Electronics/ battery use and charging.

[DLT]

Theory is then, that paralleled batteries will always equalise to the lesser battery unless controlled?

Yes, wire any two batteries in parallel and they will equalize themselves. I don't know what you mean by "unless controlled". They are either wired in parallel or they aren't...

But, lets not lose perspective. Paralleling batteries is common place and, provided they are reasonably matched (size,type,manufacturer,age) any equalization issues are going to be so small as to be "in the noise" so to speak (essentially undetectible), provided you wire them in parallel and keep them that way... You only get large differences in charge level when: you only draw from one, you only charge one, or one finally dies. The first two are avoided by keeping them wired in parallel...

Of course, if you want a dedicated starter battery, or two separate banks, then you can do that too. Just don't switch to "both", when both batteries are not in the same state (fully charged/discharged). Of course, you may need to switch to both to charge them both from the same source at the same time, but that depends on wiring and how you charge, etc...

Basically, I'm advocating either keeping the batteries wired together as one bank in parallel or keep them separate as two separate banks...

What you don't want to do is start with fully charged batteries, run one down, and then switch to both. This could cause a high current draw from the charged battery into the discharged one, which could be well beyond the rating for your switch, which could let the smoke out. (You all know that all electrical components actually work on smoke, right? Let the smoke out and they don't work anymore...) This could also cause a fire, which is really bad on a boat...

Instead, the better thing to do (assuming you are not trying to keep a dedicated starter battery fully charged) is to switch back and forth regularly, thereby bringing both banks down together... You can also just run one bank down at a time, but here you're setting up the dangerous situation if you accidentally switch to "both".

Me, I keep a dedicated starting battery. I can use it to run anything I want, but I don't... It's whole purpose is to handle the motor's needs, oh and to power a backup automatic bilge pump, should the need arise...

My house bank is three batteries paralleled together. They stayed paralleled. Together, they act like one BIG battery... There is likely some equalization amoung them, but I still have 3 times the capacity of the weakest (assuming the weakest pulls the other two down), so its just not an issue for me...

I can parallel my house bank with my starting battery, but I don't ever plan on it and hope I never see a situation where that looks like a good idea.

[50/50]

I appreciate your responses.

My mate who's helping me wire my boat is an electrician and whilst fully undertsands the theory and the principal of what I want to do with the parallel battery scenario probably hasn't run this like some have here.

My take on this now, having listened to experience, is that, as long as the batteries are pretty well balanced, (i.e. same size, age and well maintained) is that coupled together in parallel, you can expect a fairly reliable return of useable power.