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How to Stop Car Battery Drains?

How to Stop Car Battery Drains?
Update Time:2017-08-02

There are plenty of energy-robbing devices on your car that are supposed to go to sleep when it's parked, and sometimes, not all of them go to sleep. Here's how to give them a tranquilizer.


Step 1

Start with a fully charged battery. Be sure the radio and lights are shut off. Unplug your cellphone charger, GPS or laptop. Close the door or hold the dome light button down with a wedge. Remove the key from the ignition. Now use a jumper to bridge the battery's negative post to the clamp to preserve any memory and to keep from activating the antitheft code if your radio uses one.

Step 2

Now you can put your ammeter in series with the post and clamp. When you remove your first jumper, the meter will read the current draw from the battery. Here, 610 milliamps is way too much for comfort.


Got a light in the glovebox? Maybe it's not turning off. Pop open the door and see if the bulb is hot, but don't burn your fingers. Perhaps the trunk light stays lit. You might need to get into the trunk and have someone you trust close you in to see if it's going out. Or just touch the bulb to see if it's hot. (The first way is more fun.)

Step 4

Once we narrowed the parasitic drain down to a single circuit, we used this test cable, which plugs in to the fuse box (rahamtool.com) to patch our ammeter in to the circuit. Disconnect, remove or turn off all the loads on the circuit one at a time until you identify the culprit


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.


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.


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.

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