Over-wintering batteries in off-grid cabin -- what choice to make.

Fenachrone · October 2021

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 batteries, I would leave my charge controller on, my inverter off (and consequently all loads) and three of my six 245-watt panels on. This would extend about six months until I arrived and switched 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, the belief that leaving them this way in a fully charged state does not affect their ultimate longevity. And the potential advantage of leaving the system entirely off is that the charge controller (an Outback MX-60, now 14 years old) can't malfunction and endanger the batteries, as small a chance as that might be. 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.

I could do the same thing with the AGMs: Leave one string of 145 watt panels on. But here in this winter climate the arc of the sun is low and charging is dramatically reduced. So, leave both strings on? Completely shut down?

Anyone have thoughts on which way to go?

RCinFLA · October 2021

2-3% per month self discharge rate on AGM is reasonable at cooler/cold temps.If you fully charge before leaving should make it.

Marc Kurth · October 2021

I have many customers with applications in extreme northern climates that survive very nicely, fully charged then shut down/frozen for months on end with AGM batteries.

littleharbor2 · October 2021

After fully charging, physically disconnect the batteries to ensure absolutely no drain whatsoever. You'll be fine with AGM's

Riley · October 2021

Good topic: I have the same question except with FLA bank -
Here's my story: I have a remote off-grid cabin that is seasonal.
By that I mean that I don't use it in the winter time. So it's all powered off (inverter off) in the off-season. I actually open the breaker to the inverter from the FLA battery bank.
I always leave the cabin with the batteries fully charged.

I thought that opening the breaker, the batteries were basically not connected to anything but on closer inspection, I realized that the charge controller was still connected.
Because in the winter time, snow can sit on the panels for weeks at a time and the sun is very low in the sky... can be nothing coming from the panels and I was concerned that the charge controller would be drawing current from the batteries for extended periods of time with no recharge (probably very low leakage currents so maybe no big deal).

This year when I realized this and was heading out the door, I tried disconnecting the breaker for the charge controller so there would be zero load on the batteries for the next 4 months....
But on that sunny day, I got a fault from the charge controller which wasn't surprising because no batteries were connected. I wasn't sure I wanted to leave it in that fault state so I reclosed the breaker.
I didn't want the batteries to get over charged or cycled with a bulk/absorb so I turned down both bulk/absorb to the Float voltage. Meaning the the charge controller would just do float 100% of the time.

What do you guys think I should do? Open the charge controller breaker and not worry about the fault? Leave on float and not worry about it?
or ??

Thanks much for your guys expertise.

Riley

BB. · October 2021

A couple of folks here put a couple (100 Watt or so) panels on a vertical south facing wall with a simple PWM controller... As long as your south wall does not "drift" for months at a time, it works nicely to keep even an FLA bank charged.

Let the batteries get very cold, the self discharge much slower, and you can go a couple months without float charging.

PWM controllers use very little energy when not charging... MPPT type controllers can take more power (Tare losses) and significantly discharge a battery bank if the panels are snowed over.

Say your MPPT controller takes ~ 1 Watt of power. 2 months of "no sun" (spec for Outback FM 80):

1 Watt * 24 hours per day * 60 days = 1,440 WH of discharge
1,440 WH / 12 volts = 120 AH of discharge

-Bill

706jim · October 2021

I left my system on charge over the winter with a Trace C40 PWM controller for 25 years. No problems.

I now leave my system on charge over the winter with an Outback FM80 MPPT controller for the past three winters. No problems.

And I'm sure it gets as cold here as any place else in the country. (As low as -40)

Panels covered with snow for about three months when I can't get at them to clean off the snow.

ramloui · October 2021

I'm in my 9th winter with the same setup. My panels are organized in 3 strings. Only 1 string gets shifted to steeper angle. Notice how the vertical panel shed snow. Batteries are always 100% when I open the cabin in the spring.

Image: https://us.v-cdn.net/6024911/uploads/editor/wh/7kyj6ae7kj5d.jpg

Riley · October 2021

I think I am going to find a way to simply disconnect my MPPT controller in the winter time. Given FLA self discharge is very low at low temperatures, this should work fine.

wellbuilt · October 2021

I just turn my system off when I leave around Christmas and if I’m snowed out I’ll try to snow shoe up around feb 15th so 6 weeks and I’m all ways close to fill charge or above .
1 watt on a fm80 is not much I would be tempted to leave it on .

Most years we get a warm up twice a year and the snow slides off

BB. · October 2021

More or less, for every 10C below 25C, the "thing" (aging, self discharge, etc.) is reduced by 1/2...

25C = ~ 1 month between charge cycles for stored FLA batteries (typical specification)
15C = ~ 2 months
5C = ~ 4 months
-5C = ~ 8 months

Also works for "hot"

35C = ~ 2 weeks between charges

etc...

Regarding leaving anything with a "minor" power draw (such as 1 Watt) for weeks or months can be a problem with battery banks that are not under normal solar/utility charging conditions.

-Bill

Over-wintering batteries in off-grid cabin -- what choice to make.

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