I just recently purchased a 2000 Volvo S40, runs and drives perfectly. However, if sitting parked somewhere overnight, the battery will be dead.

This is what I know:
The alternator is fine
No electrical systems were left on (glovebox light, parking lights, trunk light, etc)
It has an aftermarket alarm in it.

Any ideas of what could be drawing enough amperage to drain a brand new battery in 24 hours?

 

Here's the URL for the sticky on the 850 board

https://volvoforums.com/forum/volvo-...g-101-a-42655/

It could be one of several things - a failed switch or relay, fault ignition switch, a short in the voltage regulator or even be the alarm system. The place to start is to see if any of the fuses are passing current with the key off. (there's a small amount of current drawn to power the clock/ecu etc but very nominal (ie .02A ). If you don't see anything drawn through a fuse then you may need to isolate the alternator/voltage regulator etc.

 

Maybe your new battery is at fault. Guess you could try disconnecting a lead overnight, reconnect and see how much cranking power you have.

 

Aftermarket alarm? It has a built in alarm. Are they not getting along?