It’s an unfortunate fact that car batteries will lose their charge and drain just by sitting in a parked car or even when disconnected and sitting on a shelf.
This is unavoidable without being hooked up to a quality float charger like this one on Amazon to keep it at a 100% state when not in use.
If you’re wondering why a battery drains and how long you’ll have until your battery will die, then you’re in the right place. I’ll cover all of that below.
If you’re just here for the quick answer with a law of averages, then I will hook you up with that now.
A typical car battery will drain in 2-3 weeks and be unable to start your car due to the parasitic draw from the car’s electronics. With the negative terminal disconnected, a healthy car battery will only self-discharge at 5% per month.
Let’s get into the details!
How Fast Your Car Battery Will Drain and Why (Charts)
The primary reason why your car battery drains when it’s connected to your car (even though you haven’t driven it) is due to the fact that all of the electronics on the car are connected back to the battery and they never truly turn off.
Your satellite radio, GPS system, anti-theft system, and radio presets all slowly drain your battery even when the doors are shut and the key is removed from the ignition.
This is known as “parasitic draw”.
On newer cars the parasitic draw is slightly higher than on older cars due to the increase of electronic systems. A normal parasitic draw is about 50mA or 0.05 amps, but the range can be anywhere from about 0.03 to 0.085 amps (30-85mA).
If your car gets above 50mA, then it might be completely normal for your vehicle — especially if it’s new — but you might want to look into it further by running a parasitic draw test or having a mechanic do it for you if the battery seems to be dying sooner than you’d like.
Here’s a very helpful video on how to do the test yourself (not mine).
If your car is between 50mA and 85mA, then it’s best to drive your car at least weekly to reduce the risk of not being able to start your car at all due to a dead car battery.
I have a chart below where I estimated the approximate days that you would have for any given car battery.
How Fast Will a Car Battery Drain (Parasitic Draw)
|Parasitic Draw||200-315 CCA, or 40-60 RC, or 36-46.2 AH||315-550 CCA, or 60-85 RC, or 46.2-58.8 AH||550-1,000 CCA, or 80-190 RC, or 58.8-111 AH|
|0.03 Amps||50 - 64 Days||64 - 81 Days||81 - 154 Days|
|0.04 Amps||37.5 - 48 Days||48 - 61 Days||61 - 116 Days|
|0.045 Amps||33 - 43 Days||43 - 54.4 Days||54 - 103 Days|
|0.05 Amps||30 - 38.5 Days||38.5 - 49 Days||49 - 92.5 Days|
|0.055 Amps||27 - 35 Days||35 - 45 Days||45 - 84 Days|
|0.06 Amps||25 - 32 Days||32 - 40.8 Days||41 - 77 Days|
|0.07 Amps||21 - 27.5 Days||27.5 - 35 Days||35 - 66 Days|
|0.085 Amps||17.6 - 22.6 Days||22.6 - 28.8 Days||29 - 54 Days|
At the top you’ll be able to see where your car battery rating would fit in. Simply look at the sticker on your car battery and find where you’d be by using CCA (cold cranking amps), RC (reserve capacity), or the equivalent amp hours based on my extrapolations from a battery charger manufacturer’s data that I did from a previous article which you can check out here if you’d like to see how I got those calculations.
Remember, the results are in the total days until your batter you will be 100% discharged, so you might want to cut that time in half or by 75% so that you still have a chance to start your car. Once you get below 50% your odds of turning over the ignition are going to drop drastically.
The formula for these calculations was to take the parasitic draw in Amps and multiply it by 24 hours to get the amount of Amps per day being drawn. Then, we take the equivalent amp hours of a battery based in its RC or CCA rating (see my article link above) and divide it by (parasitic amps * 24 hours). The result is the total days until 100% discharge of the car battery.
You’ll undoubtedly have a battery rating that doesn’t match an exact value in this chart, so you’ll have to estimate within the values that make up the range for your given battery and the amps being drawn on that range.
How Fast a Car Battery Will Drain if the Terminals are Disconnected
It’s easy to think that if you disconnect your car battery that you can avoid the parasitic draw and preserve your car battery’s health, but that’s not the case at all.
Car batteries, which are lead-acid batteries, are just like any other battery in that they undergo self-discharge. What this means is that the chemical reaction inside of the battery reverses itself from a full charge to a state of discharge slowly over time as the result of not being connected to a float charger which keeps the battery topped off at 100%.
Luckily, lead-acid batteries have a fairly slow self-discharge rate and you’ll lose about 4-5% of your overall charge on a monthly basis.
Heat will increase the rate of self-discharge as it lowers the internal resistance within the battery, so if you store it in a 90 degree Fahrenheit garage your battery may lose more than the 4-5%.
If you plan on disconnecting your car battery for any reason such as to go on vacation, or maybe you just don’t use a certain vehicle throughout the winter, the best a way to keep your battery 100% peak shape is to hook up a smart charger that has a float mode that will keep your battery in optimum health while not in use.
I recommend this charger seen on Amazon, which is excellent for car, marine, and golf cart batteries, and I have used this one on a daily basis for over 5 years without a hiccup on my battery bank and for quarterly car charges as well. It’s extremely user-friendly and is simply a set it and forget it type of charger. Hook it up, walk away, and you can come back and three months or more and your battery will still be in perfect condition.