Parasitic Battery Drain
If a known good battery continues to die overnight or in a short period of time, suspect a parasitic drain. This is excessive current above the acceptable range of ~50mA. Set the multimeter to measure amperage 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 drain a small amount of current and 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, current can be measured as it flows through the meter. If the current 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 make sure nothing is left on and to look for the simple things first.