Adrian79IS

Many modern inverter/charger designs use EMI filters and Y-capacitors between the AC side and the DC side. Those create a small AC leakage path to chassis/earth. A high-impedance multimeter will show this as ~10–60 VAC even though there is no low-impedance source behind it. That explains your 25 V AC reading from battery terminal to earth. Put a resistor (e.g. 10 kΩ) or a load such as a 60 W bulb between the measured battery terminal and earth, then re-measure AC. If the 25 VAC collapses to near zero under load, it was a phantom (capacitive) voltage. If it stays, you may have a real leakage/fault.
Ensure inverter chassis, battery cabinet, mains earth and RS-485 shields are properly bonded to the same protective earth. Poor or multiple earth references cause common-mode voltages and comms issues. Tighten earth terminals and check that earth conductor is continuous to the distribution earth bar. Ensure RS-485/BMS cable is twisted pair, with the shield terminated to earth at one end only (usually the inverter end). Add a ferrite clamp on the comms cable close to the inverter to reduce injected HF noise. Confirm proper RS-485 termination and biasing resistors as per the battery/inverter manual.