Parasitic Battery Drain
If a known good battery continues to die overnight or in a short time, suspect a parasitic drain. Excessive current above the acceptable range of ~35mA - 50mA. Use an ammeter to perform a battery drain test.
Note that clocks and accessories drain a small amount of current while the vehicle's at rest. Also, onboard computers consume a small amount of current. They usually take time to power down after the vehicle's been shut down.
By connecting a multimeter in series between the negative battery terminal and the negative battery post, the current is measured as it flows through the meter. If it is above specifications, the circuit causing the drain can be identified by locating and removing the fuse that protects that circuit.
Sometimes there is more than one accessory on the draining circuit, and an electrical diagram is needed. Most of the time, these drains are caused by human error. The trunk wasn't shut correctly, or the light stays on when the glovebox is closed. It's a good idea to make sure nothing is on that should be off, and to look for the simple things first.