There are considerations for both parallel and series configurations.
Neither is perfect and both can be made to work well.
Series Batteries:
With series, the primary concerns are all around keeping the multiple batteries balanced. If one starts out with a lower state of charge, that will define what is possible for storage for all the batteries in the string.
My guess is that if you start with Balanced LiFePO4 cells, you will be fine for a long time to come.
Note: There are battery balancers that are designed to balance batteries in a string but I have no experience with them.
Parallel Batteries:
The primary concern with parallel batteries is uneven ware due to slight voltage differences due to different resistance values of the path to the batteries.
The leads between cells will have small voltage drops as shown in the (not recommended) diagram below.
![1628459045335.png 1628459045335.png](https://diysolarforum.com/data/attachments/59/59307-6043d357c7389e47dea806ec047dc46a.jpg)
In the above bad example, battery 4 will provide more of the current than the other 3, battery 3 will provide more than 1 & 2 and so on.
Over time, this means battery 4 is worked harder and presumably will wear out quicker. This problem is very real with lead acid.... I have seen debates about how real it is for LifePO4, but I just assume it is a problem.
The good news is that there are ways to wire parallel batteries to avoid this.
It turns out that if you wire two batteries like this, they remain balanced as long as the interconnecting leads are the same size.
For 4 batteries, this is a way to wire them that keeps them balanced:
![1628459748853.png 1628459748853.png](https://diysolarforum.com/data/attachments/59/59310-b0dcb144787bb06369a314b93200e462.jpg)
Putting 3 heavy wires on one battery terminal can be problematic, so this is an alternative:
![1628459856225.png 1628459856225.png](https://diysolarforum.com/data/attachments/59/59311-4cfb5b57d8a8e42ef2271913a1d6671a.jpg)
Finally, you can always just wire each battery separately:
The other approach is to use very heavy duty bus bars that have such low resistance that the voltage drop is negligible. Some Battel Born batteries have a unique post layout that allows Busbars between the batteries. I try to show this in the image below:
![1628460868513.png 1628460868513.png](https://diysolarforum.com/data/attachments/59/59315-b3ff54cb1fcd561bbdc213845d6af9ae.jpg)