[PoleKing]

Simply put, a car or van battery and a leisure should never be connected in parallel (or series) as they have different discharge and charging rates.

But why, Ross?
Whats the worse that can happen?

[rosskesava]

But why, Ross?
Whats the worse that can happen?

Nothing serious would happen at all.

The worst would be either shortening the life of one or both batteries and one or both not charging up properly.

The best would be likewise.

[PoleKing]

I'm still thinking I'm gonna take that punt though.

[robert mitchell]

1000 ltrs,  2x 500 ltrs, 400 ltr and 350 ltr.
One of the vans with the 500 in does less than 3 miles a day, his round is so compact he moves his van sometimes only twice a day. He spends less than £10 per week on fuel.
The only extra charge the batteries get are when he is filling up (from our static system) and having a chat maybe 20 mins per day.
NEVER had any problems.
The 1000 ltr van gets used on larger commercial jobs and can sit all day without moving more than a couple of hundred metres, again never had a problem.
A split charge relay simply stops your secondary battery from drawing power from your vans battery. It does not boost or aid the charging system. From my experience (auto electrician) they cause more problems than they are worth.

So you are saying a split charge relay doesn't charge your second battery ?

It stops the second battery drawing power if the  main battery is not fully charged , when it is fully charged as it usually is on a vehicle the second battery will be getting a charge .

My scr has been brilliant .

[David Kent @ KentKleen]

A SCR stops your secondary battery from drawing any power from your vans battery when the volts drop below a certain value (i.e when its not running). This enables you to run the second battery completely dead and still have your van battery to start your van.
Connecting a positive and negative from your vans battery to the second battery is all you need to charge the second battery. You do not need a SCR unless you are worried about not been able to start your van.
Hope that clears that up.