Stu, I appreciate your reasoning for hooking up the house bank first to the alternator, and that is perfectly acceptable if that is your preferance. My preferance, however, is to first and foremost, have the start battery fully charged and to eliminate any possibility of accidently discharging it. I have been anchored in some remote locations where I wanted to be darn sure that engine would start when it came time to lift the anchor. My power requirements are such that a dead house bank would not be a serious matter. So far, this upgrade has worked out great for us.
Tom,
The start battery is almost always at 98-99% charged anyway. Feeding it first is not necessary and can only serve to limit the house bank depending upon your ACR.
Starting the engine requires very, very little from a battery, many folks over think this and believe it requires a lot more than it really does. The peak in-rush current draw for a split second on my 44HP four cylinder Westerbeke is about 160 amps. My 44hp four cyl diesel engine does not even consume .1 ah from a 240 ah bank for starting! The starter motor on my Westerbeke is a 1.5 kW an the starters on the M-25's are .8 kW.
Any deep cycle bank will have way more than is needed in MCA's to crank a small diesel aux. For over 20 years I have started my engines off my house banks including a very large Cummins diesel in Downeast cruiser.
Battery acceptance and state of charge are the real questions. We must ascertain just what or why boaters thed to believe they would be limiting the recharging ability of the engine battery by first routing to the house bank and not the start.
As I mentioned above small diesel engines really don't take much battery capacity to start. In the context of battery acceptance you would really need to discharge a start battery considerably before it could even accept a charge.
We also need to keep in mind that the minute that ACR makes, the current from the full battery (your start battery in this case) starts dumping its charge and what the alt is putting out, into or through the ACR, and into the low bank along with what the alt can put in until max acceptance is met. This is one reason I prefer a Duo Charge or an Echo Charger because the full bank can't be bled off or cause a relay to make/break, make break.. With wet cells this is not a huge deal because of "acceptance" but with AGM's you can see a HUGE and FAST transfer of current between banks fast enough to drop the start battery voltage below the "combine threshold" long enough to open the relay, it then closes again and can repeat a few times before everything levels out.
With an ACR type relay it does not matter as much where the alt is connected except that if you feed the reserve batt and it combines with a much bigger deeply discharged house bank it can cause a drop in voltage that can make the relay cut in and out many times before the house voltage can be sustained without cutting out the relay on low voltage. Feeding the house bank first avoids this game of cut in/cut out, cut in/cut out..
A fully charged start battery might wind up at 95-98% of capacity, after a worst case scenario start up, that perhaps involved more cranking than normal. This, to an alternator, is basically seen as a fully charged battery and it will react accordingly with low output limited by acceptance.
I have actually pulled my stop lever, closed my seacock and cranked enough, intermittently of course, to have water-locked my engine if I had left the seacock open. I still used only about 1% of the 240 ah banks capacity leaving me at a 99% state of charge. Even one full minute of cranking with a 150 amp starter draw is still only 2.5 amp hours from a battery.
On even the smallest group 24 start battery, with roughly 65 amp hours available, this still leaves you at over a 96% state of charge. Considering the acceptance rates of wet cell batteries is roughly 20-25% of the 20 hour rating the MAX a 65 ah group 24 wet cell will accept, when it is flat out dead, is about 16 amps. The 16 amps only happens of course when the battery is basically dead. At 96% SOC you'll be lucky to force even one or two amps into it. Even if you were to consume 5% of the ah capacity your battery will still barely be accepting 1-2 amps at the most depending upon the start bank size and chemistry. AGM's of course accept a lot more than wet cells do.
By wiring to the start battery first, and sensing it, you can actually cheat your whole system of alternator output. This is why Xantrex designed the Echo Charge. They recognized that the start battery will rarely if ever need more than about 15 amps in reality will usually only need an amp or two. Their directions are clear that you should wire to the house bank first and then use the Echo to feed a start battery or other aux battery.
One reason I really like the Balmar Duo Charge is because a start battery generally does not need the 14.2-14.4 volts the house bank does, as it resides almost 100% of the time above 95% SOC, or what we refer to as float voltage range. When a start battery is sitting at 98-99% SOC a device that can send 13.2-13.4 to the emergency/start batt and 14+ to the house bank, is a smart device indeed. ACR's and the Echo Charger don't do this but the Duo does. ACR's and the Echo are voltage followers. Combiners combine and thus the bank becomes one homogeneous voltage thus over charging is not possible unless you have a single stage regulator with a high set point and constant 14.6 volts out.
There is nothing wrong with an ACR, they work well, but I agree 100% with Stu in that your alt and charging system would be better served by wiring the alt directly to the house bank first, not the start battery first.
Hope that made sense..?
P.S. Here's how I buttered my batts up and slid them in on my old 310. This was during the test fitting. I was able to get a battery box in around the start battery..
