Advice needed - batteries are draining faster than expected - off grid pv system

[Davidlinken]

Hi!

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 total 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 (DC), 240 Ah / 12,91 = 18,58). Some losses from the inverter and cables are expected, but say 16 hours approximately. I wanted to run the batteries to 50% charge, i.e. approx 8 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!

Best regards

Davidlinken

[SunEagle]

Hello David and welcome to Solar Panel Talk

My first response is that your usage calculations may be incorrect. Are those loads using 24V or some other voltage since you have an inverter in the mix? You should be calculating the watt hour usage and then determine the % discharge of your batteries to see if you are over discharging them. Calculating the Amp Hours used can get you incorrect information.

A 240Ah 24V battery system should safely provide about 1500watt hours a day (240Ah x 24V x .25 = 1440wh) without discharging them more than 25%. That 1500 wh should get you more than 5 hours of usage for you 310 watt load and not drain you batteries below 50%.

It is possible that you have miscalculated your true watt hour usage or due to a parallel wired battery system not discharging or charging them equally. That could cause one of the batteries to fail which would draw down the system voltage to what you are measuring.

[Mike90250]

To recharge a 24V battery, requires at times, 30V from the charge controller. You may be hitting some voltage limits of the PV voltage being (58V) too low for the charge controller to effectively downconvert to charging voltage. There should be some document for the controller listing desired voltage ranges for proper MPPT function.

The other thing is heat. You are paneled to the max for that 30A controller, and if it's warm/hot the controller may be going into thermal limiting.

[NorthRick]

I think part of the problem is that the amp hour ratings on the batteries are usually for a 20 hour draw down. Your calculated draw down is 18.6 hrs but doesn't include losses or the inverter draw, so your load is likely higher and, hence, the draw down period shorter. The faster you draw on the batteries, the lower their capacity.

[Davidlinken]

Thank you for your feedback, appreciate it. The charge controller is the classical bulk, absorb float with a bulk on 28.8 V. We charged the batteries again to what we think is fully charged because the charge controller registered float and the individual battery voltage did not change. We then left the batteries without charge or load for 18 hour and re-measured and gained the following result: all batteries had 12.82 and the battery pack had a voltage of 25.64. Is it correct to assume the batteries arw fully charged?

We then put on a load of only 5 lamps with 36 W each resulting in a total load of 180 W. We then measured the battery voltage every hour. This is the following result from battery 1 to 4 and then total volt

Hour 0: 12.45, 12.43, 12.49, 12.50 and 24.96
Hour 1: 12.41, 12.40, 12.44, 12.44 and 24.87
Hour 2:12.37,12.36, 12.38, 12.38 and 24.77
Hour 3: 12.30, 12.30, 12.34, 12.35 and 24.64
Hour 4: 12.25, 12.25, 12.28, 12.29 and 24.55
Hour 5: 12.20, 12.21, 12.22, 12.25 and 24.48
Hour 6: 12.13, 12.15, 11.17, 12.20 and 24.35
Hour 7: 12.4, 12.7, 12.13, 12.18 and 24.27
Hour 8: 9.45, 11.84, 12.18, 12.23 and 24.24

This time the lamps never went off but we are very confused with this last measurement and was wondering if you know what could have happened?

We disconnected the batteries again from load and charge and checked the individual as well as the total energy pack voltage which resulted in:

12.38, 12.38, 12.45, 12.46 and 24.91

According to two separate data sheets the battery should be about 70-80% charged.

Would appreciate any help you can give us.