Bergos wrote:

which of the red cables currently attached to the main battery go to the alternator and starter and which power the rest of the in the car? Photos will be appreciated [/b]

Not sure about your MY 2007 but there should be one thick red cable (to starter and alternator) and a thinner brown cable to "the rest".

At least from MY12 it looks like this



At the plus connector there is a 500 A fuse and a 50mm^2 red cable routed to the front and a 10 mm^2 brown one going over to the other seat box.

Bergos wrote:

If I just put an automatic voltage sensing relay to separate the two batteries do they have to have the same parameters? Can I have a 100Ah main battery and 60Ah leisure battery? Will the charge well or the alternator will get crazy and one will be overcharged?

That's almost exactly what I installed a few months ago and it works fine. My main battery is the standard LR one and alongside I have a 70Ah leisure battery, linked by a voltage sensing relay. It's worked fine, so far.


Click image to enlarge


Click image to enlarge Darren

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Cupboard wrote:

That's a very small second battery, in UK conditions most decent fridges will draw in the order of 1Ah/h, go abroad and you can easily double that. My Waeco CFX75DZW was using about 2.5Ah/h with it set at -10C and +5C in 30-35C ambient last week (it uses a lot less than that when it's colder and thanks to some careful planning I've just parked it up for 4 days and it only used 10Ah to keep the fridge cold at a 20C day/10C night ambient).

Given that you don't want to be dropping a battery below 50% DOD, you're only going to be running a fridge for a day or so in the UK, less if the car is parked somewhere sunny. I've used all the available capacity of my 85Ah battery with my old 40l Snomaster parked for a day in Turin just set to 5C, but it was running pretty much continuously and used more energy doing less cooling than my Waeco.

There's a good logical argument for completely forgetting about a split charge system and just running a pair of batteries in parallel. The argument goes something like this. The cranking amps of a battery don't really drop much at 50% state of charge compared to 100%, unless there's something wrong with your engine if you can start it at 100% you can start it at 50%. Last winter I cold started my car at -5C using a 7 year old battery that had been sat for 3 weeks with a 50mA drain on it. Not as low as 50% but probably not much more than 60%.

Given that it's not good to take more than half the charge out of a battery, a "100Ah" battery has a usable capacity of 50Ah and something like 1000CCA. If you have two such batteries in a conventional dual battery setup your fridge has 50Ah to use and your starter has 1000CCA to use. If you forget about the split charge system and connect them to each other, your fridge now has 100Ah to use (half of the 200Ah total capacity) and your starter has potentially 2000CCA to use. The situation has improved for both leisure use and starting, and the whole setup is simpler. That's how trucks do it.

Would I want to do it like that? The issue is if you accidentally use more energy that you're expecting then you could end up in the middle of nowhere with both batteries flat, and that's an issue.

You might come across people telling you to spend a fortune on a DC/DC battery charger. They have a place but for most people they're a massive waste of money and will slow down your battery charging.
Finally, state of charge monitoring is at best rubbish. There's so much handwaving. The battery voltage tells you approximately nothing, state of charge monitors exist and are better but they're far from perfect. This is a screenshot of mine (a Victron) from a couple of weeks ago.



Click image to enlarge

One thing you could do is put two big batteries in parallel and use the relay on this to disconnect loads when the state of charge drops to save the starter circuit.