The power battery consists of some 40 cells connected in series to produce some 280 volts dc.
The charging current is in series too, so any resistance in any single connection , just only one, will reduce the charging current going through the battery.
Hence the battery will never have its full power.
Simultaneously when the battery is called upon to deliver power to the converter and then to the inverter, will not be able to deliver the full current possible and available.
The fact that you get the alarms when it rains or when there is humidity in the air, indicates bad contacts on the battery connections, power cable connections etc.
I would strongly recommend that you access the power battery, check for any signs of corrosion on all the terminals and disconnect each cell, measure the voltage across each one and record it. You should also check the current delivered through a resistance to limit the maximum current delivered during the check.
To do this work you must have the knowledge to work on medium voltage circuits.
If you do not have this knowledge, it is very dangerous and it can be lethal if you touch the dc battery terminals. There are special insulated gloves you wear for this purpose.
The Techstream diagnostic scanner would be a great help in pin pointing the problem.
I would be very interested to do this work for nothing but I do not live in uk.
If you look at a similar topic you will find a contribution made by Richard. He repaired the power battery for somebody and the problem was corrosion on the connectors of the battery cells.
He has the experience the knowledge and the equipment to help you.
Wishing you the very best of luck,
Chris.