OK, your main problem is that the battery voltage will vary with charge level.
It will vary from some multiple of (maybe) 3V (discharged) to some multiple of 4.2V (charged).
When you say 25.2V fully charged, this probably means it has 6 cells in series. This will give you 25.2V when fully charged, dropping to 18V when approaching discharge.
You have three problems:
- The higher voltage when fully charged. 25.2 vs 24V is pretty trivial, and whilst it's not guaranteed, most devices rated to operate at 24VDC would hardly even notice the small increase in voltage.
- The lower voltage (18V) when the battery approaches discharge. If the device is normally battery powered, it may operate on the reduced voltage. However it is posible it won't. Your need to boost voltage may be an issue.
- Limiting discharge. Unless the battery pack does this itself you will need to turn off the load when the battery approaches discharge or you will very quickly destroy the battery. A similar problem exists when charging -- overcharging is also dangerous.
Of the three problems 2 and 3 are likely the most significant, 1 may not be an issue. Have you considered these?