Hi Franz.
Fist off, because your battery is in the rear of the vehicle, while 8B&S ( 7.9mm2 ) will allow your auxiliary battery to charge, if the auxiliary battery is low, the current draw it will try to place on the 8B&S cable will cause a fair bit of voltage drop, slowing the charging and increasing the amount of driving time needed to recharge a low battery.
For batteries located in the engine bay, a single positive 8B&S
cable is fine, but with the battery in the rear, the distance means you need thicker cable to reduce the voltage drop. Here TWIN 6B&S ( 13.5mm2 x 2 ) is a much better choice.
As above, it is better and gives a guarantied good earth every time if you run the from battery to battery.
YES, the SC80 allows all batteries to be charged at the same time, UNLESS the common voltage has dropped below 12v and the SC80 has turned off ( cut-out ).
In this case, if the battery charger is connected to the auxiliary battery(s), only the auxiliary battery(s) will charge.
If the charger is connected to the cranking battery, once the charge voltage rises above 13.1v, all batteries will then be connected to the charger.
Yes and this is one of the main advantages of the SC80. If your have only done a short drive and your auxiliary battery is in a higher state of charge when you turn the motor off, the auxiliary battery will help to put some additional charge back into the cranking battery.
While this feature was not a designed intention, owners of D3s, D4, and RRSs who were getting “LOW VOLTAGE” warnings before they had one of my systems installed, because of the back feed, the messages stop occurring because the system improves the state of charge of the cranking battery and this also helps to extend the operating life of the cranking battery
You need circuit breakers on the positive cables that come from any battery in a dual battery system because if you have an accident where the cable is shorted out you could end up with a fire if each battery is not protected by a circuit breaker ( or fuse ).


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By the way I also installed a Maxi fuse between the extra alternator charge cable and between the 2 aux batteries with a 100 amp fuse.Hope this helps...



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