Trying to fix a low voltage cutoff issue with my RV based inverter

[Hedges]

Seems to me your issue isn't battery technology, but rather available charging.
Could be tired batteries, but at 18 months that would likely be the result of insufficient charging.

Lead-acid may have 70% round-trip efficiency (more or less) so lithium gives a bit more usable Wh.
But probably more PV watts (more panels or tilted orientation) is what's needed.

With lithium, shouldn't have to worry about degradation due to not fully recharging. But you do want to avoid bumping along empty.

 

[skftw]

I'm actually not the original owner of these batteries, so I can't say for sure how they were used/abused in the past. The previous owner of this RV had put very few hours on the generator, didn't swap out any of the 25+ lights inside with LEDs, and said he bookdocked almost exclusively, simply claiming warranty on the batteries each year as they failed. I would guess that he ran them way lower than 50% SOC and I'm missing a fair bit of the rated capacity on these as a result. Given that it'd be $200ish to just replace them like for like, I'd rather throw a few hundred extra into getting the better tech and doing this properly. I was hoping that with them being only a year or so old he couldn't have done a ton of damage to them, but who knows.

I did go slightly oversized with the MPPT controller in anticipation of possibly needing more power down the road. If I'm not getting solidly into absorption/float modes each day then I'll definitely throw more panels on the roof.

 

[Hedges]

Wet-cell FLA want a particular charge rate for best life. A charge controller putting out exactly that (for better yet monitoring a battery current shunt and regulating that) with PV panels aimed at both morning and afternoon sun would be ideal.

Lithium, according to specs, is tolerant of current over quite a range. However, they have maximum charge rates that vary with temperature and state of charge.
Maybe you can get current to tail off above a particular SoC by setting appropriate absorption voltage.
You also have to reduce or halt current below some temperature. That temperature isn't zero degrees C unless you have relatively small PV.
I think setting a higher minimum temperature for charging is the easiest way to do that.

Since you have or plan on Victron MPPT, see if it can work with shunt and regulate battery charge current even with much larger PV array supplying inverter loads. That feature seems to be lacking in most other brands.

Going to assembly DIY lithium?

Before giving up on the FLA, check their specific gravity and see if they need equalizing.

 

[skftw]

I ran the equalization cycle on the Victron and it actually made a substantial difference on the following day's charge. Sitting idle, the production logged was about 110Wh less than the day before. It seems that must have fixed a low cell and allowed the normal float charge to occur with less actual power. Pretty happy with that result.

I did order 4 of the Lishen 272Ah cells to DIY a battery pack. I haven't decided on a BMS yet but I have plenty of time to figure that out. Even if not 100% required, it should be a nice upgrade. I'm not too worried about temperatures. I'm in Phoenix and won't be camping in the snow much at all. I might relocate the batteries to be inside the RV to help with temperature range. There's not a great location that would ensure they actually get heated, but it would at least alleviate some wind chill.

 

[Hedges]

With 300W of PV, charge rate will be under 0.09C for 272Ah 12V pack. That makes it pretty benign over a wide temperature range.

Did problem of morning low-voltage disconnect when starting fridge get cured, just with equalization charge?
"occasionally had issues"
Sounds like a number of nighttime usage cycles have to occur before you have confidence that doesn't happen any more.

 

[skftw]

I haven't tried it yet. The RV is currently parked (in the sun) with the fridge turned off. But since even that daily float charge was affected, it appears that the equalization cycle did at least something productive.

I'll have at least 2 more long trips before the lithium cells arrive, so I can certainly test this out a bit. I still need to get a hydrometer to really see what's going on inside those lead acid batteries in the meantime.

 

[Hedges]

Check the rest voltage of the battery, a few hours after charging, like in the morning before sun hits the panels. Also measure battery temperature.
That should show if all the cells are full charged, comparing to documentation.
But I'm not sure how cell voltage is expected to change with age.