Frozen flooded batteries

[Sunking]

It is tricky to get up on the roof to dink with the panels, but that is something I've considered.

I am mulling buying a 50W panel that I can mount in a south-facing window for the winter.

That will turn that panel into a 5 watt panel.

[Sunking]

You are below 50%.

[gardner]

Raise the panels to a near vertical position for the winter

It is tricky to get up on the roof to dink with the panels, but that is something I've considered.

I am mulling buying a 50W panel that I can mount in a south-facing window for the winter.

[gardner]

That chart is for +25C. At low temperatures the OCV for a given SOC drops. Below -15C it drops quite steeply.

[Sunking]

The day before they froze they had an OCV of 11.7V at a temperature of -25C -- that is approx 40% SOC or so. They were by no means fully charged, but not totally discharged either. Even a 80% charged FLA will freeze at around -40C.

No 11.7 volts is roughly 20% SOC. They should never be allowed to go below 50% or 12.1 volts. Click on the chart.

battery-state-of-charge.jpg

[Willy T]

Raise the panels to a near vertical position for the winter is what most do with a remote unmonitored site. They won't be snow covered.

[gardner]

The day before they froze they had an OCV of 11.7V at a temperature of -25C -- that is approx 40% SOC or so. They were by no means fully charged, but not totally discharged either. Even a 80% charged FLA will freeze at around -40C.

[Sunking]

If the battery Specific Gravity got low enough for the batteries to freeze means they have been in a discharged state for a very long time and have sulfated. Meaning your batteries are toast. No way to revive sulfated batteries. In extreme cold climates you should be looking at using AGM batteries. But even AGM batteries will dioe if left in a discharged state. They just do not freeze and crack the battery jars and let the electrolyte leak out.

[gardner]

I have an off-grid installation with a pair of US Battery 230Ah golf cart batteries. It's remote and I can only visit once in a while in the winter. I do get SMS messages from the system 4 times a day. The problem is that the solar panels get covered with snow and the system stops charging for months at a time. The loads are tiny -- a few dozen mA -- but it's very cold and over the course of a few months the SOC gets down to the point that the batteries can freeze at -25C or so. Freezing, obviously, really knocks the capacity out of them and they never regain their original vigour, if they survive at all.

Any wisdom about recovering/reconditioning a battery that has been frozen? What sort of actual damage occurs? Would replacing the electrolyte or doing a long equalization charge likely help?

I am looking for some ideas to safeguard the batteries against freezing. One idea is to locate a small 20W panel in a location that can't accumulate snow and use that to provide a topping charge.

If I have to replace my batteries in the spring, would switching to AGM help? I don't know how those stand up to freezing, but if they are significantly superior, I would give them a go.

Any other ideas?

Thanks