Hello Ann
So if you have 25v at the battery and 17v at the main contacter.
You have to check the circuit and trace where the voltage is dropping.
If you can check the battery under load, eg: operate the hydraulics and check the voltage at the battery terminal, also flip open the battery caps, look for electrolyte boiling while operating the hydraulics, try to load the battery.
If you know what you are doing you can do this directly from the hydraulic contacter.
Good luck