Maximum - 12 volt Deep Cycle Batteries Connected in Parallel

As long as the conductor has intrinsic over ampacity the length issue is of minimal concern. One 8-D connected to a 12" length of 2/0 cable is essentially going to see the same delivery as an eight foot length of 2/0. Where ampacity comes into play is at high volumes of amperage. But four 8-D batteries may feed a 40MT 12-volt starter that may demand 600+ amperes. Then length will amplify voltage (resistance) differences. But high amperage is not the issue in this case.

Point of illustration: My 12 2-volt cells furthest connections are 26' from the inverter and 29' from the chargers. I had to calculate voltage drop not only to the furthest (weak sister) cell, but the total length of the circuit. Fifty eight feet. Only 500MCM cable could meet that spec. I had forgotten I used 500 MCM. It's been so many years. I have been incorrectly thinking I had used 250 MCM which itself is 30% larger than 4/0. 500 MCM is close to double the circular mil. The cable sheathing is white braided telomer fluorocarbon (Teflon). I remember the spool weighing close to 440 lbs. Stiff. I had to bend the wire mechanically.

I have 4 8D AGM's and many of the Prevost conversions use 6 or 8 (usually 4D) without issue. You need to ensure you have the wiring done right and that you have enough charge capacity to bring them back up after usage. While my setup is neg. to neg. to neg, the best setup would be the pos and neg wires from the charge source to a junction where 4 identical length cables would go to each terminal.

The phone companies have been paralleling massive battery banks feeding entire CO's for probably 80 years, but one of the big differences is they don't cycle them like we do

It is not optional to use bussbars on both positive and negative. This means you have to do it because the risk of well intention bad connections grows unacceptably large.

Blue Seas and Marinco have good buss bars. Use Genuinedealz for making up custom cables. Personally I do like like the stud connectors on RV batteries. I use the post for duty that pulls greater than 100 amps.

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Wear your reading glasses knothead Mex. (Corrected spelling of http://www.genuinedealz.com)

No limit to how many you can have in parallel, but as stated a practical max of 4 is about optimum.

Most important consideration is that the batteries are all of same strength. Don't mix old weak ones with new strong ones. Weak will sap strong.
Best to have identical batteries, all of the same age.

i have (5) batteries of the same kind and age, wired parallel
been that way for almost 14 months, works great

I have a set of 4 batteries in a compartment that I run in parallel. Each battery is connected to a positive and a negative bus, not to each other.

I have read the practical maximum is four in parallel. Technically you can have as many as you like but the chances of trouble start increasing. To keep them balanced you may need a bus. As a minimum attach the main feed vs ground at each end of the stack.