Your battery case will conduct a charge between poles. (little known fact)
If this is a minor discharge and you want pease of mind buy Battery Minder on ebay. A recommendation to do a 2 amp trickle charge should be avoided because a constant overcharge will degrade your battery

 

Orangeblossem
I had a Lotus Elite circa1969. I put a dash switch on the fuel pump. and wired it backwards. Off was on, on was off. Unfortunately, no one tried to steal it. I would have loved to see a thief's face in the middle of an Interstate with a dead car! I would do it to my XJS, but I'm getting so old I'm afraid I would forget the switch, car, and everything else.

 

I too had battery drain. Pulled each fuse one at a time. Found nothing. Forty milli amp drain seems to be the norm. I just keep a battery tender plugged up.

 

No guys a battery drain is not normal on these cars and no you shouldn't need a tender or disconnect. Mine sits for up to 4 to 5 weeks w/o being started and when I hop in it starts just fine. Find the problem and resolve it not just put some bandaid fix and assume that it's just "normal" for these cars to do it. Most common drain is the original power antenna shorting inside due to the water drain over the circuit board.

 

I have the same issue with my battery being completely dead after a week of no driving. To me it seems like there are small electrical drains taking place when it sits. For example, I open the door and the alarm immediately chirps which makes me think the alarm must always be creating some sort of drain. There's that flashing red light that's always on by the steering column, but I really have no idea what I'm speculating about as I'm a complete novice with this vehicle and am not an expert in electronics...but I am experiencing the battery issues. What about the battery itself? Do these vehicles need some sort of special battery like maybe one of those hybrid dual purpose batteries that have the deep cycle component? Sorry to bump a two year old post, but what is mentioned here is relevant to my issue. Cheers!

 

Your battery should not be dead in a week unless you have a week (sic) battery. The easy way to trace a clandestine discharge is by pulling fuses and if I remember correctly, a meter between the positive side of the battery. When you pull the right fuse the meter shows that the discharge stops. Then you will know the circuit the discharge is in. With the XJS you could also remove one of the power lines in the engine compartment. That would cut your fuse pulling exercise in half. Hopefully, some other owner will help with this explanation.

 

Just a short heads up for you U.S. guys. Harbor Freight has a battery trickle charger for under $10.00 maybe even less. It's made in China (of course) but seems to work well on my other car. I have a Battery Minder on my XJS.

 

Check that the boot light is going out when the lid is shut. This is the most common cause of a drained battery. If the plunger switch actually works, the lid may not be close enough to it when shut. The remedy is to stick a small piece of rubber onto the lid so the switch makes better contact with it.

The alarm should not drain the battery, and the red flashing LED uses almost no current.

 

The Harbor Freight Trickle Charger is called: "http://www.harborfreight.com/automatic-battery-float-charger-69955.html." and costs about $9.00