What Happens If You Put Smaller or Under-Sized Battery In Your Car?

Many people want to replace their faulty or damaged batteries with smaller car batteries they have lying around. However, there are some things you need to know about putting an undersized battery in your car before you do that. 

Typically, if you put a very undersized battery in your car, the car won’t even start. Even if it does, the battery will drain rapidly, the battery may affect the electrical performance of your car, and it may also damage the alternator or even the battery itself!

So, there are many downsides to using an undersized battery in your vehicle, both performance and safety-wise. However, we have provided an exact guideline to follow to find the right size battery for your car. Read below to learn more!

What Happens If You Put An Under-Sized Battery In Your Car

What Should Be Considered Under-Sized Battery In Your Car?

First of all, we need to clear out that we are all on the same page here and define the term undersized. 

Usually, battery size can refer to the physical size of the battery and the capacity of the battery. Let’s discuss a few things here!

Physical Size 

The actual physical size of the battery matters because modern cars are designed with a battery box, which is designed to fit a specific model and size. 

So, if you buy a very small-sized or too big-sized battery, the battery will not fit your car! You will probably find the battery to fit the battery box properly or not even be able to strap the battery to the vehicle. 

This can cause many safety issues and can be very dangerous. So, You should take the physical size of the battery very seriously. This physical size is referred to as the “Group Number” of the battery. 

The group number tells you the battery’s width, height, and length. This way, you can look for a battery with the exact dimensions that your car fits. 

Battery Capacity

Battery capacity is usually measured in two different systems. One is Cold Cranking Amp or CAA, and another is Reserve Capacity. These are measures to calculate the battery’s electrical resources.

Cold Cranking Amp: CAA refers to the number of Amps available for 30 seconds at 0°F. The temperature is important because batteries behave differently at different temperatures. 

Reserve Capacity: Reserve Capacity refers to the number of minutes the battery can deliver 25 Amps to the car at 80°F. 

Both CCA and reserve capacity express the same thing but in different languages. So, you can consider a battery undersized if the actual physical size is smaller than the battery box or if the battery capacity is lower than the original batteries designed for the car.

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Problems Of Using Smaller or Under-Sized Battery In A Car

We don’t recommend using an undersized battery in your car. Usually, you can’t fit a physically undersized battery properly in the battery box of your vehicle. 

Using a battery without a proper fit can be very dangerous because you do not want a loose battery flopping around in the battery box and leaking all over or possibly even causing a fire!

However, even if you can fit the battery, it will probably not be enough to run your vehicle properly. Either the car won’t start at all, or some critical electrical components won’t work if it does.

Because a small battery will drain rapidly from your car accessories, even simple tasks like cranking the engine or running the radio can drain the battery!

If you’re using your vehicle regularly daily, the battery won’t have enough time to charge properly. A larger battery can withstand that pressure, but an undersized battery won’t be able to take that toll. 

So, an undersized battery will affect the performance and safety of your vehicle. 

Effects on Performance

Batteries power modern cars’ alternators, fuel injection systems, and many other electronic components. So, an undersized battery will run all these components on low electricity.

The car may not start because the battery cranks the cold engine and heats the fuel, and even if the car starts, all the electronic components will barely be working! So, the effects can be listed as follows:

  • The engine is not starting or is very slow cranking
  • Damaging electronic components
  • Bad fuel injection performance
  • Battery draining very fast
  • Very bad performance at a colder temperature

Effects on Safety

Modern cars have a full built-in electronic system that takes care of most of the behind-the-scenes work in your vehicle. So, using an undersized battery will affect these systems in your vehicle. 

Unfortunately, this also includes your safety! The battery powers the safety components in your vehicle. Such as

  • Oil Change Indicators
  • Anti-theft Systems,
  • Tire Pressure Indicators.

So, the effects on safety can be huge. Also, an undersized battery will probably toss around the battery box. This can damage the battery and the box itself, cause an electrical shortage, and catch fire!

So, we suggest using the recommended battery size in your vehicle’s owner’s manual. Do not compromise your safety for a better price!

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How To Find The Right Battery For Your Car?

Batteries are similar to tires because there’s a lot of code that doesn’t make much sense, and there are a lot of choices. So, choosing the right battery can be confusing if you don’t know exactly what you’re doing. 

However, there are some key things to remember when buying a battery to find the most suitable one for your car. 


On almost every car today, 12V batteries are the standard. However, very older vehicles used to have 6V batteries. 

But you’re probably looking for a 12V battery. 


The battery compartment on most modern cars doesn’t have much space for batteries, so choosing the correct size group is critical. However, batteries come in a variety of sizes. 

Typically, a number is used to represent the size group. Below is a BCI battery size chart (all measurements are in inches): 

BCI GROUP SIZELengthWidthHeight
2410 1/46 13/168 7/8
24F10 3/46 13/169
2712 1/166 13/168 7/8
3410 1/46 13/167 7/8
359 1/166 7/88 7/8
H6 (48)12 1/166 7/87 9/16
H8 (49)156 7/87 3/16
6512 1/167 1/27 9/16
7810 1/47 1/167 11/16

The size group contains the battery’s overall measurements. It will probably be challenging to close the car’s hood or battery cover if it is too tall, wide, or lengthy to fit in the battery compartment. 

Each battery should have its BCI group number on the battery. So, check your older battery for the group number. Then, you won’t have to measure the battery or the compartment. 

Verify the right specs a second time before buying a battery. Also, you can find the complete BCI Battery Size Chart here.

Terminal Position

There is a positive and negative terminal on every battery. They need to be placed correctly depending on the cable design of your car. The terminals can be on the top or the side. 

Also, the terminals can be on the top but have their polarity reversed. So, if you buy the wrong battery, the terminals might be in the wrong place or the right geography but flipped positive to negative. 

The problem is that the cables won’t reach the terminals if the position isn’t correct. The BCI number on the battery also specifies the terminal position of the battery. So, that should be your easy way to understand which battery you need. 

Cold-Cranking Amps

It would help if you thought Cold-Cranking Amps as the power the battery can offer the car. So, the battery can have very high CCA, but it won’t affect the car at all because the car will only take as much as it needs. 

That’s why buying a battery with more CCA than your original won’t cause problems if you can fit the battery properly in your battery compartment. 

However, having a low CCA will result in safety and performance issues, as we mentioned earlier. 

Also, you should check the warranty and price of the battery compared to competitor brands before buying any car battery. These should last you a long time before you need any changing!

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As we discussed the problems you might face if you put an under-sized battery in your car, our suggestion would be to not use an under-sized car battery for the long term. 

Long-term usage can damage the electrical components of your car to such an extent that you may have to change more parts later! Also, an undersized battery will be a headache whenever you try to start your car!

So, buy the right size battery for your car and properly maintain the vehicle for optimum performance and safety!

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