OK I know a little bit about battery discharging process that is, I know that the open circuit voltage of battery decreases as SOC goes down, and the voltage is less than the Voc when connected to a load (so we have an internal resistance which rises as the battery gets emptied) I also searched and found more complex equivalent circuits (considering diffusion voltages). but these all model a battery acting as a source but How does a battery behave (in terms of circuit elements) in charging process (as a load) and what is it's voltage? I mean is it the open circuit voltage (disconnected from charger) or the charging voltage? I'm asking because I'm a bit confused about solar charge controllers. as we all know they are not voltage sources and behave somehow like current source so if we connect a battery directly to a solar panel (like Self-regulating PV systems) how can we determine the operating point of the system? Thanks
A battery connected directly to the output of a solar panel will hold the solar panel's output at the battery voltage. If you look at the I-V curve of a panel, you most likely will find that the maximum output power does not occur at the battery voltage. This means that you will not be able to extract the maximum power from the panel. Also, you wouldn't be able to control the charge profile of the battery, which can cause ill effects to the battery. This is the reason for a device called a Maximum Power Point Tracker (MPPT), which you can google. I know this doesn't answer the question asked in the post, but will hopefully help to set you in the right direction.