Inverter fault alarm under minor load, bad batteries?

[whomba]

*Question* - I think one, or both, of my batteries are bad. Looking to either 1) confirm I need new batteries 2) figure out how I can 'make them healthy'. I'm also looking to understand how I can prevent this from happening in the future.

*Problem* - My Renogy 3000w Pure Sine Inverter is beeping when under load (around 20w from charging 2 lamps, 2 phones and water pump). Based off my research it appears it is likely because the battery has low voltage, this is because my renogy battery monitor is showing the battery as being ~ 10.9v under load. The Charge controller is flashing error code `E3`, Battery health display shows 96-98% full.

*Background* - I've recently purchased an off-grid cabin which has a solar setup in the mountains in the pacific northwest (think lots of clouds in the winter). It appears that the prior owner bought everything from Renogy, the current system has:
1. 3000w Pure Sine Inverter
2. 40a Rover mppt charge controller
3. 4x - 100w Solar panels
4. 2 lead acid Interstate Batteries, groupsize 31t, with a creation date of 6/22
5. Battery Health display (shunt?)

We go out to the cabin once every couple of weeks, outside of that, the entire system is on/connected but no load is on the system. Approximately 2 weeks ago while I was there, everything was fine and worked as expected. I ran my cpap machine all night, charged a bunch of stuff and it worked great. This past weekend after we turned on the water pump, and plugged in our usb lamps (total load < 20 watts) after 30 - 60 minutes, the inverter began beeping and shut down. Ambient temperature in the cabin was ~ 40ºF The following day when there was a tiny bit of sun (charging < 10w) I tried again - this time 20w was fine, but bumping up the load to ~ 30w caused the voltage to dip, fault alarm to go off and inverter to shut off. The battery health shows it's 96-98% full, under load of 12v marine water pump, the voltage on the batteries show a dip from 11.6v ish to 10.8v.

So, at this point, I'm trying to determine if the batteries are bad. I am unsure if
1. I just need new batteries (and should swap those lead for LiFePo4)
2. I can 'make them work' by attaching to a generator to charge them up (didn't have a generator to get them charged, so haven't tested)
3. something else?

Additionally, is there anything I should be doing *differently* in the future to prevent this from happening? This is my first time doing solar, so unsure if this is par for the course.

It's quite unfortunate that I don't have internet out there and it's a few hours away so my feedback loop for testing is a bit miserable, so I'm writing this not near my system.

Attachments:
1. Pic 1 - health display sitting idle
2. Pic 2 - entire system

Obligatory - no wires are loose, nothing is hot, etc.

 

[robbob2112]

What does a meter show the actual battery voltage is? Battery health displays get reset either automatically when the charge hits a maximum voltage or they have to be reset manually. If you haven't been resetting it, or checked the health of the batteries it may show full when it is in fact empty.

Also, lead acid? have you checked the water in all the cells and if low refilled with distilled water? Never mind, I don't see any way in your pictures to check the water in the cells.... if you have been overcharging them the water will boil away and vent through the hole in the middle on the left side of the batteries.

You might pick one up and see if it weighs more than the other or if they weigh less than they did when new - use a scale - lookup the battery specs if the weight isn't printed on them.

 

[GXMnow]

This past weekend after we turned on the water pump, and plugged in our usb lamps (total load < 20 watts) after 30 - 60 minutes, the inverter began beeping and shut down.

What is the ratings on the water pump? I highly doubt any water pump will be less than 20 watts.

 

[whomba]

@GXMnow you are right, my mistake, 12v dc w/7.5am draw @ 50psi. When it kicks on while i was testing, the health meter shows a 12w draw.

@robbob2112

What does a meter show the actual battery voltage is?

didn't have a voltmeter up there :/ it's packed for our next trip.

If you haven't been resetting it, or checked the health of the batteries it may show full when it is in fact empty.

Interesting, I'll try to find out how to reset it. Plan on printing out the various manuals before i head back up there.

You might pick one up and see if it weighs more than the other or if they weigh less than they did when new - use a scale - lookup the battery specs if the weight isn't printed on them.

Ok, no idea how much they should weigh as I didn't buy them. I can bring up a cooking scale though

Additional video of my wife turning on the water and watching the voltage dip:

https://imgur.com/a/X8PLOqQ

 

[Mattb4]

You could do all kind of effort to see if the batteries are recoverable but being that they are ~2-1/2 years old they likely are too heavily sulphated to bother. Simple voltage readings will not help because they could have a surface voltage indicating full charge but once a load is put on them it will sink out of inverter operating range. I am guessing you do not have a battery load tester but you could take them to a battery store and have them test them.

Time for new batteries is my guess.

Hard to prevent simple sealed lead acid batteries from failing at around 3 years used in a environment where they do not get fully recharged at times and no equalization charge is made every few weeks.

 

[whomba]

@Mattb4 that is where my fear / feelings were headed too - So pivoting to some LiFePo4 Battleborns or similar seems like it might make sense?

Ignorant question - the rest of my system shouldn't care about swapping in new non-lead acid battery - is that right? Meaning, i know i can't mix & match a lead battery with lifepo4, but, the inverter / mppt don't care about the battery type changing as long as the volts / amps are within range. Correct?

 

[robbob2112]

Do your research, battleborn is very good at marketing... but the batteries 1/4th the price are as good or better

 

[GXMnow]

Additional video of my wife turning on the water and watching the voltage dip:

The battery monitor is obviously not measuring all the the load current, it sits at just about 1 amp for that whole clip. It keeps showing full charge because it is not seeing any discharge current.

I agree, the batteries have probably lost most of their capacity. Sulphated plates basically act like much smaller plates. So the active area of a cell is greatly reduced. It will charge up and act fine, but the true capacity is only a fraction of what it should be. Unless it has caps to add water, equalization is not a good idea as it will boil off some electrolyte. It is normally only done on flooded wet cell batteries. It will ruin Gel or AGM batteries very quickly. Sealed or VRLA will vent some vapor and lose fluid until they are killed.

What are the conditions at this location? Does the space stay above freezing? If it does, I would recommend going to an LFP battery. But if it drops below freezing, that can cause some issues. LFP cells should not be charged below freezing as it will damage the cells quickly. Stopping charge helps, but then you are not getting energy. They also have batteries with built in heaters, but they pull some of the charging power and you still need them in an insulated space to hold in the heat. If the batteries need to deal with freezing temps, especially when you are not there, it might be best to go with quality flooded wet cells and do a proper equalization and watering schedule.

 

[GXMnow]

the rest of my system shouldn't care about swapping in new non-lead acid battery

Lead Acid pre sets are typically pretty close for LFP, but you want to make sure that Equalization is turned off. Depending on how old the units are, they may have a preset for LFP. The important settings are in the charge controller. Mainly it's the Absorb voltage and the Float voltage that you want to be sure are safe for your batteries. The inverter won't care, but hopefully it will shut down above 10 volts, if not, you might also want to get something like a "battery protect" which will shut it down at about 11 to 12 volts. If it runs lower, the BMS should still protect the cells, but it is not good practice. This of BMS protection as a fuse. It should only trip when something goes wrong as a last resort.