Here are a few things that can drain your battery-- anything that is still connected to your battery with ignition off - including automatic light sensor circuits.
Alarms
Aftermarket alarms are notorious for sucking even healthy, fully charged batteries dry within a few days. If you have any non-factory alarms, it’s the first thing you should check. Be aware that there may be more than one connection to the car’s electrical system, and some aftermarket installers may use, ahem, non-industry standard splicing techniques. So you may have to simply follow the alarm wires to see where they go. More expensive alarms tend to be less problematic, but maybe that’s because more expensive alarms are installed by better, higher-paid technicians.
Stereos
OEM stereos are usually not problematic. Aftermarket stereos, the kind with giant, finned boxes and their own finger-thick wires directly wired to the battery, can be. With a power lead bypassing the car’s electrical system, they go into standby mode, waiting for the main radio head unit to tell them to wake up. In standby, they’ll draw only a milliamp or three. If they fail to go into standby, or if the DIP switches on the amp are set incorrectly, they can draw as much as several hundred milliamps, even though they’re not producing any actual noise. Or music.
Proximity Keys
Lots of new cars are available with proximity keys. They’re a great convenience—all you need to do is walk up to your locked car with the key in your pocket or purse. As you approach the door, the locks pop open automatically. Plunk yourself in the seat and, with the key still in your pocket, thumb the starter button and drive away.
Guess how these things work. There’s a radio receiver that continuously listens for the key’s frequency. When the receiver hears a signal at its assigned frequency, it wakes up to see if the key is the one that matches the car. That draws more current for a minute or two, until the receiver abandons the possibility that it’s about to unlock the door for master. This might be an issue if you leave the car parked for many weeks without starting it. Imagine the confusion of a car parked near the elevator door in a busy parking structure. Every proximity key that walks past makes it sit up and beg, draining your battery for a few minutes. Soon, dead battery.
This issue is even more profound in the case of hybrids like the Toyota Prius. The key receiver operates on 12 volts, as do most of the accessories. More importantly, the 12-volt battery operates the main computer that controls everything else, like the door locks. The Prius starter, however, operates off the 280-volt traction battery—but the main computer has to be powered up for anything to happen. Since it doesn’t have to start the engine, the 12-volt battery of the Prius is very small, 38 ampere-hours. So while a 64-ampere-hour battery in a normal car can last for weeks of a proximity receiver checking for the right key, the Prius battery can run too low within a few days. Bottom line: Turn off the proximity-key function, from the Prius’s electronic dashboard, whenever you’re parking it longer than overnight. It might be possible to do this on other vehicles as well. Check the owner’s manual.
