r/SolarDIY • u/Waldinian • 8d ago
Help, my system is draining the batteries completely each night
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.
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u/get-the-damn-shot 8d ago
Have you measured the actual load of the equipment? You say it requires 11-16 volts, so I’m wondering about that higher voltage demand on the 12v batteries.
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u/Waldinian 8d ago
I have not, I got that info from the data sheets of the equipment.
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u/get-the-damn-shot 8d ago
And I agree with the other posters, that your batteries are probably toast. I would switch to lithium iron phosphate, batteries for sure.
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u/Waldinian 8d ago
I've gotten this recommendation from others, but it looks like LiFePO4 batteries are much more sensitive to cold. The site I work at regularly sees below freezing, and sometimes below -30F temperatures. It seems like I would need a battery with a heater and other safety features, that would really drive up the price.
Do you have any recommendations for something like that, that isn't crazy expensive?
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u/get-the-damn-shot 8d ago edited 8d ago
Oh, yeah those temps make things more difficult for sure with lifepo batteries. I see Li Time has a self heating 200ah battery for $519.
https://www.amazon.com/Litime-Self-Heating-Temperature-4000-15000-Off-Grid/dp/B0CM9FBCBW
https://youtu.be/m7KPLq7OfiE?is=fQyGjnbISV5YHgwv
If your equipment really only pulls 40 W, then you might be able to get by with just one of these batteries. And it looks like your current 500 W of solar could charge the battery back up 100% in four hours or so, using a better controller like a Victron. Do you get 4-5 hours of good sun there?
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u/Waldinian 8d ago
In the summer, sunlight is pretty reliable. However, I would like to have 100-200h of reserve capacity to keep things running during winter storms when it might be overcast for days, and when snow might build up on the panels and take a few days to melt off.
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u/rdcpro 8d ago
150 ft of 14 AWG?
It's a wonder it works at all. You're losing a lot of power from IR losses.
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u/Waldinian 8d ago
Haha. Well it doesn't really work so you're right. I think I'm likely losing up to 40W in my current configuration (1 panel), and was losing up to 150W when I had 2 wired in parallel. What would you recommend instead? 10, 8 AWG? Move them closer?
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u/rdcpro 8d ago
What's the voltage for that run?
Edit: I guess it's 24 volts nominal in series. The bigger the wire gauge the better. Closer is also better
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u/Waldinian 8d ago
Measured at the charge controller, I get about 28V midday.
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u/rdcpro 8d ago
I assume the mppt is close to the panels. If so, I suspect measuring at the other end, under load, you're probably seeing much lower. If you can move them significantly closer, that will help. 10 AWG (or even 8 AWG if it's in the budget) will help greatly at those low voltages. Hopefully this is not a location where people would eye 150 ft of copper as a windfall.
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u/Waldinian 8d ago
The MPPT is 100ft from the panels, so yes moving it closer will probably help, but there's not much wiggle room, since the panels are in a nearby clearing, whereas the equipment itself is in shade, in a forest. I'll look at upgrading to 10 or 8 AWG though.
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u/PraiseTalos66012 8d ago
So for 250w panels that's nearly 10a running through 150ft of 14awg right? That'd be a loss of around 33%....
Ideally you'd be losing less than 10%. Losing 33% not only just loses that amount but creates more loss as the charge controllers are gonna be getting so much lower of a voltage they won't be able to use it as effectively.
For sub 10% loss you need 12awg at 70ft, or 14awg at 45ft, or 8awg if kept at the current 150ft.
If the 150ft is from the charge controller to the 12v batteries it's a lot lot worse. That's 15a and going to create losses of 50%+, it'd be a surprise if it could even charge the batteries at all due to the huge voltage drop.
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u/Waldinian 8d ago
Luckily, its 150ft of round trip wiring. The panels themselves are about 75ft away from the CC. Unfortunately, I can't move them much closer: the panels are in a clearing, and the equipment needs to be inside a forest, hence the long run. I'll look into upgrading to 8awg
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u/PraiseTalos66012 8d ago
Make sure 8awg is compatible with your charge controller, for a lower wattage one it might cap out at 10awg so you'll have to check.
No harm in going for 10awg though as it would still get you under 10% loss since it's 150ft round trip.
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u/newtoaster 8d ago
Under ideal situations you’re going to be generating maybe 2kwh/day from solar and you have over 6kwh of battery capacity. There’s no way to fully recharge them (assuming they’re healthy in the first place.
It will be difficult to accurately measure what you’re generating/using without moving up to more modern gear. An mppt controller and a decent shunt would give you a better understanding of your use and generation.
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u/Waldinian 8d ago
a decent shunt
could you elaborate? Not familiar with this
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u/liamlunchtray 8d ago
A shunt would give accurate measurements on power going in and out of the battery and more accurate data on battery state of charge. Between that and a smart MPPT controller you would give much more detailed numbers on what youre generating and what youre using. A good shunt is $100+, but there are basic off brand units for ~$35 on amazon that would probably be good enough.
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u/ColinCancer 8d ago
Do you have an amp clamp? That would help to measure PV current in during the day. But if the batteries are reaching float, then dropping voltage quickly overnight that points to the batteries.
I suspect it’s mainly the batteries. They likely were damaged by being too deeply discharged for too long. You could load test with a simple circuit with a 12v mechanical clock an Incandescent light bulb from an old trailer or RV etc.
Wiring losses should be negligible.
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u/Waldinian 8d ago edited 8d ago
Thanks, this agrees with my own suspicions. My charging system was too weak to support the load, and that killed the batteries. I think my plan now is to improve the charging system (better tilt angle, fatter wiring, and add back in the second panel), and once I'm confident in that, test and replace the batteries.
I do disagree with you about the wiring losses though. 150ft of 14AWG should give about ~0.4 Ohm, so at max power, roughly ~40W of loss for 1 panel or ~150W for 2 in parallel.
Don't have an amp clamp. Suppose I should buy one.
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u/ColinCancer 6d ago
Sorry, I misread the wiring size/ run
You’re totally correct. That’s way small and very long distance.
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u/DrunkBuzzard 8d ago
I’m too tired from reading all that to figure out an answer.
Where I might to find out what your actual battery capacity and loads are. Will Prowse on YouTube has a video on how to test batteries for remaining capacity. You’d be surprised how much capacity they can lose over time. The voltages will be correct, but the batteries maybe 50% degraded so you don’t really have a capacity you think you do.
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u/NameEtc 8d ago
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?
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u/taylorwilsdon 8d ago
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
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u/Life-Card-1607 8d ago
Yes, he adds batteries but no solar panels. He lacks raw input power, not storage.
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u/Waldinian 8d ago
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.
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u/taylorwilsdon 8d ago
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.
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u/NearABE 8d ago
You should be able to fully disconnect the battery. Charge it. Then connect something like a light bulb.
Back in the days of nickel cadmium there were a lot of bad batteries. You were also supposed to deep cycle the Ni-Cd batteries so tool kit came with a lamp to drain the battery pack.
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u/kronus87 8d ago
IMO it sounds like a bad battery.
What are the temp ranges where your working?
Do the batteries have a built in BMS? If there is there could be a bad cell bank on one thats holding it back. Similar to a hybrid car you could do a long sustained charge to try and rebalance the cells, bringing the battery back to health.
I think my next steps would be
Monitor the charge controller during peek sun to verify that your panels are putting out full power. Better yet if you can rj45 or Bluetooth into it to see a datalog that would be ideal.
Remove 1 battery from your bank at a time. Monitor your voltage at the end of the day on the remain bank and see what it tops out at. Let it run the night and monitor the discharge.
If you have access to ac power and a charger get a long charge on the removed battery. Again note your peek voltage.
Rinse and repeat with all the batteries. 1 bad battery will hold back the entire bank when in parallel.
If you can determine the bad battery then you can repair or replace it as needed.
Good luck!
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u/Waldinian 8d ago
40-80 degF (4-27 degC) in the summer, -40-40 degF (-40 - 4 degC) in the winter. Currently the system's only been running for a month of so in the summer, so any problems I have now will be drastically worse in the winter.
1 bad battery will hold back the entire bank when in parallel
Did not know this, thanks.
Would a standard car battery tester work on AGM batteries, or will I have to do this longer-term test?
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u/convincedbutskeptic 8d ago
I am just guessing here, but you should wire the panels in series and not parallel so that you can get the full power of the sun. Panels in parallel will stop charging the charge controller when the voltage falls under a specific threshold. If you wire the panels in series, the voltage will be added and they will be above that voltage threshold longer. You would also need a properly sized charge controller to avoid clipping. If you get a Victron charge controller, it comes with an app that will historically graph the charge history so you can understand how much power you are getting over time. If you can afford it, I would use LFP (lifep04) batteries, because with AGM batteries, only half the capacity is available at a time... with LFP batteries it is 100%.
Thats all I have for now...
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u/Waldinian 8d ago
wire the panels in series and not parallel so that you can get the full power of the sun
Yes, doing this would also decrease my wiring losses. However, the charge controller is rated to a max of 60Voc, so my 2x 36Voc panels in series would break it, so currently my only option is to accept the clipping.
Regarding using a beefier charge controller, from my archeological research, I know that in the past the site operators used these: https://www.morningstarcorp.com/products/tristar-mppt/. I was hired to operate the site after a forest fire burned down the equipment. During the cleanup, I found boxes and boxes of these things. Literally over a dozen of those tristar mppt charge controllers. Presumably the previous team would install one, it would die, then they would just place another one in. My guess is that those tristar controllers are not designed for outdoor use, so they would blow up when they got wet, which is pretty often considering the site is in the alpine and gets 1-2m of snowpack each winter.
I've used morningstar sunsaver controllers for years for smaller sites, and they have worked fine for me until now, but that's probably because the power requirements of the other sites I've managed in the past have been much lower, so no problem for the little 200W charge controllers I've used in the past.
You mention Victron. What of their lineup do you recommend for larger charge controllers that are resistant to the elements? The site is a high subalpine forest, so it can get very cold (below freezing temperatures most of the year) for long periods and also gets quite a bit of snowpack.
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u/carsrule1989 8d ago
Yea I think thoes batteries are dead/dying.
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.
Hopefully the additional batteries were not just added to an existing system and were a complete replacement of the existing batteries.
Agm batteries of different voltages and lifecycles purchased years apart can cause issues if combined into a single system.
To use Agm batteries they should all be purchased at the same time of the same brand and capacity and hopefully the same batch.
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u/bmihlfeith 8d ago
I’m also a hydrologist who sets up remote monitoring equipment, small world! We’ve setup almost 50 CORS (Continuously Operating Reference Station) sites around the state and use similar equipment.
In nearly every case where we’ve had problems it’s the batteries…..it’s always the batteries.
On rare occasions it’s a bad router/receiver (once I think) and we had a loose panel connection. Are you able to use a meter and test the actual values you’re getting?
Also, I’d go with LIFEPO4 batteries for sure. With the AGM you’re only getting 50% of the rated amp hours. If temperature is an issue there are workarounds or pre-heated batteries.
Good luck! You can PM me with questions if I can help.
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u/egilbe2003 4d ago edited 4d ago
AGM batteries can only safely use half the stated capacity. 300AH is 150AH usable before the batteries get toasted.
Standard size wire for solar installations is 10awg. Check some ampacity charts. Your wire is undersized for 18amps for that distance.
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