Can a Battery Drain with the Negative Cable Disconnected?

If you’re going on vacation or you don’t plan on using your car for a few weeks or months, you’ve probably heard that you should disconnect the negative cable on your car’s battery to prevent it from draining.

Once that’s done you should be good to go right?

Not so fast.

A car battery will naturally “self-discharge” at a rate of 5-15% per month with the negative cable disconnected but is the best option for long-term parking if you cannot hook it up to a charger. Leaving the battery hooked up will drain it at 20% or more per week!

In this article I’ll dive into how a car battery will still lose some of its charge despite disconnecting the negative cable and best practices for keeping your battery ready to go when you returned to your vehicle.

Let’s get stated!

Why a Car Battery Loses Charge Despite Disconnecting the Negative Cable

Like I mentioned above, simply disconnecting the negative cable is not enough to stop your battery from losing charge.

Don’t get me wrong, it is definitely better than nothing and it will go a long way in preserving your battery’s charge while you’re away.

disconnect negative terminal car battery

There’s something that every battery undergoes and that is called self-discharge. You’re probably familiar with that when you buy a pack of AA batteries and it will say something like “Will keep 90% of its charge after 5 years”.

If you weren’t using the battery for those 5 years, where did the 10% go?

The answer is self-discharge.

When a battery is not connected to a smart charger on float mode, which essentially keeps it at a perfect 100% state of charge, then it will undergo self-discharge. Self discharge means that your battery is chemically reverting from a state of charge to a state of discharge because that is technically easier for the batteries chemistry and that’s where it naturally wants to be.

The rate at which your battery does this is dependent upon the type of battery that you have, and car starter batteries (whether they are sealed or flooded) are categorized as lead-acid batteries.

Healthy lead-acid batteries typically will self-discharge around 4-5% each and every week that they are not being used and not hooked up to anything.

Those numbers are derived from Progressive Dynamics’ website and Battery University’s website.

I believe those numbers take into account a new battery with perfect health and at room temperatures.

My 4-year-old battery in a car that was parked on 110 degree pavement with the negative cable disconnected lost 15% of its charge in 10 days without driving after fully topping it off with a 3-stage charger.

I believe that batteries will lose about 5-15% on average per month, but I’ve also read accounts of them dropping as much as 30% in a month. However, You also need to consider the amount of sulfation that has built up inside the battery and if that sulfation is causing an internal short within one of the cells.

Back to the main topic. When you disconnect your negative terminal and only the positive is connected then the circuit is no longer complete. That is true. If you were to leave both cables hooked up, your car would likely be dead after about 2 to 4 weeks due to “parasitic draw”.

Parasitic draw is all of the computer systems on modern cars that will draw energy from your battery even though your car is turned off. Things like your anti-theft system, the clock radio presets, and much more slowly sip away at the battery to keep their settings even though your car is powered down.

Don’t get me wrong, disconnecting your negative terminal on your battery is definitely a going to prolong the battery’s eventual demise and you most certainly are going to get much more time out of it.

If you had a full charge on your battery you’d only have 2 to 4 weeks depending on your make and model of car if you left both terminals connected (to lose about 50% charge). In that same I’m on a time you’d only lose about 5-15% charge by just disconnecting the negative terminal.

If you were leaving for a month or two or even longer and lost 5-25% of your battery’s charge to self discharge, you might think that that’s not too bad. After all, you’ll still have enough juice to start your vehicle and it’ll top right off again, right? Yes and no.

That leads us into our next topic.

Sulfation is the #1 Enemy of Your Car Battery

The tricky part about any battery losing its charge that will catch up with you later is that when a battery is entering a state of discharge it starts to immediately sulfate and the further you go into a state of discharge the faster it will sulfate.

Sulfation is when the lead plates inside of your battery get coated with sulfates (a white coating) due to the chemical reactions during the state of discharge.

This coating of sulfation is soft at first and if addressed promptly it can be flaked off your batteries plates by recharging. Deep-cycle batteries have a protective alloy on the plates that is more forgiving, whereas car starter batteries can be permanently damaged to one degree or another regardless of whether or not you recharge them immediately. That’s why car batteries can only be deep-discharged about a dozen times before they will no longer hold a charge, and deep-cycle batteries can do hundreds.

However, if a battery sits in a state of discharge for any length of time, then this sulfation begins hardening into a crystallized state and it becomes nearly impossible to get off of your lead plates — especially car starter batteries.

lead sulfate on discharged battery plates
White sulfation all over a lead plate on the right when compared to the one on the left. Picture courtesy of Wikimedia Commons

These sulfate crystals increase the internal resistance of your battery which means that it’s harder to actually charge it and get your battery to a full charge, and your battery is not as efficient at releasing current one it’s demanded.

Over time, each and every time you discharge the battery the sulfates build-up to a point where they can actually bridge the gap between the plates which will cause an internal short and kill your battery, or they can simply cover the plates enough that your batteries rendered any effective

Best Way to Maintain a Battery When Not in Use

If I were not going to be using my car for a month or less, I would have no worries about just disconnecting my negative cable from the battery. Of course, I would use a charger if I had access to it just to keep my battery in perfect shape, but disconnecting he negative is just fine for all intents and purposes.

If I were leaving on vacation longer than a month and I had access to a garage, I recommend hooking up a battery charger while you’re away. This charger and maintainer, seen on Amazon, I have used for over 5 years on a daily basis for a battery bank and I also charge my car batteries monthly fully to keep them at an optimum state since I don’t trust the alternator to fully charge them.

recommended battery charger
I trust this baby to keep my battery backup system topped off and ready for when the lights go out.

This is a true “set it and forget charger”. You simply hook up the red cable to the positive terminal and the black cable to the negative terminal and that’s it. The charger will bring your battery to a full charge and then enter a “float” state will keep your battery perfectly topped off at 100% indefinitely. You can leave this on for months and be good to go.

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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