It would be helpful if you show two schematics, one for charging (before switchover), and one after switchover.
Questions:
Do you wish to use one or more relays?
I dont want to use mechanical relays because my application must be ultra low consumption.
What is the source charging voltage?
There is no charging mode, the batteries are of 9Ah and i will use until 4 2*2 parallel by pair. These are the Batteries:
https://www.amazon.com/Ultralife-ER26500-Lithium-Battery/dp/B00HKJ2GC4/ref=pd_sim_sbs_121_3?ie=UTF8&psc=1&refRID=A44VSMFY6P6K8QQHEE21
According with my calculation, i dont need to charge the Batteries.
Where is load attached?
The Load is Attached after the D7 Diode... Look at the image attached VCC.
Do you need for the negative end of the battery to be 0V ground?