Some Facts
Perhaps we should establish some facts:- 1) The link "http://www.powerstream.com/Wire_Size.htm" shows wire gauges and resistances AND AC frequencies below which there is 100% of the current into the wire. Battery charging is DC (direct current) so depth of penetration is irrelevant because it is always 100%. 2) If the distance through the boat is 20 feet, the connection between alternator and batteries requires TWO wires 20' long however it is configured - though there will probably be a large capacity negative wire in place already between battery and engine to handle the starting current. In which case Mainsail #1 is correct. 3) On safety grounds we MUST be able to start the engine quickly. The engine battery may be discharged because of having starting or charging problems or for other reasons. So the safe and sensible way is to give the engine battery priority of charging; i.e. alternator to start battery. This way it gets recharged soonest ready for the next start up. This may, in part, be mitigated by being able to use the battery isolator switch(es) to connect the house bank to the starter - but who knows what condition they may be in after a night or so away from shore power. 4) Given 3) above then there will already be heavy positive and negative wires between battery and starter so it only needs a short length of heavy cable (100A rating 6GA) between alternator and starter. 5) Any thought of connecting batteries directly to alternators without passing through a battery isolator switch is asking for a fire when you are not aboard regardless of whether there is an in line fuse. 6) Boats need stranded conductors because, unlike a house, they "work". In my experience Main Sail's other advice #2 thru #7 et seq is good advice too. I have a 55 amp alternator on my Yanmar 37 HP with a total of 450 AH batteries and never seen more than 40 Amps via a smart alternator regulator, even after several nights away (30% remaining charge). I have a Link 2000 - like Link 10 but also controls Freedom Combi.
Perhaps we should establish some facts:- 1) The link "http://www.powerstream.com/Wire_Size.htm" shows wire gauges and resistances AND AC frequencies below which there is 100% of the current into the wire. Battery charging is DC (direct current) so depth of penetration is irrelevant because it is always 100%. 2) If the distance through the boat is 20 feet, the connection between alternator and batteries requires TWO wires 20' long however it is configured - though there will probably be a large capacity negative wire in place already between battery and engine to handle the starting current. In which case Mainsail #1 is correct. 3) On safety grounds we MUST be able to start the engine quickly. The engine battery may be discharged because of having starting or charging problems or for other reasons. So the safe and sensible way is to give the engine battery priority of charging; i.e. alternator to start battery. This way it gets recharged soonest ready for the next start up. This may, in part, be mitigated by being able to use the battery isolator switch(es) to connect the house bank to the starter - but who knows what condition they may be in after a night or so away from shore power. 4) Given 3) above then there will already be heavy positive and negative wires between battery and starter so it only needs a short length of heavy cable (100A rating 6GA) between alternator and starter. 5) Any thought of connecting batteries directly to alternators without passing through a battery isolator switch is asking for a fire when you are not aboard regardless of whether there is an in line fuse. 6) Boats need stranded conductors because, unlike a house, they "work". In my experience Main Sail's other advice #2 thru #7 et seq is good advice too. I have a 55 amp alternator on my Yanmar 37 HP with a total of 450 AH batteries and never seen more than 40 Amps via a smart alternator regulator, even after several nights away (30% remaining charge). I have a Link 2000 - like Link 10 but also controls Freedom Combi.
[Sorry about the spacing, but that's how it came in from my word processor] ***************** ***************** COMBINER SETUP FOR ALTERNATOR CHARGING Here’s what I have: Alternator is wired to the house bank (via the house bank post on the battery switch, could just have easily been physically wired directly to a bank as you have done, same connection, longer run for me) (I only have two banks, one battery switch, 1, 2 or Both or off, #1 bank is starting battery (a measly 60 Ah, and three wet cells for a 365 Ah house bank). Starter is wired to the common post of the battery switch to permit switching if the starting battery dies; to start the engine we place the battery switch (in my case) on #1 - our starting bank. Inverter charger is wired to directly to the house bank. Combiner charges both banks when either charger or alternator is on. However, different from yours, I’ve wired the alternator charging to the HOUSE bank, NOT the starting battery. Reasoning is this: from the West Marine battery combiner installation manual: “The act of starting an engine uses very little electricity - perhaps two amp hours - so engine starting batteries are almost fully charged. House batteries, on the other hand, are frequently deeply discharged, since their purpose is to is to power loads until you discharge them. If the alternator is connected to the engine battery, all of the charge current that goes into the house bank must run through the Battery Combiner, since the engine battery does not need to be charged. It is a more efficient solution to connect the alternator output to the house battery bank(s) so the combiner does not have to pass as much current.” I do recognize that when I turn my instruments and refrigerator on once the engine is running and the switch is still on #1 (my starting battery) that I’m draining down from the starting battery, not the house battery. The way I look at it is this: the alternator is putting more back in than I’m taking out. Remember the size of the wiring to and from the combiner - it's a heck of a lot smaller than the battery cables. Why run current through smaller wires? In addition, you noted: “Since I start out with full batteries when I go sailing or cruising I reasoned that I first want to replenish the starting battery in case I need to start it again for some reason.” First, once you’re “out there” awhile away from daily shorepower, your house bank WILL be lower than the starting bank. I don’t think that one could guarantee that starting out with full batteries will work or happen all the time. Think of your proposed cruising dreams in Belize. Think :”I’m gonna be on the hook, nowhere to plug in”. What you always want to do is keep the house bank up as high as possible, since once you’re away from the dock, more’s coming out of that bank than the starting bank, after all, you only just used two amps in about 30 seconds to start the engine. The second you turn on your refrigerator and instruments, you’re pulling 5-8 amps out of the house bank right away. Second, replenishing the starting battery is done almost immediately through the combiner once the alternator starts, since you’ve only taken less than 2 amps out of it. With a 120 Ah starting battery, using the 50% rule, and with 20% charging inefficiency fudge factor, you have 60 * .8 = 48 amps available to you. Divide by 2 amps per start and you’ve got 24 starts even before you even have to charge the starting battery at all! And once the engine is going, you put those 2 amps right back but, but where they’re REALLY needed is in the house bank. My suggestion is to carefully check the combiner literature. I understand your concern about making sure that the starting battery works, but first the combiner will do it, second you can always use the house bank to start the engine if for some goofball reason the starting battery does down.