Connected a battery maintainer to the battery of my 96 XJS and a red trouble light appeared. This never happened before with other cars. I removed the maintainer and hooked up another one. This one seems to be working. I heard a sort of pop from inside the car and the maintainer seems to be working. I'll check on it later today. The battery is about two years old and the car was parked about 3 weeks ago. Is there any reason I should be concerned? It is hooked up correctly.

 

If you suspect a problem with your car's electrical system (like an excessive power drain), then I'd first disconnect the battery, remove it from the vehicle and test the maintainer on the bench while keeping an eye on it (and an ear - listen if the battery makes a sound like its boiling or the maintainer hums loudly). If it works off the car but trips when its connected to the car, then the problem is likely in the car. If it still trips when out of the car, then its either the battery or the maintainer. Then check it connected to another known good battery to rule out the maintainer. But don't connect it and run off and leave it unattended or overnight until you're satisfied that whatever the problem is has been resolved. I.e. start each test in the morning and check it frequently throughout the day.

I don't know which unit you've got, but in my younger days had some close calls with "cheap" battery maintainers, like boiling over and exploding, blowing the caps off. Thankfully learned my lesson about saving money before burning the garage down. Now I have all CTEK units because they're supposed to be smart enough circuitry to shut themselves off if they detect a problem that could be dangerous. It was once a frustrating weekend trying to diagnose why the battery on one car (not one of the Jaguars) kept tripping the CTEK units off. Turned out to be something simple, but a cheaper unit might have just kept charging and done serious damage.

 

Is it a AGM battery?

 

It is not an AGM.