From HeatSpring.com PV Boot Camp
Sean, can you expand on why paralleled strings of batteries will not charge/discharge uniformly?
If you have multiple strings of batteries, the connections to the strings will have different resistance. This can be because of a longer conductor, a better or worse connection, etc. Nothing is perfect and one connection will have less resistance and that string of batteries will be used more than the other strings, which will cause that string to wear out first. The more strings, the more likely the problem.
Another problem with multiple strings is that there can be parasitic currents. Even when you are not using the batteries, there can be some flow between the strings.
It is best to have the fewest strings, such as one.