How Long Can A Car Battery Sit Unused?

Whenever leaving on a business trip or vacation or leaving your car in storage for the winter, your battery may sit unused for an extended period of time. In these cases, there is always the question of how to correctly store the car battery, if it’s worth using a smart charger and if it’s stored discharged, how long can a car battery sit unused? In this article, we will answer all these questions to help you store your battery without having to buy a new one when it’s time to use it again.

How long can a car battery hold its charge?

Many car owners ask themselves how many days can a battery keep its charge while being out of the car without the need to be recharged. It is necessary to understand that for the preservation of the battery charge, the temperature at which it will be stored is essential. If it’s stored in subzero temperatures, the battery will discharge much faster than at room temperature. However, it should be taken into account the fact that a discharged battery, being left in the cold for an extended period, will fail, since the density of the electrolyte inside is significantly reduced, which will lead to crystallization and ultimately damage to the plates.

If you store the battery at optimum temperature, the time during which the battery will stay in good working condition will depend on its overall capacity. Natural current leakage of a car battery is about 10 mAh. Given this figure, you can easily calculate how long your battery can be stored without needing to be recharged.

For example, your battery has a capacity of 50 A/h, to find out its permissible discharge, 50 should be divided by 3.3 which gives us 16 A/h – this value will be the minimum at which the battery will be in working condition. Now you can find out how long it will take for the battery to reach this value. To do this, subtract the result from the total capacity and divide this number by 0.01. The formula is as follow;

  • 50 A/h-16 A/h=34 A/h
  • 34 A/h: 0.01 A/h=3400 hours

3400 hours equals 141 days during which your battery will be able to hold the required charge.

If the battery is in the car and connected to its onboard network, this period can be cut down to about half of it. The fact is that components in the circuit keep drawing current even when not in use, contributing to the leakage of the charge. If the car has an alarm system, it’s enough to discharge the battery in around 10 days in the winter, and about 20 days in the summer. Obviously, different types of batteries will react differently. An AGM battery and Deep-Discharge models, for example, can hold the charge significantly longer.

There is also an alternative that can significantly simplify the storage process and extend battery life. To do this, you need a device called a smart battery charger or car maintainer. This small device connects to a regular 12v socket and keeps the battery charged at an optimum level, preventing it from discharging and thereby failing. In this article, we will not consider this option in detail, but we advise you to check this battery maintainers guide on what to look for to choose the best model.

Storage of the battery in winter

In case you store your vehicle for the winter, you’ll need to properly store your car battery if you want it to work again when summer comes around again. One of the best tricks is to store the battery at home, leaving it in a warm closet. If you don’t have enough space to keep it inside, you still need to remove it from the car and plug it to a battery charger until it is fully charged – this will allow the battery to stay in shape a lot longer. Sand the battery post to remove any trace of oxidization and apply a small layer of dielectric grease.

When spring comes back, clean the battery posts with brake cleaner and fully charge the battery.

New battery storage

If your car battery was manufactured a month ago and has never been previously installed, it should be enough to put it on a slow charge for 22-24 hours. After charging, put it in a dry and warm place with low humidity, for example, in a closet. Repeat the charging process every 2 months until you plan to use it again.

Note! The battery must always be stored and kept in the same position in which it stands in the engine compartment. Accidentally flipping over the battery and allowing the transfusion of the electrolyte, can short circuit the battery. In this case, it will poorly hold the charge and will be unsuitable for further operation.

Even a new battery should be periodically charged

If the battery was made two or three years ago and just stood in the warehouse without charging, then the situation is much worse. First, batteries immediately start to drain upon manufacture. Secondly, if the battery stood without charging for a long time (a couple of years), its power falls by about half. Accordingly, you should never buy a battery built more than 1 years ago. Even the worst and weakest Chinese battery will be many times better, more reliable, durable, and qualitative than a brand-name battery, standing two years without a charge.

To test the battery after an extended downtime, you can use special battery testers. But if such a tool is not available, drive your car for about an hour and the battery should be fully recharged. Then turn off the engine and try to drain the battery using the car’s accessories. Turn on the headlights, heated rear window, listen to music, etc. If within half an hour the battery dies out or if the car can’t be started anymore, it means that the battery will need to be replaced.