We have a pretty early S-Type, registered May 1999, which we bought 4 years ago.  We quickly discovered that it was prone to discharge the battery overnight - not always, but more often than not.  A cheap solution has been to fit a Richbrook battery isolator, £9.95 plus postage.  While out and about, the battery lasts at least 3 hours, but we disconnect if the car is being left for more than that.  

Hi Marianne, no car should discharge its battery so quickly, there has to be a significant parasitic drain happening. Have you checked the obvious like boot lights remaining on? Interior lights are more obvious over time, so I assume you have ruled those out.

However, there is one interior light that is a little harder to spot and that is the light in the vanity mirrors, behind the sun visor. If your car is fitted with vanity mirrors and appropriate lights and also has the sliding screen across the mirror, if you don't close the sliding screen to cover the mirror, the vanity light remains on, causing a large battery drain.

N6 JMX

12 hours ago, Racerlady said:

We have a pretty early S-Type, registered May 1999, which we bought 4 years ago.  We quickly discovered that it was prone to discharge the battery overnight - not always, but more often than not.  A cheap solution has been to fit a Richbrook battery isolator, £9.95 plus postage.  While out and about, the battery lasts at least 3 hours, but we disconnect if the car is being left for more than that.  

I would suggest you check the “alarm chirp sounder”. This is by and large the most common fault with discharging batteries. Disconnect it and then see if the battery dies.