Source: https://forum.solar-electric.com/discussion/354493/advice-needed-batteries-are-draining-faster-than-expected-off-grid-pv-system
Timestamp: 2019-04-20 06:54:47+00:00

Document:
1 Victron Energy BlueSolar MPPT Charge Controller 100V, 30 A.
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!
Are you trying this test at nite when the sun is down? So batteries only, or when there is sun so that you are trying to run this load off the solar array ?
If batteries only, Your issue might be connections or a bad battery / battery cell. Are the batteries new?
Are they sealed batteries? If Not, Do you have a Specific Gravity (SG) tester - Hydrometer? can you test the batteries? If the batteries are new, did you do a commissioning charge and Equalization (EQ) (Do NOT EQ sealed / AGM batteries), And then testing the SG to confirm they are fully charged? What is the SG of each cell when Fully charged and Equalized?
Did you make your own battery cable connections? Under load, do you have a way to test the temperature of each connection using an Infrared thermometer?
- the spec sheet @Photowhit linked to doesn't appear to specify waveform type, and a "pure" sine wave type likely would. If a "modified" or square wave type, it may have trouble with some loads, causing extra heating in devices as well as the useful work (light, motor rotation). In some cases, this can be enough to significantly shorten device life as well as using more power.
- flourescent lights and motors can be hard loads to run. Your 310w load number seems to be the wattage rating for the work done by each light/fan. What the inverter needs to supply is enough power in VA (volt x amp) to do that work. In a purely resistive load (eg incandescent light bulb), these numbers could be quite close. With other load types (your motors and lights), VA can be much higher. Modern strip flourescents with high power factor electronic ballasts tend to be not too bad, but older designs can be surprisingly hard to run.
To see if and to what extent one/both of these are contributing to the issue, you may want measure DC current coming out of the battery to supply the loads, with a shunt based meter or a DC clamp meter.
Thanks for all your answers, I really appreciate it!
These are 12,9 DC amps, I was a bit unclear about that. Yes, 9 hours should be approximately correct. There will be some losses from the inverter and cables, but maximum 10-20% in total. I did the run during daytime, but with the pv panels disconnected from the charge controller. The batteries are new, sealed lead acid batteries. I have reconnected all the battery connections, checked the peeling and taped them tighter together. I have no possibility to check the temperature unfortunately.
Yes, your calculation is correct, but it should still not exceed the capacity of the inverter.
The inverter is a pure sine wave type, so these types of loads should not be a problem. However I did not now that about motor and light loads, thank you! I will try without the fans connected to the system. I am not sure if I can measure this current, but I will check if I have the tools to do so.
What I have done now is that I have recharged the batteries until the charge controller indicated float. I then disconnected the batteries for 24 hours to measure the open circuit voltage. It then had a voltage 25,64 V for the whole package, and 12,82 V of each battery, which should mean they are 100% full. I have then connected the load, now only consisting of the lights, I removed the fans in case the motors are too heavy to run. After connecting the load, the battery pack had a voltage of 24,96 V and each battery between 12,43 and 12,5 V. I will now measure the voltage every hour for half a day, and see what happens. Hopefully the system will last longer. I will then once again disconnect, wait 24 hours and measure the open circuit voltage again. If this voltage shows somewhere close to 70-80 % full, the system should be working as expected. I will get back to you with an update, thank you once again!
Inductive loads such as motors and magnetic ballasts may have a negative effect on the inverter itself, but won't increase the total power taken from the batteries. Sounds like you're doing the right thing by fully charging before doing a load based calculation, the load will draw the voltage down dynamically, it's the resting voltage which is important, when all loads are disconnected, this gives some idea of state of charge. The whole procedure will take some time, mathematical calculations may not translate directly to real world findings, that's to be expected, doing actual testing will reveal any shortcomings, should there be any.
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
You should buy a DC, rms, clamp current meter. About US $50.
Thank you for your replies, I will take them into consideration!
So, I measured the voltage every hour for 8 hours yesterday. The measurements are attached below.
As can be seen, the voltage dropped significantly the last hour for battery 1, and went up for the battery pack as whole. Do you have any idea why this might have happened? The lights still worked fine.
These numbers should indicate that the system had approximately 70-80% left of the capacity, which should mean the system works somewhat fine. The battery drop on the last measurement is strange though, do you have an idea of why this might have happened or what is wrong?
You disconnected the load, but was the solar array still connected?
Looks like there is some imbalance between the two strings, one is discharging deeper than the other, this could be due to configuration, I'm guessing the diagram in post 3 is not the actual wiring. Reading this link will answer many questions, the current during charging and dischargeing should be equal or very close in each string. This, if not corrected will lead to the premature loss of capacity of the string doing the most work.
In additional to checking and correcting under-load balance with a clamp ammeter, battery balance will typically be improved by connecting the mid-point of B1/B2 to the mid-point of B3/B4 (assuming B1/B2/B3/B4 labeling in your diagram). Measurement and swapping batteries around is even better. Adding a HA01 balancer also helps.
When using a voltage measurement to determine SOC, it must be done with a long rest and no-load/no-charging . Under these conditions, 12.38+12.38 and 12.45+12.46 is impossible for two parallel strings. But who knows how you are identifying B1/B2/B3/B4. Could be 12.38+12.45 and 12.38+12.46 which is quite possible.
Note that @t00ls advice assumes flooded batteries, but I didn't see a battery type specified.
If not flooded (i.e. sealed - gel or agm) type, they should definitely NOT be equalized, as the higher voltage overcharge could easily ruin them.

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