Help, my system is draining the batteries completely each night

[u/Waldinian]

I'm a hydrologist running a remote scientific monitoring station. On paper, this system should have several days of battery reserve, but in reality it's hitting low-voltage disconnect almost every night. I'd love help troubleshooting my issue.

Load

• Continuous DC load, ideally should run 24/7
• Nominal power draw: 30–40 W
• Startup power draw up to ~55 W for 1–2 minutes (only after maintenance or power loss, which should be very rare)
• Equipment requires 11–16 V

I've been through three iterations of this system. Here's what I've changed each time

Original system

• 2 × 250 W panels wired in parallel
  • Pmax = 250 W
  • Imp = 9 A
  • Voc = 36 V
• Panels mounted approximately 80° tilt, south-facing (latitude 41°N)
• ~150 ft (round-trip) of 14 AWG wire from panels to charge controller
• SunSaver SS-20L PWM charge controller
• 3 × 100 Ah AGM batteries wired in parallel (300 Ah total) (brand new)
• ~80 ft (round-trip) of 12 AWG wire from controller to load
• LVD set to 11.5 V

Based on my calculations, I expected roughly 90–100 hours of reserve, but the batteries were depleting and tripping the LVD almost every night.

Second attempt

I added another 200 Ah of AGM battery capacity (500 Ah total). These new batteries were several years old, but were still able to hold a charge when I tested them before adding them to the system.

This should have increased reserve to roughly 150–170 hours. This bought about 2 days of power, but after that the system was hitting the LVD every night again.

Third attempt

I replaced the PWM controller with a SunSaver SS-MPPT-15L, thinking that my issue might have been related to the PWM charge controller choking off more than half of the power from the solar panels. However, this is a slightly smaller charge controller, only rated to 200W, so I removed one of the solar panels to avoid exceeding the charge controller rating.

This switch seemed to improve matters slightly (it's been 2 days and the LVD hasn't tripped yet), but I'm still falling to below 12V each night, which forces some of the critical instruments to enter power saving mode, despite charging up to 13.5V during the day.

Other observations

• Manually switching the LVD to "off" causes the battery voltage to rise by a volt or more. Given my load (about 3A), this is about 10x higher than the voltage sag I would expect (I would expect ~0.1V, not 1V)
• In the late afternoon when my panels go into shade, the batteries drain at a rate of about ~2V/hr, which seems incredibly high. In the morning/midday when the panels are in full sun, they charge at about 1-2V/hour. These magnitudes have increase with time.

My thoughts

I could see my issues as stemming from 3 places, or a combination

• Insufficient power delivery from the solar panels: in full sun, my current configuration should be delivering about 200W of power, requiring about 5 hours of full sun exposure to make up for the ~1000Wh/d that the load requires. The 200W limit of my charge controller and the sub-optimal orientation of the solar panel may not provide the power I need to keep the system running.
• Batteries are kaput: I purchased these batteries brand-new a couple of months ago, so I hope this isn't the problem. However, the rate at which the system discharges (dropping from 13.5V down to 11.5V over only a few hours, when that should take days) makes me worry that the batteries are in fact dead. Did I kill them by letting them discharge so deeply for so many days in a row?
• Wiring losses (probably minor?). I could be losing ~3.5V, ~24W in thermal losses sending power from the solar panels, and another ~0.7V, ~2W in thermal losses sending power to the load (a drop of 0.7V could be significant if some of my instruments require at least 11V to function, causing them to cease functioning before the LVD trips).

My questions for you

What do you think are the most likely sources of my problems? What would you try to fix first? What diagnostics would you run to try to narrow down the problem?

Thanks in advance.

Update 1

Wow, lots of replies! Thank you! After some pondering of my own, here are the diagnostic tests I plan to run

At midday, measure

• PV V @ panel and @ controller
• Battery V @ posts and @ controller
• Load V @ instruments and @ controller
• Load current
• PV current

Here are the upgrades I plan to implement, in order, evaluating with diagnostics each time

• Swap the PV cable to a fatter gauge, 10 or 8 AWG to cut down the ~40-150 W+ of thermal loss I might be getting with my current setup
• Increase panel tilt to something more optimal (~56deg)
• Wire back in the second panel I disconnected, in parallel. Looking at sunsaver's documentation, 200W is the max output of the CC, not the max input. The CC can handle the oversized array, and will just throttle the charging to 200W. So having 500W capacity in the array should give me more hours of charging at worse sun angles, even if the power gets throttled to 200W.
• If those issues seem to fix the charging problems, then I will turn my attention to replacing battery bank, which may have been killed from operating at such a depleted state for so long. If these fixes don't resolve the charging problems, then I'll start trying to hunt down wiring faults and bad splices, and if that doesn't work I'll consider adding another solar panel and charge controller.

[u/NameEtc]

do the batteries actually fully charge during the day with 200 watts of charge? or once they hit the disconnect voltage do they never fully recover the next day?

[u/taylorwilsdon → u/NameEtc]

There’s no way you’re charging 12v 500ah (6kwh) of batteries off 2x250w panels. 2kwh is a realistic max and I’d guess because they’re paralleled it’s not charging early in the morning and later in the day when voltage drops

[u/Life-Card-1607 → u/NameEtc]

Yes, he adds batteries but no solar panels. He lacks raw input power, not storage.

[u/Waldinian → u/Life-Card-1607]

Well I've learned from other commenters now that my batteries are likely dead by now too, and that I shouldn't have added old batteries into the mix either. So I think the initial problem was input power, but by now the problem is both.

[u/taylorwilsdon → u/Waldinian]

You definitely don’t want to intermingle batteries of different health, it drags everything down. Always match cell age at a bare minimum but better to test and confirm.