Since I got my XJR I'm keeping the mileage reduced on my 06 XK8.
Not driving it as much I figured I would put a battery tender on it.

Went out today after it setting for two weeks and when going in reverse got a long beep and no parking assist. Coming home got long beep when key was turned on then again when going in reverse and no parking assist.

Wondering if this is coincidence or could the tender have fried something. I know these cars need exercise. Maybe she's just pissed at the lack of attention.
2X

 

She's just pissed. I usually have a tender on out xjr and the xk is the "beater" car. Even with that I still spend more time on the bike if I need to go somewhere. Provided the tender is working fine it uses less volts to keep the battery charged then the alt. does.

 

I have used a battery tender on my XKR ever since I stopped using it as a daily driver (April 2009). As TreVoRTasmin states the tender's low voltage and amperage should not fry anything. Not sure which brand you are using but if you are using a true Battery Tender (by DelTran) pay attention to the lights on it. If you have your car on there long enough to switch from (Red) charging to (Green) storage mode you'll probably notice that everything functions properly, if it is still red you may get a few "gremlins". I frequently got a DSC error when it was not fully charged, on the tender with a green light that has disappeared completely.

Also on our XKs it is my understanding that if you lock the car while it is on the tender you'll get better results as the BPM goes into a deeper sleep mode.

For reference I am using the: Battery Tender Plus 12v 1.25 Amp 4 Stage Smart Charger BT-021-0128

 

I keep a Battery Tender on all my occasional use cars. I've never been greeted by a drained battery. I've also never documented anyone having any electrical problem brought on by using a Battery Tender. I know that some Tenders are not designed to work with AGM batteries [check to see if you are using an AGM battery]. If you are using an AGM battery make sure your charger is compatible. All my Battery Tenders (3) are hard wired to the battery. I have no experience using the lighter socket as a charging port on a Jaguar.
RB