What's going on with my battery

[Lawson]

I have a small, unheated cabin in the "canadian north" where I have a 100w solar panel kit connected to a charge controller who is then connected to an uninsulated AGM deepcycle battery.

I installed all this during the summer and everything seemed to be fine and dandy. However, when I went to check up the setup in late January (last checkup was in octobre), I found the battery not even outputting enough to power the LEDs that the charge controller uses. After confirming the charge controller was still functional (with a different "heated up" battery I had at that time) and that the solar panel was still outputting correctly with a voltmeter, I unplugged my AGM battery from the setup.

When I went back last week to check up, I found the AGM battery still pretty dead. However, when plugging it back up to the setup, the charge controller tells me it's over-voltage and that the battery is deeply discharged.

I'm confused about this situation since AGM deepcycle batteries that are full at the start of winter shouldn't be losing much charge during the following months and everywhere I read they tell me they are pretty much "maintenance free".

Should I have not unplugged the battery in january? Should I have not plugged it at all at the beginning of winter?

I would be happy to get any insights on this issue.

[bcroe]

All batteries self discharge to some extent, a FLOAT level of charge is needed some or all of the time.
Snow on your panel could have prevented this, and any even very small drain (your LED?) could
eventually discharge the battery. Bruce Roe

[Lawson]

My solar panel is vertical (not the best positioning, but the one I had at my disposal). However, it's very possible the snow could have covered it for large periods of time. I find it surprising that a couple of LED lights used by the charge controller could have completely emptied a fully charged 70 AH AGM deep cycle battery. Could I be wrong about this?

Would you suggest to simply unplug the battery in the autumn since AGM batteries seem to self discharge minimally during winter.

[BoloMKXXVIII]

If practical, I would disconnect the solar panel, then the battery and take the battery with you if you plan to be away for an extended length of time.

[Lawson]

I thought about that, but it's quite heavy and the cabin is deep in the woods with no car access for about a couple miles. I'm trying to find the best way to keep it there.

Options are :
1 - Leave it plugged into the solar panel and risk it being completely discharged because of the LED usage (which I still find weird)
2 - Unplug it and let it self discharge normally (which is minimal if I understand Gel batteries)

I'm leaning towards option 2, but I'm not convinced since I still don't understand how it could get so discharged when I applied option 1 this winter.

[bcroe]

Batteries self discharge, LEDs or not. Leaving them for half a year even disconnected is risky.
I would check that there is zero drain when the panels are disconnected, the clock in my car
used to kill mine over winter.

Keeping a panel vertical might be the best way to keep the snow clear (most of the time), but
that will not work if sitting on a horizontal surface (roof) that piles snow around the bottom. A
panel must be elevated many feet. Some larger panels will be able to generate FLOAT current
even under clouds, just make sure the equipment has no drain at night, etc. I doubt a 100W
panel is up to maintaining a 70AH battery in unfavorable conditions, the battery experts will
have more to say. Bruce Roe

[Xplode]

My solar panel is vertical (not the best positioning, but the one I had at my disposal). However, it's very possible the snow could have covered it for large periods of time. I find it surprising that a couple of LED lights used by the charge controller could have completely emptied a fully charged 70 AH AGM deep cycle battery. Could I be wrong about this?

Would you suggest to simply unplug the battery in the autumn since AGM batteries seem to self discharge minimally during winter.

Those little LEDs are sadly capable of draining a battery pretty good. And if the battery is in the great white north's northern area, the -20C temps will help reduce the battery capacity/output as well.

A basic 3mm LED (which is probably whats on a cheap charge controller) will run around 1.8VDC @ 20mA. Or roughly 0.036Watts. Doesn't sound like much, but consider that the cheaper controllers might just be using an inline resistor to regulate that current down. When not charging (ie battery voltage of about 12V) the resistor/LED combo would have to drop 12V @ 20mA, or about 0.24W... X24Hours x 30days = 172 Watt hours... Your battery only holds about 840Wh total when charged, and at normal temps... So you can see how one or two LEDs could kill a battery over the course of a few months.

There is also, I'm assuming, an internal microprocessor or at least a few chips that are handling the PWM functions when the sun is up - so even if the LED is powered through an efficient regulator, the other chips are still going to chew up some power. And it all adds up over time.

If you've got a digital multi-meter, try using the Ammeter function and hooking it up in series with the battery and charge controller. See how much the controller is wasting when the panels aren't producing.