I am referencing the spice circuit for the LTC4000-1/LT3845A Demo Circuit - Battery Charger for Three LiFePO4 Cells with a Solar Panel Input (20-60V to 10.8Vfloat @ 10A Max) in order to design my own circuit for a 305W solar panel with Vmpp=36.3V and Impp=8.41A. I am also referencing the Lead acid battery design under the "Typical Applications" section.
In the data sheet it shows how to set the Vin voltage for a certain level which is needed to optimize the solar panel for its Vmpp value. I am not sure where to measure the input voltage to see the voltage change in order to confirm the IFB pin is correctly setting the desired voltage level. I have designed my system to have a Vmpp of around 36.3V but as I increase the voltage source to a higher voltage (ex 41V), the input voltage does not drop to the Vmpp value of 36.3V and rather just drops 3.33% of the input voltage source if measuring at the "IN" pin or the "VIN" pin of the LT3845A. Is this due to the input voltage source not modelled as an equivalent circuit of a solar panel? If so, is there a model or sub-circuit I can use in order to simulate this feature of the LTC4000-1?
It also mentions how to monitor the input current of the solar panel using the IIMON pin which is proportional to the current across RIS. However, when I try to calculate this current after the battery has charged, it is only around 0.51A.
I have also problems programming the absorption voltage level when including a third resistor RBFB3 in the BFB feedback sub-system. The battery will only charge to the float voltage (not the absorption) and the charge current will drop but the voltage will not drop to the actual float voltage I have set as the charge current declines to C/10.
Below is my circuit: