battery charger question

[dubob]

Bob, I guess you have not got into fligging or fishing without running your two gas motors. I do that in the Spring and Fall most every year now, so I can understand what Richard is doing plus I sometime run up to three finders off one battery, one for a stand alone GPS and one for fish finder from the transom and the third from the bow for side scan, on my electric trolling motor. Two units running at the same time is bad enough but adding a third unit really draws the juice, so for that reason alone, having one battery for just my finders makes sense. That way I'll never get stranded. This isn't a problem if I run my kicker but it seems more and more I'm fligging, so this setup makes sense for my needs.
I have a two bank charger for my two electric trolling motor batteries, can't remember the brand name for sure but it could be from Cabela's. I run a second, single bank charger that I bought from Harbor Freight that is just for the fish finder battery. This setup works for me but my forth battery is just for my main motor and like you said, it charges that battery, so I don't usually have to charge it.

Your correct Curt, I hardly ever still fish anymore.  But let me provide a little more information on lead acid batteries (same holds true for Gel Cell or AGM batteries as well).  Lets assume you are running a Gp 27 marine, deep cycle battery with a rating of 100 amp hours (AH).  That rating is based on a standard that assumes a draw time of 20 hours will consume all 100 amps, or 5 amps per hour.  Lets also assume that all three of your sonar/gps units will draw something less than 5 amps total.  I'm going to guess between 3 and 4 amps would be about right.  Since the draw is slightly less than 5 amps, then it stands to reason that the time it would take to consume the 100 amps the battery is rated at will be slightly longer than 20 hours.

100 AH/5 amp/hour draw = 20 hours

100 AH/4 amp/hour draw = 25 hours

100 AH/3 amp/hour draw = 33.33 hours

If you're using a Gp 24 at a rating of 75 AH, then substitute 75 for 100 above and do the math.  I doubt seriously that you would be fligging for that length of time and thus create a dead cranking battery.  I'm not sure what outboard you have on your boat, but my Mercury puts out 50 amps which will recharge very fast.

There is a very good read at this link that explains a lot about batteries without a lot of technical jargon if you're interested.

Battery Basics: A Layman's Guide to Batteries

One last tip about batteries for use in marine applications.  Make sure you use DEEP CYCLE RV/MARINE batteries only.  Ordinary car batteries will die an early death when used in applications involving constant deep draw downs.