volts per cell causes unnesseccary stress.
For that reason chargers have charge termination logic.
There are different methods for a charger to determine that the battery is full.
Tail current
Charge is terminated when charge current is less than or equal to a configured tail current value in amps.
Absorption timer
Charge is terminated when a configured absorption timer reaches its configured value in seconds, minutes or hours.
Voltage sense leads
This is a less common method. The charger has a separate set of leads that are attached to the batteries terminals. This allow the charger to know the voltage from the batteries perspective. This allows the charger to compensate for voltage drop over the current path between the charger and the battery and also to terminate the charge based on the voltage at the battery terminals.
Comunication protocols
Battery and charger communicate by a data protocol to control the charge process.
I can't say much more about this as I don't have any experience with it.
Float
After the charge is terminated the charger can optionally provide float voltage.
LFP batteries do not require float voltage but it can used as power assist.
At the start of float phase and depending on the configured float voltage the battery services the load and the charger stands by.
As the battery services the load its voltage is drawn down toward float voltage.
As the battery voltage appraches float voltage the charger increasingly takes over the load from the battery.
Eventualy the charger becomes the primary charge source and the battery is maintained at float voltage.
Its like a soft landing for the battery.
If the charger is shut off the battery will instantaneously service the load.
The highest "safe" float voltage is aproximately 3.35 volts which is just slightly below the full resting voltage on an LFP cell.
Engineering is all about tradeoffs, depending on your usage model it may be advantageous to configure float voltage higher or lower than 3.35 volts.
Cycling
LFP batteries don't like to be full.
Even resting at voltages above aproximately 3.375 volts is an avoidable stressor.
LFP batteries also don't like to be empty.
LFP batteries do like to cycle.
Setting the float voltage lower allows the battery to cycle.