There are two batteries on a modern XJ8. One is in the trunk area.
The battery is under the saddle. The red cable goes to the terminal marked positive (+) and the black cable goes to the terminal marked negative (-).
Red, positive + cable to Positive + battery terminal. Black, negative - cable to Negative - battery terminal.
It is usually red.
When connected to the cable which normally runs to the starter it completes the circuit. Electrons flow from the negative terminal to the positive terminal.
From battery to starter solenoid
its the red one conected to the positive terminal
There is no way that the positive battery terminal was ever connected to the chassis. This would be a dead short and would fry the battery. Negative to chassis is correct, but positive to chassis, no way. That positive cable goes somewhere else.
In most properly wired cars, a red cable goes to the positive terminal on the battery, and a black cable goes to the negative terminal. If color coding doesn't work, negative is the ground cable, positive is where your power is going.
The battery post positive terminal is where you connect the positive cable. Connect the negative cable to anyplace on the engine or chassis. Connect positive first and remove it last.
Red is positive (+).
Always the negative cable first.
Find the positive battery terminal. Follow the thick cable from that terminal and it will lead to the starter solenoid.