Every car, even when turned off, draws a small amount of electrical current from the battery. This is known as parasitic draw, and it powers essential systems like the car’s alarm, computer memory, and remote locking. But what is an Acceptable Draw On Car Battery, and how do you know if it’s excessive and draining your battery?
An acceptable parasitic draw is generally considered to be between 25 to 80 milliamperes (mA), or 0.025 to 0.08 amps. Modern vehicles with more electronic systems might lean towards the higher end of this range. However, older vehicles with fewer electronic features should ideally have a draw closer to the lower end. Anything significantly above 80mA could indicate a problem and lead to a dead battery, especially if the car sits for extended periods.
Alt text: A multimeter is connected in series between the car battery’s negative terminal and the disconnected negative battery cable to measure parasitic draw, displaying a reading in milliamperes.
Several factors can cause excessive parasitic draw. A common culprit, particularly in older vehicles, is a failing voltage regulator. The voltage regulator controls the voltage output from the alternator. If it malfunctions, it can continue to draw power even when the engine is off.
One car owner experienced this firsthand with a 1990 Lincoln Mark VII. They were facing a persistent issue of the battery draining overnight. After ruling out common problems like the alternator and starter, they discovered a significant 3.6 amp draw. Through systematic troubleshooting, disconnecting various electrical components, they pinpointed the voltage regulator as the source. Cleaning the regulator’s connector temporarily reduced the draw, but the regulator soon overheated and failed completely, confirming it as the problem. Replacing the faulty voltage regulator resolved the excessive draw, and the battery drain issue disappeared.
Other potential causes of excessive draw can include:
- Faulty relays or modules: Relays stuck in the “on” position or malfunctioning electronic control modules can continue to draw power.
- Aftermarket accessories: Improperly installed or failing aftermarket accessories like alarms, stereos, or lighting can contribute to parasitic drain.
- Glove box or trunk lights: Lights that remain on due to a faulty switch can drain the battery over time.
- Corrosion and wiring issues: Damaged wiring or corrosion can create unintended electrical paths, leading to current leakage.
If you suspect your car has an excessive parasitic draw, you can test it using a multimeter. The process involves disconnecting the negative battery cable and connecting the multimeter in series between the negative battery terminal and the cable. The multimeter will then measure the current draw in milliamperes. It’s crucial to allow some time for the car’s systems to “go to sleep” (usually 20-30 minutes) after connecting the meter to get an accurate reading of the resting parasitic draw.
While diagnosing a parasitic draw can sometimes be straightforward, tracing the exact source can be complex and time-consuming. If you are uncomfortable performing electrical testing or are unable to pinpoint the cause, it’s best to consult a qualified mechanic. They have the expertise and tools to accurately diagnose and repair electrical issues, ensuring your car battery remains properly charged and reliable.
In conclusion, understanding what constitutes an acceptable draw on car battery is crucial for maintaining your vehicle’s electrical health. By being aware of the normal range and potential causes of excessive draw, you can proactively address battery drain issues and prevent unexpected breakdowns.
