Dual battery setup

[the evil twin]

snip...

Stupid question though, wouldnt it be better when winching to have the main battery disconnected from the auxillary so that if you screw up and completely drain the main battery you can still link them to be able to start?? and if you are going for this model, instead of having a permanent link ability in your car you could just get a set of jumper leads which then means you can use them to weld, help other people and restart yourself of your aux.

Not a stupid question at all. As I mentioned in the thread earlier there are always alternative views and at the risk of getting off topic even more...

It is easy to forget that the battery is also part of any electrical circuit. What that means is that any/all of the current flow (other than thru the Alternator circuit) is also flowing thru the Battery. Now, batteries esp in this day and age of "just good enough" engineering hate prolonged high current because batteries have internal resistance and that means heat is generated, sometimes a lot of heat and heat changes the physics of a battery very significantly.

So, a quick hypothetical. Lets say the winch is pulling 250 amps and the Alternator is contributing 50 amps (hypothetically remember) that means the sustained current flow thru the battery is 200 amps. If a second battery is paralled in then the batteries will load share relatively equally but not exactly as even two identical batteries will have slight internal resistance differences. This means 100 amps each. So what does that get us?

The effect of high current draw on batteries isn't linear IE 400 amps isn't twice as bad as 200 amps, it is much worse and 200 amps isn't twice as bad as 100 amps, it is again, worse. The relationship is exponential albeit not savagely so but definitely a consideration depending, I stress DEPENDING on what the user is doing. The effect is also worse in most 'deep cycle' variants as opposed to 'all rounders' and least damaging in 'crankers'.

I always consider reducing the current draw when winching not just for the Winch but also to stop giving the batteries a harder time than needed and quite often use a snatch block because (given the above) it can be quite beneficial to Battery long term capacity and longevity to winch for longer at a reduced current and load.

Bottom line is that there are a lot of variables and no right or wrong way to look at it really. I probably use both methods 50/50