almost started a fire

[wine_guy_3]

I have a cabin in Northern Wisconsin.
6 100 watt panels feeding 4 different  charge controllers.
One  for main battery  bank, three others to keep the ATV, mower, and tractor batteries topped off.
BAD IDEA.

I normally go up there every 2 or three weeks.
Due to the -25 degree temperatures, I  didn't get up there for about 3 months.
When I walked into the garage, there was SMOKE coming up from one of the ATV batteries.... a LOT of smoke.

I assumed the charge controllers would (essentially) shut off when the batteries were full. Evidently not.
I wrecked one battery and almost burned the place down.

I ain't gonna do that again.
Panels are now disconnected whenever I leave.
Just thought I would pass along my experience.

[bigrockcandymountain]

Sounds like the battery had an internally shorted cell or something like that.  The charge controller would be trying to charge and the short would be draining all the power and making heat.  It's lucky you caught it when you did. 

There are some charge controllers with a battery saver mode that might be better for your system.  Or you could just turn down the charge voltages when you leave.  Leaving batteries without charge for long periods isn't ideal either. Unhooking the panels is certainly an option. 

[SparWeb]

That was a close call!

The charge controllers will throttle down the current when the batteries seem full, as you assumed.  However, the way the CC figures out that they're full is simply by tracking the trend of current demanded to maintain a given voltage, and holding the voltage (this is for lead batteries, different for NiCad and Lithium, but you're talking about ATV and tractor starting batteries).

This is a problem when the CC is fooled into thinking that the battery is discharged when actually something is wrong that's accidentally holding the voltage down.  Then the CC responds by opening the throttle and passing as much current as it can - with disastrous results.

I will second the suggestion from BRCM to unhook most of the panels but add my 2 cents: not all of them.  If you can hook up just enough panels in a way to offer a small trickle to keep them topped up, long-term that will help them pass through the -25 temps.  I have a few of these 1-square-foot panels for jobs like this.

One thing about batteries (bear with me if you may already know this) is that they are more resistant to freezing when they are fully charged.  Mine get it worse than yours, and have to take -35F once or twice every winter (not this one it seems, touch wood).