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The cable run from the roof panels to the controller should not be included with the cable run from the controller to the battery in terms of sizing the cable. Two different considerations. Here is why.
Your roof panels are probably producing 17 or 18 volts. Any small voltage drop getting down to the controller will only mean you will have slightly less watts (amps x volts = watts) at the controller. My Bigfoot had 8 AWG already installed to the fridge vent for solar so I was stuck with it – no way to change it. If you already have 8 AWG from the roof to the controller I would leave it. Just might mean a few less watts for the controller to process than 6 AWG. No big deal. And often the wiring to the j-box on the roof from the panels is 10 AWG anyway.
But from your controller to the batteries cable size is a big deal and voltage drop should be minimized. If you look at the Lifeline AGM manual it specifies 14.41 volts for absorption at 70 degrees F. Lets say your controller set for AGMs puts out 14.41 volts for absorption. If you have a 2% voltage drop from your controller to your battery, that is 0.29 volts. That means your batteries are only getting 14.12 volts for absorption. Not good.
So I would leave the 8 AWG to the roof as is. But determine size of the cable from the controller at your distance of 9 feet to the battery. I think you will find that the cable size will not be that bad. You will have to use the max amp and voltage AGM output of your controller for your variables. It won’t be the 17 volts your panels are producing. It will be closer to the 14.41 absorption volts. And the amperage will be based on what your controller produces from your panels. With an MPPT it will be more amps than the panels produce because the controller reduces voltage from the panels in order to increase the amperage to the batteries.
I used 6 AWG in my system from the controller to the battery. And check your controller – it may only take certain size cable. For my Victron 100/30 MPPT, the max cable size was 6 AWG anyway. I like the smart phone control for the Victron MPPT. And now I think on certain models they have the Bluetooth transmitter built in to the controller. But getting the dongle is no big deal either. The graphics are better now, too.
Here is the voltage drop calculator I use:
I ran the 16 amps at 17 volts for 15 feet using 8 AWG from your roof and you will only have .30 volts drop. That means your controller will see only 4.8 watts less (16 amps x .30 volts) due to voltage drop on the 8 AWG from the roof. Not a big deal.
Don’t know where you live to get your Lifelines. I googled it a lot when I needed mine and ended up ordering and picking them up at a marine supply house in Seattle, so do not limit yourself to RV supply houses. I did not see too much variation in the low end of pricing – no screaming deals. But you do want to stay away from places that mark them up too much. Freight cost is a big factor. I would not have them shipped directly to your house. A dropped battery along the way and you may end up with a bad battery. I think the freight deliveries to larger supply houses are more reliable for heavy, fragile batteries – JMO.
Hope this helps.