Multi batt charing?

Question on multi charging 12v batt
I have an on board charger for my bass boat. I use 2 marine deep cell batt for my trolling motor and one starting batt for my outboard. My new GPS/Sonar electronics will drain the starting battery. I want to install a fourth smaller (lawn Mover type) 12v battery. My on board charger only has three outputs for three 12v batteries. Currently I use all three. Can I split one output so that it will charge Two 12v batteries. The Starter batt and the smaller 12v batt from one output lead?

 

doing this only ties the batteries together. this is what you're trying to avoid my question is about your motor it may already have a second charging system on it that just needs to be hooked up to you other battery and it's isolated

 

Four batteries on a bass boat?
By the end of this, you'll have a battery for starting the motor, a battery for running the GPS and fishfinder, and a pair of batteries for the trolling drive. That's getting to be an awful lot, and if your elec gear is hungry enough to kill a starting battery, it'll drain a lawn mower unit flat in half the time. And while it may seem convenient to have the systems more or less isolated from each other, you might find it to be rather irritating when one runs down before the other and you have to start up the engine to recharge.
Might I suggest taking a bit of inspiration from larger craft here and reworking the system a bit. Have one starting battery for starting, and one large deep-cycle "house" bank for running everything else- trolling motor, electronics, etc. (Your trolling motor is 12 V, not 24, right?)

 

Hah. Yea too many batt. Hope I still got room for them big bass!
Thanks for the reply.
So I do have a 24v trolling motor. I might look into hooking my hummingbird fishing system into my trolling batteries now. I just don’t know if I can since I have the batts wired in series.
I don’t like dropping the voltage on the main starting motor. The hummingbird manual does suggest using a separate batt. I don’t think the hummingbird will completely drain the batt but will make a big enough dent in the voltage to deter the 150 horse motor from turning. The smaller lawn mower type batt won’t have to turn any motor so I feel it can be a good solution. I know 4 batt does sound crazy. I just don’t like running cold cranking batt too low. Wish I could find a small rechargeable deep cycle marine batt for around 50 bucks.
So I guess try to hook up my fishing system to my pre existing trolling batts or buy a forth batt. I just might need a way to charge that forth batt but it seems that I will need to introduce a separate portable charger for the forth.
Thanks for the whim of inspiration though.

 

you can do all this by using blocking diodes, they wont let all the batts reach full float 13,6 v but they stop the current running backwards so a full batt stays full
they cost abt 60 a piece, and chuck in some isolating switches too, so you dont build yourself a bomb)