by MtnDon » Fri Dec 29, 2017 9:53 pm
The change from bulk charge to absorption charge occurs when the battery voltage builds up to the preset voltage; current constant. I don't know the nuts and bolts of how that is achieved by the charge controller, but it is a user selectable voltage on the CC I use (midnight Solar products). Charge current should be as high as practical to get as much energy replaced as quickly as possible, but not to exceed the manufacturers specs. Once the battery voltage has hit the set level it changes to the absorb profile. That voltage level also usually has a time requirement to prevent system fluctuations from causing unwarranted changes.
Once the voltage in the battery has risen to the set point, then the charger changes to the absorb stage profile and holds the voltage constant, or nearly so, while the current decreases. The current tapers lower and lower and I believe is a function of the lead-acid battery internal resistance. I could be wrong on that. Low resistance cells such as lithium don't need an absorb stage. Lead-acid need the absorb stage for the lead plates to slowly soak up the remaining charge. The duration of the absorb stage is often set to be a timed event. Some charge controllers allow the end of absorb to be set by how low the current flow into the battery falls. That is determined by total battery capacity. Using "end amps" allows the absorb charge time to be 'just long enough' to ensure a full charge and not unnecessarily long as can be with a timed charge.
Then the float takes over.
Is your design an MPPT or a more simple PWM?