question

Hello @Hullukala, the primary issue is a matter of the internal support, damping material arrangement, cell arrangement, and BMS mounting methods of the batteries. Obviously with the old Pb batteries, you couldn't put them on their side because the electrolyte would leak out, which doesn't apply to LFP batteries; in the case of LFP batteries, whether or not they are recommended for installation on their sides is a matter of their internal construction.

Victron LFP batteries are not recommended for installation on their sides because they don't have the proper bracing to ensure that the battery will survive such a configuration in the long-term (such installation requires an entirely different support lattice for the cells and BMS). Vibration, for instance (not that that's a concern in your particular case) is a primarily up-down movement, and when the battery is upright that movement is in accord with the internal bracing of the battery and the cell arrangement, but if the battery is on its side then the up-down movement of common vibration is perpendicular to the internal bracing and will be more likely to cause premature failure of the battery.

In your case, vibration is obviously not a huge concern, but the principle remains that the internal bracing, cell arrangement, and BMS mounting location as well as all damping materials used are all designed -and warranted- for maximum lifespan when installed in the upright position. Installing the battery on its side may lead to premature failure, and such failure would not be covered by warranty because it would be considered a result of improper installation.