let's go a little further on this,shall we...

charging systems,we have "amperage" and "voltage",amperage,that's the force-think of it like pressure.voltage will vary,it will allways be hgher than battery voltage,which is 12.6v.you should see a charging system voltage of 13.5-14.5,somewhere in there...voltage is controlled by a regulator,if voltge was allowed to climb,fuses would pop.amperage is a little different,works like this,amperage will flow as needed,batteries "run down",amperage is gonna be higher when it starts,then gradually taper off-think of it like this,the battery is like your checking account,you write the mortgage check,a big one,account's low,you keep putting the money back in,then you taper off and put the money into the savings account instead-get it
there's "laws" to electricity,"ohm's law" is one of these laws.I=V/R what this means is,I is the amperage,V is volts and R is resistance,what this all means is,divide resistance by voltage and you will get the answer to how many amps are flowing in a circuit.
with all this,the amperage is gonna be the same in a circuit,again,menaing,this charging system and the batteries,the amperage flowing in this circuit is the same through out.this is a circuit,you've got 2 batteries,a charging device,and a ground,a complete circuit.a completel circuit,by definition,has a power source,with postive and ground,wiring,the resistance,or "load" and ground-the return path for the circuit.the amperage is the same through out the circuit.again,with all that,the batteries,both are getting the same amount of amperage.batteries will equalize,and a low charged or dead battery will discharge a good battery.

running power leads from a battery,batteries should have battery cables only going to them,nothing else,nothing should be hooked directly to a battery's positive terminal,except for cables.all connections should be made behind the battery switch,and a seperate ground buss system should be in place for the grounds.all poer leads need to be fused per abyc codes,and coast guard cfr's,as well as common sense.it's not required,but,i like a positive shut off system for ship's power.a boat is a moist environment,right ? water is an excellent conductor of electricity,i could tell you a few stories concerning wet 12v wiring causing all kinds of problems,not limited to "stray current electrolysis".you want a positive shut off system for you batteries.
if you're running a boat offshore,you need to limit the potential for problems as much as possible,common sense right ??you keep up on the engine(s),make sure you've got a full load of fuel,all these good things right this is another thing to look at on your rig.mabey,this is the year you're gonna try and make an overnite run to the canyons,or the inshore tuna grounds,your plan is to use the 12v spreader lites to lite up the deck and atract the squid ? you've got a wiring system like the one i described,1,2 all and off,you're gonna have a problem,that would be dead batteries...i've lost count at how many times i've been in the canyons for the nite and in the morning,guys are on the radio calling for a "jump start" or a "jump pack"...these are guys who need to look at their electrical systems,there's no excuse for this,there's a better way and a safer way...
and for the record,trying to run 12v lites for the nite,it's not the best approach,you're gonna need a few sets of batteries,just to power the lites,those 500wspreader lites use alot of power and they will kill a battery quick-try it for yourself,hook 2 up to one of your boat batteries,do it in the garage if you pulled the batteries,see how long they stay lit...we'll talk about a better way to lite up the boat for the nite...