If you’ve ever planned to go on a vacation and have a car to leave behind, you might’ve asked yourself the question of, how long can a car battery sit unused?

While It’s hard to say exactly how long a car battery can sit unused, a very rough estimation would be around 20 – 60 days with two months being its limit in proper conditions before it goes flat

Keep in mind that a car battery needs at least 12.2V (50%) to start, so although your car battery can last up to 2 months unused, it would be fully drained and not ready to be put back in the car.

So if you’re wondering how long a car battery can sit unused before it’s impossible to start the car, cut all the numbers above in half, but even that doesn’t tell the full story.

What you need to understand is that many factors contribute to how long your car battery can sit unused, those include:

  • Battery Age/Health
  • Battery Type
  • Battery Charge Level
  • Ambient Temperature 

Battery Age/Health

Car batteries have a limited lifetime. Even in perfect conditions, batteries naturally degrade until they can no longer supply enough power to start an engine. This process can be accelerated if you take short trips often since it would not allow the battery to fully recharge.

This is because anything below 12.4 volts (75%) is harmful to the battery and will speed up the deterioration process. So obviously the higher the age and deterioration level, the shorter your battery will be able to last unused.

If your car battery is brand new, the maximum it will be able to sit unused is about 2 months before it completely goes flat (considering you have good/perfect conditions for the battery)

Battery Type

Regular Lead Acid:

A regular lead-acid type battery, whether it is flooded, sealed, etc. will generally cost significantly less than an absorbed glass mat battery (AGM). However, it will not hold a charge for as long and is less able to tolerate a deep discharge.

AGM:

Although more expensive. An AGM battery can offer a relatively long service life, stands up well to cold temperatures, and has a low self-discharge rate

Charge Level

The charge level of your car battery is one of the obvious factors. If your car was sitting at 80, 70, 60, etc, percentage. It would not be able to sit unused for longer than a fully charged battery.

On average, car batteries lose about ~0.5-2% charge every day in normal conditions without being charged by the alternator/generator. Batteries tested include:

  • Interstate Batteries 12V SLA/AGM
  • Honda’s OEM Battery
  • Kirkland Flooded Battery

Most cars do not have a battery charge indicator, so the easiest way to read the charge level is to use a multimeter. While not accurate, it is enough to get the job done.

Note: Voltage Variation

Voltage can vary slightly between sealed lead-acid, flooded, gel, and AGM deep cycle battery types and also between brands. For more accurate results, make sure to turn off your car

ChargeLead Acid VoltageGel VoltageAGM Voltage
100%12.70+12.85+12.80+
75%12.4012.6512.60
50%12.2012.3512.30
25%12.0012.0012.00
0%11.90-11.90-11.90-

One of the leading causes of lead-acid battery failure is leaving them discharged (below 12.4V or %75) for long lengths of time. Discharged batteries will sulfate, and are at risk of freezing, either of which can permanently destroy a battery.

Terminology: Sulfation

Sulfation is caused when a battery’s charge drops below 100% for long periods or undercharging. Hard lead sulfate crystals fill the pores and coat the plates, making it very difficult or near impossible to charge the battery

So if you are storing the battery for an extended period of time, one of the best ways to prevent damage to your car battery is to make sure the voltage never drops below 12.4 volts. We recommend using a battery maintainer for that.

Ambient Temperature

The colder the battery, the lower the voltage/capacity, meaning in cold environments your car will have less voltage and capacity than if it was warm. The chart below can help you understand the severity based on temperature.

At 32°F(0°C), the car’s battery loses about 35% of its strength. And at 0°F(-17°C), it loses up to 60% of its strength while your engine requires nearly twice as much power to start. This can severely hurt the battery capacity. Make sure to store the battery in a dry environment at room temperature if possible

Related: Does a Car Battery Drain Faster In Cold?

How To Prevent a Car Battery From Going Flat When Unused?

Now that you’ve read the factors that go in the time your car battery can sit unused, You’re probably wondering how to make it last as long as possible, and we got you covered.

1. Use a Battery Maintainer

This is the only true solution. With a battery maintainer (also known as a maintenance charger or trickle charger), your car battery can last as long as 5 years! instead of 2 months without it. But requires an extra investment in a separate charging system and constant access to an outlet.

In simple terms, a battery maintainer is an efficient charger that gives juice to your battery every so often to prevent damage. According to autobatteries.com, there are two types of battery maintainers:

  • Traditional Float Chargers: Traditional float chargers provide constant voltage with tapering amperage to the battery even when it is fully charged. The typical floating charging voltage ranges from 13.0 to 13.8 volts.
  • Fully Automatic Chargers: Fully automatic multistage or multistep chargers monitor the battery and charge it as necessary. Multistage maintainers will charge at varying voltages and varying amperage. Some of these multistep chargers are also capable of working well as battery chargers.

If it is not possible to use a battery maintainer, disconnect the battery from the vehicle during storage to prevent the vehicle from discharging the battery. Always provide a full charge with a battery charger before storage, then check the battery voltage every month or so, charge if it falls below 12.4 volts. Also, when possible, store your battery in a cool, dry location.

2. Keep Your Battery Warm

This isn’t a solution, but more of a way to maximize storage time in a cold environment. To keep your battery warm, make sure to:

  • Check your battery’s health, make sure it doesn’t have corrosion.
  • Use a battery maintainer or keep your battery fully charged to prevent the battery from freezing
  • Put your battery in a thermal blanket (battery warmer)
  • Use a block heater (if you will store the battery inside the car) which should come with most modern cars.
  • Alternatively, you can put your battery in a garage.

If Your Battery Is Sitting Inside The Car Unused

If you’re leaving your battery inside the car, instead of removing it, it would last roughly 1-2 weeks before it goes flat. If you want to know why that is, we need to explain a couple of battery ratings.

RC and AH are the standards that most battery companies use to rate the capacity of a battery. You would generally find the label for each on the battery itself.

Reserve Capacity(RC):

The RC label on the battery stands for Reserve Capacity. The number on this rating determines how many minutes the battery will last with a 25 amp draw. So, if the label says RC 100, then this battery would last 100 minutes with a 25 amp load.

Modern cars tend to have an amp load of 40 – 85 milliamps or 0.085 amps due to parasitic drain caused by the increasing amounts of tech in new cars. This would equate to 1 amp every 12 hours. multiply this by 25 and your battery would be completely flat in 14 days.

But since a car battery requires at least 12.2V (%50) to start, the actual usability would drop to 5-6 days, after which your car wouldn’t even start.

Amp-Hours (AH):

On deep cycle batteries, you would usually see “AH”. Similar to “RC”, “AH” denotes how long a battery will last if it isn’t recharged, but it’s a little more complicated.

For example, if a battery is rated as, say 8Ah, it will be able to provide 4 amps for 2 hours. If a battery is rated as 100Ah, it will provide 100 Amps for 1 hour.

This does not necessarily mean a battery will only last one hour because this whole rating depends on how much amps are asked for, or “Continues current (in amps)” if you want to be technical. So it will last 2 hours if it’s asked to produce only 50 Amps, 4 hours at 25 Amps, and so on.

If You Let Your Battery Sit Unused For Too Long

If you left your car battery to sit unused for too long and it has completely lost its charge, you will need to jump-start it, but in a slightly different way.

Note: Don’t Use Cheap Jumper Cables

Note that most cheap battery jumper cables don’t carry enough current to do the procedure below. Make sure to get good quality cables around 4-6 gages, this will get your vehicle going much quicker and will prevent your jumper cables from melting.

Before we hook up any cables make sure that you inspect the battery for any signs of physical damage, like cracks in the case, and/or leaking fluid. If you see either, do not proceed with jump-starting. If everything looks fine, locate the positive and negative terminals, each terminal is usually marked with a stamped “+” or “-“, or colored plastic caps for positive (red) and negative (black). Make sure your certain which is positive and which is negative to prevent severe battery damage. Now that we’ve got that cleared up, we are ready to begin.

  1. Connect the red (+) cable to both batteries
  2. Connect one end of the black (-) cable clamp to the negative terminal of the good battery.
  3. Connect the remaining black (-) cable clamp to a “ground” on the recipient’s car. (Anything metallic or an unpainted part of the frame/chassis.
  4. Rev the engine for about 5 minutes on the donor vehicle from about 1500 to 2000 rpm to get some surface charge on the dead battery.
  5. Try starting the recipient’s car, if it doesn’t, rev for another 5 minutes. If it still doesn’t start, there may be some failure.
  6. After the recipient car starts, remove the black “ground” cable, first on the recipient vehicle, then on the donor vehicle
  7. Remove both red (+) cables
  8. Do not shut off the engine on the recipient’s vehicle. Drive for at least 40 miles to get your alternator to charge the battery. Keep in mind that an idling engine will NOT charge the battery.

If you are removing the cables from a battery always remove the black (-) first. Never the red (+). You will see the sparks you get when your wrench touches any metal part of the car if the wrench on the positive and the negative are still connected.

The reason for connecting the negative cable to the ground rather than to the negative terminal of the dead battery is to reduce the risk of a spark near the battery. Batteries emit hydrogen gas when charging or discharging, so although quite rare, if a spark does ignite the gas, the flame will go back through the vent and the explosion will blow the top off the battery, potentially getting acid in your eyes.