1997 Questions for all other cars Car Battery

Electrical problem
1997 Questions for all other cars Automatic 140000 miles

hello,

We have a Rover 800 I automatic with a brand new battery but when we leave the car overnight, the car battery gets discharged, completely and we have to charge it all ovver again even though it's a new one. We checked if we left the lights or else turned on during the night but nothing like that.
could it be a didode problem?

thank you!

Amélie