"I've just bought 8 x 230W PV modules for off grid use. I was thinking of putting them in series (two chains of four modules) and using a MPPT controller to charge the batteries. I also have two pure sine 24V inverters (1500W and 700W).
I wonder if I could connect all those PV panels in parallel and feed the battery bank and the inverter directly."
Going back to the original theme, I did some experiments lately that showed me I can't count on the operation of the direct-connect panels to be in line with conventional wisdom, probably because of the varying operating conditions, as follows:
1. Can't always assume that the panels' voltage will be pulled down to the battery's voltage, because it may be the other way around.
2. Can't always assume that the panels will operate in the current-source region (below Vmp), because they may operate in the voltage-source region (above Vmp).
For example, I've had my small portable system out on a couple of occasions, and saw it operate in two much different ways. The system is two 60-cell Sharp 230W panels (Vmp=30.0V and Voc=37.1V at STC) wired in parallel, connected directly to a 24V battery bank (two 12V lead-acid deep cycle 105Ah batteries wired in series).
On one occasion, I had it out in about 60-70 deg F weather, with medium sunlight, and fairly high humidity. The system voltage was about 28V at best, and the charging was pretty lame (looked like the batteries were always undercharged after each daily charging cycle). I believe the panels operated in the current-source region then.
On another occasion, I had the same exact system out in about 40 or so deg F weather, good sunlight, and pretty low humidity. The system voltage came close to 34V while charging, and the batteries started boiling. They continued to boil even when I took one panel out (the voltage stayed the same and the current was cut in half, as expected). I've lost quite a bit of water from the batteries.
I took some measurements then that showed me the system must have operated in the voltage-source region, even though it does not seem possible, but that's the only way I can explain what I measured.
I haven't yet tried operating this system with any sort of diversion or PWM charge controller to keep the battery voltage in check. This would have probably prevented the runaway voltage, but still the point is that unusual things can happen without any charge control, as the system makeup and the operating conditions change.