Why is the car battery draining fast?

Updated: September 15, 2022
If your car battery keeps dying or needs to be boosted often, there are three common causes:
1. The battery's capacity is getting low. This often happens if the battery is more than 4-5 years old. As the battery gets older it can hold less charge and as a result drains faster. A newer battery can lose its capacity too, for example, when you don't drive too often and the car is parked for extended periods of time.

2. Some of the electronic devices in the car remain powered on when it's parked, draining the battery. The second cause is more likely if the battery is fairly new.

3. The alternator could be failing and not generating enough power to recharge the battery when you drive.

How long should the car battery hold charge?

If the battery is good, all electric system in the car work as they should, all doors are closed, no lights left on, and nothing is plugged into the power outlets, the car should start easily after being parked for two-three weeks or more. Larger batteries can keep the charge longer.
Tip: The battery will hold the charge longer if it's well charged up before the car is parked for a long period of time.

What to do if the battery keeps dying?

  • Check if there is anything plugged in into one of the power outlets.
    Electric device plugged into cigarette lighter socket In this car the charger adaptor was draining the battery
    Some cars have power outlets that are powered only when the ignition is turned on or in Accessory mode. Other cars have power outlets that are powered on at all times. Many cars have both types.
    If something is plugged into an outlet that stays on at all times, like in this photo, try unplugging the device and see if this helps.
  • See if anything is plugged into the OBD connector located at the lower portion of the dash on the driver's side. The OBD connector is a universal connector to connect a scan tool to the vehicle.
    OBD connector In this car, the tracker device plugged into the OBD connector drained the battery
    Some aftermarket devices plugged into the OBD connector, such as a tracker device, can also drain the battery.
  • Check if the 'door ajar' warning light doesn't stay on when all doors are closed and dome lights, the glove box light or the trunk light are turned off.
  • When the car is parked, check if the transmission gear indicator (PRNDL) shows that the transmission is in Park.
  • Third, have the battery tested, especially if it's old. In some cases, even a newer battery can fail. Read more: When to replace the 12-Volt car battery?
  • Have the vehicle tested for parasitic current draw. Any reputable repair shop with access to wiring diagrams and factory service manual or a brand dealer can check your car for parasitic current draw. If it's a newer vehicle, your local dealer should be able to assist you.

What is a parasitic current draw?

When the car is parked, some of the electrical systems such as the security system, the clock, smart access key and various modules that have memory presets, such as the radio, remain powered on drawing a small amount of electric current. In an average car this parasitic current draw doesn't exceed 20-30 milliamps (mA), which is normal.

Your mechanic can measure the parasitic current draw and see if it's within the limit specified by your vehicle's manufacturer (typically around 50 milliamps). If the current draw is higher, the affected circuit/device must be identified and repaired or replaced.

How do mechanics measure parasitic current draw

Mechanics measure the current from the
Parasitic current draw test Parasitic current draw test.
battery with a multimeter or a special tester according to the factory service manual instructions, when the car is turned off and all electrical devices and lights are off. The multimeter is set to Amps and is typically connected in series between the battery and the negative cable.

Initially, when the car is turned off, some of the electric modules stay on, drawing higher electric current (up to several Amps). However, after 10-15 minutes, all electric modules in the car go into sleep mode and the current from the battery drops. Car makers specify the acceptable parasitic current draw in milliamps and how soon it should drop.

If the electric current from the battery remains excessive even after 10-15 minutes or the time that is specified, it will cause the battery to drain sooner. For example, in the vehicle in the photo, the current draw shows 0.068 Amps which is 68 milliamps (0.001 Amp is equal to 1 milliamp.). It's too high for this vehicle and the owner had to boost the battery every time the vehicle was parked for more than 2-3 days.

Common problems causing excessive parasitic current draw

We have seen many cars where a phone adaptor or a dash cam is left plugged into a power plug that is always on, draining the battery.

Aftermarket remote start units, trailer wiring modules and audio units often cause additional load that drains the battery faster.

Another common problem is when the transmission range switch doesn't work properly. This causes the vehicle not to recognise that the transmission is in Park and the radio stays in Accessory mode and drains the battery.

 In many newer cars, some of the modules may not go into sleep mode due to a software error and may need to be reprogrammed.

Often, the excessive parasitic current draw is caused by water leaking into the electrical components (e.g., from sunroof drains) causing corrosion and shorting some wires or electric modules. Look for signs of water leakage and corrosion around wires.

A sticky relay is another common culprit for dead batteries. For example, the A/C compressor clutch relay is known to go bad in some Honda and Acura vehicles.

Faulty Bluetooth and satellite radio modules are known to cause excessive parasitic current draw.

How do mechanics diagnose parasitic current draw problems?

The first step is to check if anything is plugged into 12V power outlets or the OBD port or is connected directly to the battery or if there are any aftermarket devices installed on the vehicle. If the current draw drops to a normal level when such a device is disconnected, you found the culprit.

Mechanics also check for service bulletins, as often car manufacturers are aware of common problems in their cars. You can find some of the service bulletins online. Searching for causes of excessive current draw in your year make and model vehicle can also help, as someone may already have solved the same problem and posted the fix.
Normal battery current draw In this car, the current draw is normal (8 milliamps).

The next step is to identify the electric circuit that draws excessive current.
Each circuit in the car is protected by a separate fuse. Mechanics disconnect fuses one by one and see if the current drops.
If, for example, the current drops to normal when the "Room" fuse is removed, you know that the device that draws excessive current is on one of the circuits protected by the "Room" fuse. Then, following the electric diagram, the components on this circuit are disconnected one by one until the current drops.

This process could be time consuming, because there could be several components protected by the same fuse.

Measuring temperature of electic deviceThis relay is powered on as its temperature is higher than other relays.
Another way mechanics find the circuit that remains powered on is by measuring the temperature of the suspected components with an infrared thermometer and comparing it to the temperature of other parts around it.

For example, the relay that remains powered on, will always be warmer than other relays as you can see in the photo. If the parasitic current draw is fairly high, even the wires that are loaded will be slightly warmer compared to other wires.