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diy solar

HELP! Sudden voltage drop from 70%(12.5v or so)

tybearthegreat

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Oct 28, 2020
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Please bear with me because I'm very new to this and learning everything as I go. I will do my best to describe the issue with the utmost clarity and describe my setup to the best of my knowledge.

We purchased a converted bus from the previous owner who did all of the work including installing the entire solar setup.

The original setup:
6x 100-watt Renogy panels(I'm not entirely sure how they are wired)
40a-amp Rover MPPT charge controller
3000-watt Renogy inverter
4x 200ah Renogy AGM batteries(about 2 years old when we bought the bus)

The new setup:
Replaced the Renogy AGM batteries with four USED 138ah Valence LiFePO4 batteries.

The story:
The only thing that is 12v on the bus is the Shurflo 4008 RV water pump that's wired directly into the battery bank. The batteries were wired in parallel with varying cable lengths and gauges. The inverter was wired to the bank with 6AWG cables. We were noticing the life of the batteries was slowly depleting and we knew that would be the case with the number of cycles on them. So, we pulled the trigger on FOUR 138ah Valence LiFePo4 batteries from eBay. I wired them up in parallel and we were seriously impressed with the performance. The batteries didn't drop below 90% for a few days, but those were some nice, sunny days. I'm in the Pacific Northwest by the way. We got a few cloudy days and the batteries began to deplete a little bit more down to about 70%, but that was completely expected due to the weather. The confusing part is that the batteries dropped from 70% down to around 30% in one night and that immediately freaked us out because we JUST spent a couple thousand dollars on these batteries.

I did some research, pulled all of the batteries, charged them individually with my NOCO Genius 10 charger, and all four batteries were resting at about 13.15'ish volts. I wired them back up in parallel with all new 12-inch 6AWG, connected the communications cables on the batteries so they could properly balance, and let them sit for 24 hours or so. The bank was resting solid at 13.15v when I hooked them back up to the inverter with brand new 5-foot 2AWG cables. I removed the larger loads from the system because I wanted to introduce things slowly and see how the system performed. The only loads on the system were the 12v water pump and the lights and outlets the inverter supplies power to via the electrical panel on our bus. The only things we ever plugged in were cell phones to charge, the router/modem that's always running, and the composting toilet fan that's always running. We did NOT plug in our TV, refrigerator, or computers. All of those are currently running off of house power via extension cords.

Again, everything was great and the batteries were generally hanging out around 80-90%. Then we got another run of bad weather and the batteries depleted down to around 70%(12.5'ish volts) today. Then, out of seemingly nowhere, the batteries went from 12.5 to 11.5(30%) in only a couple hours. I've since turned off the inverter via our remoted switch in the bus and the batteries are sitting solid at 11.8 volts. It did the same thing last time as well before I topped the batteries off and swapped out the cables. For the life of me, I can't figure out what is causing the sudden drop of voltage after 12.5'ish volts. The Renogy BT app kind of sucks and there's not a lot of data that I can pull from that to help either.

I apologize for the novel, but I feel like I've tried everything and spoken to everyone I can. Renogy just keeps asking me for voltages of things with more screenshots every time I email them. I get it. The more information they have, the better, but nobody seems to be able to tell me, "Hey! It sounds like it COULD be this, so try doing this!"

While doing research, I've learned that our panels, charge controller, and battery bank aren't sized properly, but could that possibly explain the rapid drop in voltage even with VERY small loads on the system?

I'm open to any and all critique, tips, and whatever else you can throw at me!

Thank you so much for your time!
 
Are you 1000% certain you purchased new batteries from ebay?

Did you conduct any testing on them to confirm they were both new and met rated capacity?

Did you know that your inverter pulls 30W all the time doing nothing? The circuitry that makes 120VAC available on demand consumes power any time the inverter is on. 30W * 24h = 0.72kWh.

Your total battery capacity, assuming they actually meet it, is 4 * 12.8V * 138Ah = 7.1kWh.

That means that just having the inverter powered on, but with nothing plugged into it, it will consume 10% of your battery capacity every day.

40A*14.4V = 576W = the absolute maximum you will ever get out of your 600W panels due to your undersized charge controller, and that's at peak voltage. When you're charging at lower voltages, you'll get less power. Your panels, presumably flat on the roof, are likely not bringing in but a fraction of the power they might in full summer.

13.15V after a full charge is NOT at a fully charged state. It is likely closer to somewhere around 50%. You should be charging your batteries to about 14.4V to ensure they are fully charged.

Likely causes of your situation:
  1. Improper charging.
  2. Using more energy than is available.
  3. Harvesting less solar energy than you are using (basically same as #2, but from the other direction)
  4. Batteries that do not meet spec.
Can I assume you have conducted an energy audit, or are otherwise aware of the exact amount of energy you are using?

Here is my usage for the last 20 days:
1606110704929.png

Most days are around 2.7kWh - likely well beyond what your 600W array can supply. It powers the following:

AC-DC converter consuming about 100W that powers LP detector, DSL modem/router, wifi camera interface, 4 small LED bulbs and a battery charger floating my spare 12V batteries. That's it. It runs them 24/7. It also includes the idle power of the inverter. The difference is the inefficiency in FLA charging.

You can see the difference when we're actually there using power.

This is for a single 5th wheel trailer.
 
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I agree with snoobler's points. Not enough solar to get a full charge.

How many amps are being pulled from the system? Is there perhaps something running at night that is pulling more amps than it should?
 
I have a similar problem as tybearthegreat above, I have eight 270amp lifepo4s wired in series and parallel giving 540Ah at 12V. I have set my 120Amp JBD BMS to 80% giving me 432Ah. I have charged the batteries to 432 Ah, disconnected ALL devices and left overnight to rest. I do NOT have an inverter installed as I do not use any heavy accessories. I have disconnected the solar panels. I checked the batteries 24 hours later and they still displayed 432Ah, all well so far. The batteries had a balance of .003, and still do.
I then noticed that in the system it was showing, with a red triangle, an incorrect low temp setting. I reset the temp as the JBD app instructs but then noticed that the 432Ah was now suddenly showing just 302Ah a SUDDEN loss of over 100Ah. I am unsure of what to try next as I have been having this problem for about 4 months when pressing Reset Capacity, however this is the first time I have charged to 432Ah and it has given the biggest SUDDEN drop.
I am considering splitting the battery, make 2 12v 270Amp top balance, and then recharge. I do have a spare 60amp BMS which I can also try as well.
Could this be a faulty BMS controller? your thoughts would be appreciated.
 
The state of charge shown on my BMS was off until it went through a few charge/discharge cycles. I charged to full (14.4 volts) and discharged to maybe 70%. It's the charge to that high voltage number that the BMS seems to key off of.

I have two 4s 280 Ah batteries, each with a BMS, connected in parallel for 12 volts.
 
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