On my plus4, The battery is on the left hand side of the vehicle behind the seat under the parcel shelf.
Two ways to get at it. Both involve pulling back the carpets.
The easiest is through the aluminium inspection hole behind the passenger seat.
Or by removing the plywood shelf.
I hope that helps.

Ian
2011 Plus 4
Wild Mogs Reims 2019, Goodwood 2018, Isle of Man 2017, Carried on to Caramulo" 2016