most likely it is because items, such as lights (trunk, etc.) are left on. Or that there is a short in one of the alternator diodes (is it charging correctly?). Or the battery is just old and no longer holds a charge. How often between running the car? It could be tha you dont give the battery enough time to charge sufficiently.
All of a vehicle's electicity using devices drain/discharge the battery, but during the time the engine is running, the alternaror is supposed to keep the battery recharged. If the engine IS running, but the alternator is NOT providing enough current to recharge the battery then ALL of the electricity using devices discharge the battery. Some of the "onboard" computers draw some slight amount of power even when the ignition switch is off, but they do not use enough power by themselves to significantly discharge the vehicle battery. Any electricity using device which SHOULD be turned off when the ignition is off, BUT... which due to some fault [such as a "stuck" relay for example] that keeps that device still turned on when the engine is not running and the key is off will also discharge the battery. Or, just leaving the ignition switch on without the engine running, or the headlights or parking lights on, or sound equipment left on will contribute to discharge of a vehicle's battery.
from me constantly raping your cars exhaust while you sleep.
If the defect is causing a parasitic drain, yes.
Either something is on and is drawing current from the battery or the battery itself has a dead cell which will drain the battery. If the battery is over 3 yeas old, replace it.
If the battery is bad, you may only hear clicks. It could be the starter, but I would check the charge on the battery first.
would discharge in 18 hours
It typically indicates that the alternator has failed and is no longer charging the battery. Once that happens, the battery will slowly discharge and the vehicle will no longer start. The battery is simply an electrical storage device and if the alternator is unable to replace the electrical energy used LAST time the vehicle started, the battery will discharge. Automotive batteries are not designed for "deep discharge", meaning that if you allow your battery to discharge too much you are causing harm. Once the warning light comes on it would be wise for you to take care of the problem as soon as possible.
Battery and brake light coming on means the Alternator is not charging the battery . It could either mean a bad circuit connection or the alternator is just shot. Have the Alternator checked.
The voltage reg is sticking and will run down the battery.
A quick way to reset the check engine light would be to disconnect the battery for a short while
Battery light usually means alternator not charging. Most common solutions would be be a bad alternator, bad connection, or loose alt belt.
A dead cell within the battery itself will drain the battery. There is no fix for this except replacing the battery. You may also have something that is pulling power from the battery. Examples are under hood light, dome light, glove box, trunk, courtesy light, and any numbers of relays.