Finally got the internal batteries updated. I went with the Panasonic UP-RWA1232P2 lead acid batteries. these are tried and proven in industry to be robust and long life with a hefty 4.5 Ah capability (4500 mAh for those new tech folks who seem to love higher numbers)
The datasheet is now included in the Git file bundles and so far they are working great.
I have room for several in the case but for now I'm only running 2 in parallel for our current build but I think I will update this to 4 very soon. The controller monitors the battery voltage and temperature at all times with no issues thus far. We also have the optional relay power cut as shown in the schematics. I haven't found a huge need for this yet but I find it better to include such things now rather than trying to add them later.