A good primer on parallel connection of batteries:http://www.smartgauge.co.uk/batt_con.html
As a general rule of thumb, paralleling batteries is never a good thing. Also, adding a new string of batteries to an older string is even worse. A superior option is to trade in smaller capacity batteries of higher voltage to higher capacity batteries of lower voltage.
For example, in order of capacity, you could have one 12V battery of about 120AH of capacity (low), or a string of two 6V batteries like T-105s equaling 225AH (medium), or two 6V batteries like L-16s for 370AH (medium high), or three 4V L-16s for 550AH (high capacity). There are even 2V L-16s. Six of those in series would give you 1500AH of capacity.
Each example has exactly the same string voltage, but the lower voltage batteries add up to much higher capacity.
When wiring batteries in parallel, I always use identical batteries of the same age and use. I like to have a 260-390Ah battery bank which I can achieve by paralleling 2 or 3 x 12V 130Ah batteries. The downside of putting lower voltage batteries in series as an alternative is that I carry the batteries around and would want to minimise the number of batteries and also limit the mass of each battery to about 32kg.
> When connecting batteries in parallel, is it necessarily optimal to utilize the positive and negative terminal that are furthest apart for output?
No, it's almost certainly not optimal. If you use proper gauge cables, variations in internal resistance of the batteries can be greater than the cables. Unequal cabling can offset this - ie better balanced! So the right way to do it is to measure amps (clamp on ammeter) or volts (5 digit meter) on each cell while under heavy load. A measurement checks actual balance (vs assuming).
Method 3 on the Smartguage site is the easiest to understand, fuse and adjust for balance.
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