Opinions wanted regarding over-wintering batteries in off-grid cabin

[Fenachrone]

Each year, I shut down my off-grid vacation cabin on Canada's Prince Edward Island for the winter. Over the years, with my flooded lead acid Rolls S-460 batteries, I would leave my charge controller on and three of my six 245-watt panels on. This would extend about six months until I arrive and switch everything back on.

Obviously the batteries weren't cycling over those shut-down months and I know that was not helpful for their longevity. Still, they lasted well beyond their warranty and were protected against freezing. But ultimately it was time for new ones and have replaced them with four Stark SRK-533 AGM batteries.

I'm finding contrasting opinions on an over-wintering strategy with respect to AGMs. The distributor is in favor of leaving the charge controller and all the panels on. But I have read that these AGMs lose only 2-3% charge per month when stored, and even better in colder temps. Following this change of thought, leaving them this way, in a fully charged state, does not affect their ultimate longevity. The advantage of leaving the system entirely off is the charge controller (an Outback MX-60, now 14 years old) can't malfunction and endanger the batteries. With the place boarded up, there would be no way to monitor this. In the case of my original batteries, though, for their survival, it was a matter of necessity of leaving some charge going into them.

Anyone have experience with this type of situation or a thought on which way to go?

 

[Short_Shot]

As I understand it, properly float charging lead acid is the key to longevity. Assuming they are kept above freezing.

 

[JaVid]

So you leave them alone for 6 months? What is lowest temps they will see during this time and when during those 6 months will they see these low temps?

If they only lose 3% charge and you aren't getting below -10 F, from what I know, you don't have anything to worry about just leaving them unhooked. Best would be to see what battery manufacturer recommends, but a fully charged or mostly charged AGM can withstand something like -40F before freezing...

 

[Fenachrone]

Yes, six months will be the maximum, but I may be able to leave the system up a month longer, so more like five months.

The batteries are in a vented crawl space, but the winter weather low on this island doesn't get below 10F. That is only briefly, in January, with temps higher the adjoining months. I have them wrapped with 2-inch insulating Styrofoam, as I did my flooded batteries, which never froze.

 

[Horsefly]

You will be fine. Our system is in the mountains of Colorado at 9,000 ft elevation. It gets cold. We have gone 4 years with our AGMs using the same config as you: Charge controllers on, loads off, and panels feeding the SCCs. This keeps your batteries fully charged, and fully charged AGMs - just like flooded - won't freeze until something like -70 deg F.

 

[JaVid]

He likely would be fine to keep them as you suggest, Horsefly, but if they were my batteries, I would simply unplug the whole system.

Here is chart that looks like it shouldn't be any problem to unplug with those temps in January - he would have to be at 50% discharge to freeze at 5F - very unlikely to be that low in January and that weather would be extreme.

Freezing,
Percent Specific Degrees
Charge Gravity Fahrenheit
100 1.265 -75
76 1.225 -35
74 1.200 -17
50 1.150 5
0 1.100 18
<0 1.050 27

Edit : sorry chart looks so crappy - it looks better in my draft...

 

[Wellbuilt]

I have left my FLA battery’s for 3 months and when cold the stay pretty much @ full charge .
But it’s cold 20o and below .
I get snowed out of my place every 4 years or so , last time my charge controller died and wiped out my battery’s
Now I just turn it off and leave .