Can A Battery Drain With A Negative Cable Disconnected?

can a battery drain with the negative cable disconnected

Yes, a battery can drain with the negative cable disconnected. This makes you wonder why anyone would bother disconnecting the negative cable. But this practice serves a purpose. Consider the following:

1). Why Disconnect The Battery?

The battery runs all the electronic devices in your car, including the radio, clock, lights, and more. It will even start the engine when you turn the key in the ignition. Disconnecting the negative cable protects the battery.

Even when you shut the car down, some electronic appliances will continue to drain the battery. For instance, remember the last time you opened the car doors using the button on your key? How could the car respond when the engine was off? The security system was active, which meant that it was pulling power from the battery.

That current is small. But given enough time, it can still drain the battery. You can see how disconnecting the negative cable can protect the battery, especially if you’re going on vacation for a few weeks.

According to Carused JP, you may spend $100 or more replacing a dead battery. This assumes that you’ve tried and failed to jump-start it.

2). Why Disconnect The Negative Cable?

If you’re trying to protect the battery from parasitic drains, why disconnect the negative cable? Why can’t you disconnect the positive cable? Have you noticed that mechanics emphasize this issue when you want to remove or replace the battery?

They expect you to disconnect the negative cable first. Why? The negative cable connects directly to the car’s chassis. A current runs from the positive terminal to the electronics. Then it flows out of the electronics to the chassis before moving to the negative terminal.

Disconnecting the negative cable first while replacing a battery prevents sparks, shocks, and shorts that occur when you accidentally touch the chassis with a metal tool. Disconnecting the positive wire is insufficient because you can still complete the circuit by bringing a wrench into contact with a metallic surface under the hood.

3). Why Does The Battery Drain Even When You Disconnect the Negative Cable?

Drivers are quick to blame flat batteries on parasitic drains. However, if you place AA batteries on a table and walk away, they will lose a portion of their charge after a few months. This is because it is perfectly natural for batteries to self-discharge.

It is a weakness in their design. The batteries want to return to a state of discharge. But if that is true, why disconnect the negative cable? Because a car battery loses 5 – 15 percent of its charge per month when you disconnect the negative battery.

That number jumps up to 20 percent a week if you leave the negative cable connected. The difference is significant. Simply put, even though the battery drains in both cases, disconnecting the negative cable reduces the rate of self-discharge. In other words, it is better than doing nothing. The exact rate of self-discharge depends on various factors, including:

  • The battery’s age.
  • The number of charge/discharge cycles it has encountered thus far.
  • The ambient temperature
  • The severity of sulfation.
  • The battery’s capacity.
  • The battery’s health.

But even with these factors at play, you are better off disconnecting the negative cable. Otherwise, the parasitic drain will kill your battery in four weeks or less.

4). What Is Sulfation?

Why does sulfation concern vehicle owners? The term refers to the sulfate crystals that coat the lead plates in a battery. The sulfation is soft initially, but the longer a battery remains in a self-discharge, the harder the sulfation becomes.

This makes the crystals more difficult to eliminate. Sulfation makes a battery more challenging to charge by increasing the internal resistance. The more times you discharge the battery, the more severe the sulfation becomes until it creates a short, ruining the battery permanently.

This paper in the Journal of Energy Storage has identified sulfation as a prominent problem in hybrid electric vehicles. Therefore, you can’t dismiss it as a fault that only afflicts batteries in traditional vehicles.

5). Why You Shouldn’t Disconnect The Negative Cable?

The only reason to keep a negative cable connected is that you don’t have the skill to remove it. A layperson is more likely to cause a short circuit by tampering with a battery and its cables. You may damage the wires and their terminals.

The alternative is to hire a technician to perform this task. But that isn’t always convenient or financially feasible.

How To Disconnect The Negative Cable?

  • Gather everything you need, including gloves, goggles, and insulated tools (Wrench Kit). Make sure the socket wrench matches the connector bolt’s size.
  • Shut the engine down.
  • Open the hood and locate the battery.
  • Find the negative terminal. It has a minus sign.
  • Loosen the nut on the negative terminal using a socket wrench.
  • Remove the negative cable. Don’t touch both terminals simultaneously with a metallic tool.

How To Test For Battery Drain With Negative Cable Disconnected?

  • Turn all the electrical components off and take the key out of the ignition.
  • Use a multimeter to test the battery. If it isn’t full, charge it. The battery should show 12.6 volts at full charge. If it can’t attain a full charge, replace it. The battery is old or damaged.
  • Find the cable with the minus sign and unbolt it from the terminal.
  • Get your multimeter and configure it. This involves running the black line to ‘COM’ and the red wire to the highest amp input.
  • Connect the other end of the red lead to the negative cable. The black lead goes to the negative terminal. Use plastic clamps to prevent the leads from falling away.
  • Readings of 20 to 50 milliamps are acceptable. A reading higher than 50 milliamps shows that your car has a parasitic drain somewhere. Something is constantly pulling power from the vehicle.

How To Prevent Battery Drain With Negative Cable Disconnected?

  • Find the fuse box under the hood and start removing fuses. Go from the smallest amp rating to the highest.
  • Check the multimeter whenever you remove a fuse. If the readings don’t change, put the fuse back and try the next one.
  • You may need help performing this test. Find a friend that can remove the fuses as you observe the multimeter’s readings or vice versa. If you can’t see anything on the multimeter, check the fuses under the dash.
  • If removing the fuse causes a significant drop in the readings, you have identified the problematic circuit.
  • Use the fuse box chart to find the components on that circuit. They are probably causing the parasitic drain. Most laypeople can’t use wiring diagrams to locate the problem. Don’t hesitate to ask for professional help. If you have some mechanical experience but lost the manual, look for the wiring diagrams online.
  • The best option here is to disconnect each device on the circuit while checking the multimeter’s readings. The readings will drop if you disconnect the culprit. Disconnecting the correct devices changes the multimeter’s readings because it removes the drain.
  • Fix the component causing the parasitic drain. Again, this task may require a mechanic.
  • Wait an hour after parking a car before testing the battery. Otherwise, the surface charge will corrupt your readings. Leaving the car alone for an hour allows you to test the resting voltage. 
  • You don’t have to check every single component on the circuit. Try following your nose. A burning smell may lead you to damaged wires.

Best Way To Maintain A Battery When Not In Use

  • This goes without saying. Shut all the accessories down, especially if you expect the car to remain unused for several days or weeks.
  • Drive the car for thirty minutes a week. You don’t have to take it out on the road. Simply starting the car and letting it idle for a little while is enough to keep the battery from going flat.
  • Most people are smart enough to turn the radio, lights, and every other electronic component off. However, it never occurs to them to deactivate the security system. They don’t realize that the security system will continue to drain the battery. If you park the vehicle in a location you trust, you don’t have to leave the security system on.
  • Instead of disconnecting the negative cable, remove the entire battery. This eliminates the risk of unknown parasitic drains.
  • Removing the battery won’t stop it from self-discharging. The only way to protect a battery in storage is to connect a float charger. If the battery is not full, the float charger will top it off. If the battery is full, the float charger will keep it at 100 percent. Float chargers are amazing because they can stay connected to the battery for as long as it remains in storage. They won’t overcharge it.

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