I think part of the problem with batteries fighting each other, is really a reference to batteries being out of balance. Which is what I am trying to avoid but wiring the batteries as I have in the above diagrams. It is extremely important. I will try and explain with some examples.
You buy two batteries, one has an at rest voltage of 12.789 and the other 12.623. When the laternator shuts off to read the battery voltage (does up to 333 times a second), it would read the battery voltage as 12.706, the average of the two. It then sets it charge rate based on this number, and the first battery gets over charged, and the second battery not enough of a charge.
Used to see in a big truck with 4 group 31's. If one battery went bad, you had to replace all 4. Otherwise the new battery would be boiled in about a month.
Now when you guys use solenoids to separate the batteries, and only use one battery ever now and then. You create a major imbalance between the two batteries. So when they are being used together or charging together, they are getting damaged by each other and the alternator at the same time. Might not be seeing it because you are not doing it often, but it is still happening.
Even if you keep them connected full time by a solenoid, there is stil a voltage drop between the two because of the contacts in the solenoid. Plus you always have the drain/draw of the solenoids themsleves. That is a bad percentage of power to give up at idle.
I can understand concern about playing your stereo at the beach with the truck off and flattening the batteries. But I would suggest wiring the batteries together the best way possible to keep the balanced, and then spending your money on a low voltage alarm. So you know you have drop thme battery power to a point, and just need to start up recharge thme for 15-20 minutes.
When researching, also remember to try understand what the purpose of the option is. Factory is do it as cheap as possible and still have it last 3 years. If it is an aftermarket company, are they have RD, do the service the fleet/service industry, are they a manufacturer, rebadger. A lot of what you see post on the web about electrical stuff coems from people outside of the actual industry. People that really know a little about it, but in truth have no clue what they are talking about, what the product they are marketing actually is, etc, etc.
I am not pointing at individuals here by any means, so please dont think that. But I here things like "Well MG wires it that way, so it has to be right". I can see why a consumer would think that. But the truth is, MG doesnt understand what they were doing wrong to their own starters. "well but it has lasted for a couple of years" Yeah lots will, but the average is half of what it would be if done properly.
I hope my point is clear. If you absolutely need to spearatre the batteries, I would suggest a product called a battery Separater by Sure Power. 1314 I think is the part number. But there are so few applications that really should be using them.
