Hi Okie, I have been browsing your posts. Man you are on a mission and I am right there with you on some of this. This can be pretty confusing but I think I can lay it out clearly. Actually the WE7 battery bank recommendation uses 6v batteries and uses BOTH parallel and Series in the bank.
Batteries or Panels Wired in Parallel - Simple Answer. The parallel wiring connects all the positive sides of each battery together and all the negative sides of each battery together. This means the voltage is the same for all the batteries but the current is divided between them.
Example: 4ea of 6v batteries Wired in Parallel stays 6v
More Detail on Why Parallel - In a parallel configuration, your battery bank current is shared between batteries and this means that each battery is exercised less vigorously. This can increase usable capacity. Removing any single battery will not change the voltage providing you redundancy in batteries. This could really be useful in case of a battery failure (if that failure is detected early enough). Parallel is also useful in low voltage systems because it may be easier to find batteries at needed supply voltages. If you have a bad cell in a parallel battery bank, it will sap energy when other cells try to charge it (all the time, which adds up in sapping energy). This can cause heat and loss of water in the failed cell that run a battery down by itself.
Batteries or Panels Wired in Series - Simple Answer. The main difference is that series batteries combine voltages together at a common current while parallel batteries add currents together at a common voltage. This is important because lead acid battery energy capacity is significantly influenced by the current drawn from them and not by their voltage.
Example: 4ea of 6v batteries Wired in Series becomes 24v - This would be minimum recommended battery bank for the WE7 rooftop systems.
More Detail on Series - A series configuration may can use fewer cells which may allow for more load, greater capacity, or a more rugged battery bank in a given amount of space. In a series configuration, a weak cell does not sap the energy of the other batteries, any current going through the battery will just pass through a weak cell. This fact, that any load or charge current goes through all cells equally, tends to keep all of the cells at an equal state. It also means that a failed cell does not cause internal circulating currents. The failed cell may add some resistance (which does sap energy, but only when a load is applied) and, since it does not contribute any voltage, it will show as a reduced battery bank voltage.