I want to add 2 6v batteries to my 2 6v battery system . The system will remain 12v. I will add two batteries of the same brand and near the same age.  My problem is that the only place I can install new batteries is about 5 foot from existing batteries. I will use 2/0 cables and make sure both positive and negative leads are the same length . I will also be adding 200 to 300 watts of panels to an existing 280 watt system. 

anyone have any comments or suggestions 

thanks 

I think that you are on the right track. Just be sure that the two added batteries are ins series like the first two and the two pairs of batteries are in parallel. 

A trick to balance two sets of batteies (especially if they're some distance apart} is to connect the positive wires (load and charging) to one bank and the negative wires to the other, even if you have to extend one set of wires to the second battery bank.

This equalizes the total cable length between the two banks, versus connecting the + and - to just one bank with two long cables running over to the other bank,.

Trace the path from + to - through each of these examples.  The staggered connection has one long cable on each path through the batteries versus the non-staggered connection having no long cables on the first bank and two long cables on the path going through the other bank.

Edited November 12, 2018 by Lou Schneider

 you have to have the same length cables from the charge controller to both banks of batteries in order for both banks to charge and discharge evenly. You want the charge controller as close to the batteries as possible, so,if you could move it between the two battery banks and keep your battery cable as short as possible you will have a better outcome. 

I was about to hit send several hours ago when my battery died.

also like Lou said.

Edited November 12, 2018 by Lance A Lott

The more you increase the wire size between batteries the less the separation effect will be. In addition, I would do as Lou suggested and even thought of the same idea, but wasn't clever enough to put in the drawing like he did!  It may be difficult to move the positive side between the two banks but it shouldn't be too difficult to move the negative or ground side. 

Thanks to all who replied. 

Jim

5 feet should be fine.