It is very frustrating when a car battery loses charge over several days of the vehicle not being used. It is also not good for the battery and if this continues to happen in many cases replacement will be necessary.
The most common cause of the battery going dead after sitting for a short while would be a drain from the electrical system on the vehicle. Most often this can be traced back to a light in the vehicle that is either left on or will not turn off automatically.
A couple of examples would be a glove box light or a trunk light. In most cases these types of courtesy lights are supposed to turn off automatically. When they don’t the car battery loses charge.
The automatic operation of these courtesy lights is controlled in most cases by a pin switch but in some older vehicles you can still find the old-fashioned mercury switch. Testing of these items is further explained below. Click on read more.
Courtesy Light Drains Battery
These types of courtesy lights operate similarly to a refrigerator light. In automobiles the courtesy light is connected directly to the battery and receives power whether the ignition switch is on or off (hot at all times).
In the example of a pin switch or a mercury switch, power is supplied to one side at all times. There is no drain on the battery until the circuit is completed by the switch. Note if the bulb is removed the circuit would not be completed as well.
In the case of a pin switch on a glove box or trunk lid when the plunger is depressed it opens the circuit and stops current from flowing. These are usually two or three terminal switches that are easily tested by pushing on the plunger.
Testing for Battery Losing Charge
In the automobile there are some constant drains on the battery that are normal and draw small amounts of current known as a parasitic drain. An example of this would be a digital clock or even the cars computer which has a function known as stay alive memory.
When these parasitic drains become excessive the car battery loses charge and intermittent no start conditions can happen. There are several ways of performing the battery drain test. Some people use a test light connected in series with the positive terminal.
I prefer to use a multimeter or an ammeter that is connected in series with the negative cable. This is where you disconnect the negative side of the cable from the battery and connect one lead of the meter to the cable and the other lead of the meter to the battery.
A good rule of thumb is that the ammeter should read less than .25 amps with everything turned off in the vehicle including the hood light. If you need more information on how to use a meter or want to see my top picks for good ones you can visit my page about automotive meters.
Fixing a Battery Drain Problem
It is best to remove the hood light bulb from the socket when you’re performing these types of battery drain tests. If your readings are high, especially over one amp what you can do is pull the fuses out of the car’s main fuse panel one at a time and observe the meter for any changes.
For example if you pull the courtesy fuse and the meter drops down to a normal condition you have located the circuit that contains the drain on the battery. You can then grab your wiring diagram and see what loads are on that circuit.
An even better way of testing for a car battery losing charge is to use a meter with an inductive pickup. This allows you to leave the battery fully connected to the vehicle and place inductive probe around the positive cable connecting to the battery. This is the preferred method but not everybody has an inductive pickup ammeter or multimeter.
When a car battery loses charge several times it eventually loses its ability to recover from the low voltage condition. Batteries that are exposed to normal heavy drains such as trolling motor batteries or even marine batteries are known as deep cycle batteries. They are designed to be drained and charged several times. The standard car battery is not a deep cycle battery and therefore cannot handle this type of battery drain problem for long.