"I have an onboard 2 bank charger wired into them that is plugged in 24/7, is this enough to keep them safe during the winter.? Average temps we could see low 30's with some nite's low 20's "

it depends on what kind of charger you are using...

 

Not sure about the brand but it's a trickle charger

 

I realize most folks say "trickle charger" when they really mean "float charger" or "battery maintainer", but they're very different things. A trickle charger puts out a constant very low current and doesn't know anything about the battery's state of charge, so needs to be monitored because it will overcharge if left on too long; fortunately, they're hard to find these days. A float charger limits voltage to about 13.2v, which is a 12v lead-acid battery's float charge voltage, which means you can hold it at that voltage forever and keep it fully charged but not overcharge it. Those called battery maintainers are often a little more sophisticated, but do essentially the same thing.

All that to say, for maintaining 12v batteries in storage, my first choice is a simple float charger, but it's hard to find a good one without making it yourself. I've had a Harbor Freight cheapie for 20 years that worked great up until a year or so ago, when it started going way over voltage and fried several batteries before I realized it. And several newer ones I bought with the same part number go to 14v or even higher, which will fry batteries. Which brings us to...

I now have a couple of the one linked below. So far, they've worked great. They have a repair/recovery mode that's not advertised and gets only a passing reference in the manual, but actually recovered the 4 batteries the cheapie ruined (but didn't help other dead batteries that have sat around several years). They're $40, but there's usually a coupon for less, currently $28 (the second link).

4Amp Fully Automatic Microprocessor-Controlled Battery Charger/Maintainer

Amazing deals on this 4Amp 12V High Freq Battery Charger/Maintainer at Harbor Freight. Quality tools & low prices.

www.harborfreight.com

Harbor Freight Coupon DB - 4 AMP FULLY AUTOMATIC MICROPROCESSOR CONTROLLED BATTERY CHARGER/MAINTAINER - $

The best Harbor Freight coupons, at your fingertips.

www.hfqpdb.com

 

Several brands of trickle chargers out there. I highly recommend them and use them on my bike and SXS.

 

There are lots "battery maintainers". Cheap ones will fry a battery dry, seen it happen to lots of batteries.

 

as long as theyre charged up they should be fine. If they go dead the acid is basically water, the battery will freeze and split open from wikipedia
In the discharged state both the positive and negative plates become lead(II) sulfate (PbSO
4), and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water.
The discharge process is driven by the pronounced reduction in energy when 2 H+(aq) (hydrated protons) of the acid react with O2− ions of PbO2 to form the strong O-H bonds in H2O (ca. −880 kJ per 18 g of water).[9] This highly exergonic process also compensates for the energetically unfavorable formation of Pb2+(aq) ions or lead sulfate (PbSO
4(s)).[9]
If you are in doubt put them in those battery boxes and check them before spring startup.

I generally expect a marine deep cycle to last at most 4 years.

 

"Not sure about the brand but it's a trickle charger"

Charlie is spot on....many "trickle chargers" will kill a battery....best to know what you have and if it has the "smarts" to not kill your batteries...

on my boats I used "Guest" smart chargers....and on my cars and motorcycles...I use Deltran...battery tenders...

 

My trickle charger is made by Interstate and it will trickle charge until the batteries are full then turn off till the batteries go down again. Like my Battery Tender brand of charger.

 

Most newer marine chargers will float fine. With that said, I don’t leave mine plugged in 24/7, I recharge when get in from using them. Time to full will vary, but typically from 4-12hrs then unhook it. When not in use I will charge 24 hrs every month to let the charger condition them and have never seen them drop below 95% in that time. I also leave mine in the boat, un heated all winter (-10 or less not uncommon) and have never had an issue. Current batteries are 3-4 years old...4 batteries, 36v troller and start battery.

 

Thank you Waterboy that was the answer I was wanting to hear.