Thank you very much, @warpspeed!
As always, I'm saving all your posts as "application notes" for further reference!
What can I say, I've thought about using a full bridge (boost-buck) topology but I wanted to avoid the increasing number of switches.
Btw, for now I'm using an "emergency" charger (as the old commercial one had some problems). That's it, the PV panels are connected right across the battery bank (in pairs of two, of course) and I'm using some relays to "regulate" the charge current, by switching on/off a corresponding number of PV pairs.
Speaking of that, I've noticed some particular behaviour (well, it might be logical) of this "emergency charger" during cloudy/grey sky periods: the output power of the PV is very small (like 3-5% of the rated power).
It might be that because the PV voltage are way bellow the battery voltage (I have 60 cells panels, not 72). Anyway, I used to have just a little bit better results with my former commercial charger (proudly labeled as MPPT) which leads me to the conclusion that it was a dumb PWM one.
Btw, I've used the same panel configuration (30V + 30V) before.
From your own experience, what was the minimum output power of the PV panels during sun "blackout"? There is some more power in that diffused light? (more than 3-5%, that is)
Back to the charger topology: I was not afraid of charger failing as the PV panels are limited current sources hence they should adapt to the "after death" situation. That's it, they will withstand open or short circuits as well.
But, taking into account your valuable arguments, I guess I'll choose the buck-boost topology, indeed.
One further advantage: I could switch to my "emergency charger" at any time, as the panels should remain in this current configuration (pairs of two).
Once again, thank you very much for your time and for sharing your knowledge. Btw, you should write a book sometime.. a huge one!
@Klaus:
Sorry, it was my fault. I was not asking for the calculations (at least, I thought I didn't) but the advantages of one topology over another (buck/boost).
Anyway, I've did some preliminary calculation too but the best compromise (mosfet voltage/current/Rdson ratings) was for an input voltage of 60V, but that was exceeding the allowable limits for both buck or boost topologies alone.
But the answer was supposed to be that simple: a buck-boost topology.