Advice needed: batteries are draining faster than expected - off grid pv system
I'm new on this forum and this is my first post. I look forward to take part of all your wisdom. I'm currently setting up a pv system in rural Mozambique, but have ran into a problem: the batteries are draining much faster than expected. My setup is as of below:
4 panels: each 240 W, Voc = 37,11, Isc = 8,9, Vpeak = 29,1, Ipeak = 8,33
1 Victron Energy BlueSolar MPPT Charge Controller 100V, 30 A.
4 Osaka 120 Ah, 12 V batteries
1 Victron Energy Phoenix Inverter 24 V, 500 W.
The cables between the pv panels and charge controller are 6 mm^2, the cables from the charge controller to the batteries are 16 mm^2, the cables between the batteries are 6 mm^2, and the cables from the batteries to the inverter are 6 mm^2.
I've attached an image of how the system is connected below. As can be seen, the panels are connected in series and in parallel, and the same for the batteries, resulting in a battery pack of 24 V and 240 Ah.
The inverter is connected to a small electric central inside a building. When I first tried to run the system, a load of 310 W was connected in the building: 5 fluorescent lights of 36 W each and two 65 W table fans. The lights are on a 5 A fuse and the fans on a 25 A fuse (heavier equipment is maybe intended for this room in the future). None of these fuses went off during the first run.
I wanted to measure the expected performance of the system. What should be expected according to my calculations is a total power supply time of approximately 18,58 hours at full charge (310 W / 24 V = 12,91 A, 240 Ah / 12,91 = 18,58). I wanted to run the batteries to 50% charge, i.e. approx 9 hours. However, when I started to run the system by disconnecting the pv input cables on the charge controller, (the charge controller showed "float" before starting). However, the system only lasted a bit more than two hours before the lights started flickering and the fans went off.
I measured the voltage during the process. The first measurement was before disconnecting the pv panels input in the charge controller, showing a battery voltage of 27,27 V. The second measurement was before turning on the inverter but after disconnecting the pv panels, showing a voltage of 26,79 V. The third measurement was after turning on the inverter and after disconnecting the pv panels, showing a voltage of 24,35 V. I also measured after one hour, first when the inverter was on, showing a voltage of 23,75 V, then turning the inverter off and waiting a few minutes before measuring. This measurement showed 24,84 V. Then after an additional hour, the lights started flickering and the fans went off. I turned off the inverter and measured again, this measurement showed 24,0 V.
So, finally, my question is to you, what is wrong with the system? Are my calculations off? Or do you think there is something wrong with the system connections or sizing? I'm aware of that the charge controller is somewhat underdimensioned with respect to the pv panels, but I want more power on cloudy days. The specs on the charge controller still says that I'm within its limits, and the charging seems to be working fine.
I appreciate all feedback that I can get! Thank you in advance!