ATCO Royale 24 EC battery dies

I have a brand new battery and used the mower today for an hour. Then it wouldn’t even turn over  on the key.  Multimeter said battery was 7 volts.  I then tested it across the battery terminals when the engine was running fast: 11.8 volts.  Any idea where is should start in order to fix this please?

Ps: the ignition switch is not spring loaded so I may have left it on START but I doubt it.