Hi
Sealed lead acid batteries, fully charged are up to 13.8V, half of that is 6.9V.
With the LiPo 26650 batteries at 3.7V x 2 = 7.4, just add a diode, big enough to handle the current.
This one should be fine
https://www.jaycar.com.au/6a10-6a-1000v-diode/p/ZR1024
I think it drops 1V and can handle 6A.
On each side setup 2 batteries plus one diode ((2 x 3.7V) - 1) x 2 = 12.8V
No problem! I think!!
If you go with the LiFe batteries, don't even need the diode! but, it would provide protection to batteies inserted backwards.
LiPo 3.7V
https://www.jaycar.com.au/26650-rech...ipple/p/SB2315
LiFe 3.2V
https://www.jaycar.com.au/26650-life...-3-2v/p/SB2317
I hope his makes sense.
At these prices for the batteries, I will for leads to die before thinking of doing it.
Have fun
Philip