All good points. My thought was to put the VSR between the MPPT and the two battery banks, so that input voltage would only come from that source. If the alternate charging sources were engaged, would that still affect the VSR? And if so, to what end? This is really academic, as I see the simplicity of going to the house bank only with solar, since you correctly pointed out the start battery gets almost no discharge. I was just thinking of redundancy, hence the idea of using a VSR. I picked up the VSR notion from MS how to. He mentioned it in conjunction with solar power, but probably not to be used the way I've planned it.
#1 While a VSR is a "
nice to have" item, for automated charging of a start bank, I don't see a need when you have a dedicated alternator for the start battery. You also have the redundancy of the BOTH position if the start battery alt were to fail.
#2 Unless you desired more amperage during bulk, by combining the banks during bulk with the VSR, the VSR is really not doing much of anything because your start battery has its own alt..
#3 A dual-sensing VSR/ACR/Combiner does not "prioritize" anything. Think of it as a voltage triggered, automated version of the BOTH position. When either battery attains the minimum combine voltage, and time (5 seconds at 13.4V for the BEP), the banks are automatically paralleled for charging. It takes very little current to raise the voltage of a small 99% charged start battery to 13.4V. In the 30 or so seconds it will take to combine you are not "charging" the start battery first... At a high SOC, like 99%, that last 1% is horribly inefficient so that last 1% can take a considerable amount of time to replace and this is not happening in the 5 - 30 seconds it takes for most VSR's to parallel the two banks..
Let's assume your PV is capable of 5A and you have an 80Ah start battery. Lets also assume the last 1% of SOC is 50% efficient (it can be worse). Immediately your 5A is having 2.5A wasted as un-storable energy. We will now assume it took 30 seconds to attain the combine voltage of 13.4V.
30 seconds at 2.5A = .021Ah of stored energy. 1% of an 80Ah battery is 0.8Ah needing to be returned, at a 50% efficiency, for a full charge. It would take approx 20 minutes at 5A to replace the 0.8Ah you'd be down at 99% SOC. The marketing departments like to lead folks to believe that a VSR can "prioritize" charging but they really do nothing of the sort..
#4 By wiring a low current solar array to the start battery first, with a VSR, you will create a relay cycling scenario that may take take a long time to sufficiently charge to your house bank due to what is called "relay cycling"... At low states of charge, 50-90% SOC, it takes either considerable time at a low current or considerable current for a shorter time to bring a house bank to combine levels. This is why it is best to allow the house bank to be directly connected to the MPPT. The PV will slowly bring voltage to combine point and then combine with the start battery. There will be no relay cycling in this scenario and the VSR only has to pass the trivial currents the start battery needs across it.
Your start battery will always be near full, because its a start battery and starting an engine takes very, very little Ah's out of the battery.
Relay cycling works like this; Sun comes up and due to SOC of the start battery, the voltage quickly & easily exceeds the VSR combine qualifiers of 13.4V for 5 seconds.
The VSR combines after 5 seconds (possibly as much as 30 seconds) at 13.4V and now the house bank sucks voltage back below the 12.8V disconnect voltage point because the PV does not have the oomph or current to maintain a combined state charging into a depleted house bank.
After 20 seconds of combined-state the relay opens.. Repeat, repeat, repeat..... Relay cycling is why we feed charge sources to the house bank not the start bank first.
#5 Your solar controller is not made by Morningstar it's a Chinese controller imported by Ham. That controller has internal temp compensation so it really needs to be close to the batteries so it can sense the correct ambient temp.
