Battery (AGM) winter storage

[seadrift]

After this summer I will haul my boat on the east coast of Canada and leave it unvisited for 7 or 8 months. It has 3x92Ah AGM house batteries and 1x92Ah AGM start battery.

Should I disconnect all leads and leave the AGM batteries to lose the theoretical 2% per month; or install a (say) 300mA solar panel and connect it through an existing Blue Sky 2512iX charge controller to an electrical system with the conventional battery switch on ALL?

For the second case, each battery bank has a fuse at the positive terminal, but I feel a little uncomfortable leaving the electrical system live.

I guess I could add the panel, disconnect the house batteries from the boat system (the charge controller is wired directly to those batteries) and leave the start battery to fend for itself (discharge to 85%)?

Previously, I was in a position to remove the batteries and trickle charge them for the winter.

[Vasco]

It's easiest to just leave them in the boat fully charged.

[seadrift]

You are likely right, Vasco, what with the problems of tying a solar panel to the boat cover to receive the meagre/meager rays.
However, you probably know the cost of four AGM batteries in Canada...

[Vasco]

They should survive. I leave my floodeds in all winter without charging and they survive.

[ranger58sb]

We leave our AGMs in the boat and starting with fully charged whenever we winter on the hard, with bilge pumps active.

I made a point last time to NOT charge them at all during our winter, just to see how that'd work. Fine.

-Chris

[btrayfors]

Yeah, they'll "survive" the winter, and if the ambient temp is low enough they will benefit from a lower self-discharge rate.

But...and it's a possibly big "But"....how much capacity has been lost over the winter? Capacity, not voltage.

The only way to answer that definitively is to measure their capacity carefully with one of two accurate methods (20-hour controlled discharge or use of an expensive capacity tester like the Midtronics series) both at the start of winter and afterwards in the spring. Only then will you really know what damage has been done by letting them sit over the winter.

I'd bet that:

1. some real capacity will be lost, more for flooded batteries and those in relatively warmer ambient temps; and

2. most owners won't know the difference because they'll be lulled by the batteries ability to "take a charge" and/or "hold a charge" into believing the batteries are just fine.

Bottom line: it's always better to maintain batteries at full charge AND periodically boost their voltage to absorption levels in order to minimize the dreaded sulfation. Anything less than that WILL result in some deterioration of capacity.

Bill

[Cuttyhunk]

I have AGMs and they wintered on the boat in Rhode Island. I happen to own a fancy capacitance tester and tested my AGMs (which were new in the spring of 2013) a couple of times over the winter. I intended to top them off, but each time, they were still very we'll charged and the capacitance tester, which showed them at 98% in the fall showed them at 95% in the spring.

I was very pleasantly surprised by this. I did disconnect the batteries. Since I have a not-yet-located rainwater leak on the port side, I had to visit the boat often to hand pump out the bilge. I would have to rig a solar panel if I kept the bilge pumps connected.


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[btrayfors]

Cuttyhunk,

Thanks for the real-world experience note.

What kind of tester do you have (make, model)?

These numbers, if they are indeed real and reliable, are about what one might expect for AGMs stored in cold conditions over winter.

I'm sure you understand that losing 3% of capacity is not the same as losing 3% voltage! Voltage can be quickly restored when charging, while capacity loss may be permanent and irreversible.

Bill

[mbianka]

I've got two banks on board. One is the house bank made up of two group 27 gels and the other my 48 volt electric propulsion bank made up of four 8A4D AGM's They have been left on board hooked up to the solar panels via solar controllers through six northeast U.S. winters with no problems.

[btrayfors]

Originally Posted by mbianka

I've got two banks on board. One is the house bank made up of two group 27 gels and the other my 48 volt electric propulsion bank made up of four 8A4D AGM's They have been left on board hooked up to the solar panels via solar controllers through six northeast U.S. winters with no problems.

Yep, that's exactly the right way to do it. If everyone did it this way, and didn't skimp on controllers and installation, we'd not be hearing of or seeing so many problems.

Bill

[seadrift]

Originally Posted by mbianka

I've got two banks on board. One is the house bank made up of two group 27 gels and the other my 48 volt electric propulsion bank made up of four 8A4D AGM's They have been left on board hooked up to the solar panels via solar controllers through six northeast U.S. winters with no problems.

Those are your regular (summer) panels?
I don't want to leave one of my 100w semi flexible panels hanging on the cover for the winter, so wonder just how small a panel I can use.

[mbianka]

Originally Posted by seadrift

Those are your regular (summer) panels?
I don't want to leave one of my 100w semi flexible panels hanging on the cover for the winter, so wonder just how small a panel I can use.

Yes they are the same panels I use during the season. Though in the winter the aluminum wishbone boom stored on deck casts a shadow on both 12 volt 75 watt panels no doubt knocking their power output down some. But, the house batteries are still always topped up in spite of this reduction. So I think you could safely get by with one of your 100 watt panels keeping things topped up. Especially if you don't have a shadow issue like I do.

[Cuttyhunk]

Originally Posted by btrayfors

Cuttyhunk,

Thanks for the real-world experience note.

What kind of tester do you have (make, model)?

These numbers, if they are indeed real and reliable, are about what one might expect for AGMs stored in cold conditions over winter.

I'm sure you understand that losing 3% of capacity is not the same as losing 3% voltage! Voltage can be quickly restored when charging, while capacity loss may be permanent and irreversible.

Bill

Hi, Bill,

Obviously, a 3% loss is a permanent 3% loss.

My capacitance tester is a Snap-On model EECS400. Although I did a lot of research at the time of purchase, all I remember now is that this one is well-made and will test 6vs as well as 12vs (I am not wedded to AGMs -- they are excellent for my current use, but when the time comes to replace the two very heavy 4Ds, I will likely get 4 or more 6vs instead, as I hope to be cruising year-round and ease of storage will be less important to me). It has a most excellent feature in that the part that attaches to the battery communicates wirelessly with the handheld tester, allowing you to read the meter without having to stick your head into a locker, as in my previous boat. I was in trade school when I bought it, and took advantage of a very, very nice discount from Snap-On. I also have wonderful socket sets and wrenches. And a torque wrench that is a work of art.

You sound a bit skeptical that I actually have a real capacitance tester. I had the opportunity, in trade school, to play with a couple different ones, and learn about what they can tell you about your battery's condition. Having cruised in the past and babied an old battery bank, I considered the investment in this one worth it. Even at around $400. In the past two years, I've gotten quite a bit of use out of it.

[btrayfors]

Cuttyhunk,

Sorry, I didn't mean to sound skeptical....just curious. It's very common for folks to believe that their $29 hunk-of-junk tester from Harbor Freight is accurately measuring battery capacity. Clearly, that's not you.

The Snap-on is a nice piece of gear, as are most all Snap-On tools. I have a couple of 1/4" socket wrenches which, also, are works of art! Wonderful stuff.

BTW, I said, "may be permanent" because the 3% may, in part, be able to be mitigated thru equalization, if the lead sulfate crystals aren't already deeply embedded in the plates.

Bill