Design a solar charger with MPPT feature that runs in 3 modes
I'm trying to design a solar charger with MPPT feature that runs in 3 modes (bulk, absorption and float) depending on the state of the battery. The logic is implemented in a fast micro controller and the dc-dc converter utilised is a buck converter. After doing my homework (lots of readings and research) I still have a couple of questions that I find confusing.
Correct me if I'm wrong on this. Do we only use MPPT during the bulk mode? If yes, we use it only if the charging current generated is less than the current threshold?
How, and by what means can I calculate the duty ratio of my transistor during the absorption and float mode in a manner to achieve a constant charging voltage with a maximum threshold charging current? Same thing also for the constant current charging.
- While digging answers for question #1 I found that TI manufactures solar battery charge controller IC (bq24650 datasheet) that has the capability of regulating the charge voltage and the current voltage with high accuracies. They mention that they use internal loop compensators to do this. Digging in the high level schematic they have I found that the reference voltage feedback VFB and the current feedback SRN & SRP (SR is a current sensing resistor) are inputted to a compensation network. Does anyone know how this compensator network was designed? I would like to implement something similar in software (I'm not asking how to implement it in software).
- When changing the duty ratio to achieve regulation, the charging voltage and current can't be predicted because changing the duty would take us to another point on the I-V of the PV which is not predictable. Or is it?