Originally Posted by LakeSuperiorGeezer
A lot of sailboats have deep cycle batteries and an engine battery. Both get charged at the same time when the engine is running. While at the dock, I bet most people will charge deep cycle and engine batteries at the same time, and float them once charge is complete with the same charger. Have there been any problems with this method? No one has said so as far as I know. I have read that an AGM battery can be overcharged with an ordinary charger as the voltage can be too high for them.
But when these batteries are charged on a boat they are often done so via a proper marine type charger with separate bank 1 & 2 leads. These chargers often use blocking diodes in the unit so the banks are really not in parallel.
Other methods such as switching a 1/2/BOTH to BOTH are usually temporary while "charging" and while on-board.. I don't like unattended use of BOTH to charge either but people do it. An ACR type relay only combines during charging and an Echo type device does not combine nor would an isolator. The ACR automatically disconnects the bank though when charging disappears and is fully automatic in protecting differing banks from being in parallel when not charging..
As I read it the OP wanted to hard wire the two batts together then charge as one. Technically this is fine when charging, but, if the power goes out, charger fails, or freaks out, which they do, the batteries would still remain hard wired and combined even without charging present. I suspect he wanted to leave them this way for the entire winter. I replace enough failed and freaked out chargers that this is not an approach I would ever take, leaving batts on charge all winter unattended especially when it is not necessary. Most boat yards specifically ban unattended battery charging during the winter months, all with good reason..
When there is no need to do so why hard wire two batteries together of differing ages, sizes or types.... ?
Also some manufacturers, Lifeline is one, specifically advise against leaving the batteries on constant trickle charge.
But of course you don't need to take my word for it Ed Sherman the ABYC's Curriculum Director just posted the quote below on Nov 10, 2011. Gota love it he's more direct and to the point about this subject than I am...
Originally Posted by Ed Sherman - ABYC Curriculum Director
Let me begin by emphasizing one key point, there is no need or even a desire to remove the batteries from your boat! The habit some old timers got into with this procedure is right up there with setting a bettery on a concrete floor and the concrete sucking the life out of a battery like some sort of vampire. The concrete isn't the culprit and never was, and the need to remove batteries from a boat when it goes into storage for the winter is in the same mis-imformed category.
What is important is that the batteries get a full charge before you lay them up for the winter.