What Car Battery Do I Need? A Complete Guide to Group Sizes & Fitment
- by Joe Weber - updated on 2/20/2026
Key Takeaways
- One Size Does Not Fit All: Car batteries are categorized by "Group Size" (dimensions and terminal placement). The wrong size often physically won't fit or connect.
- Check for Auxiliary Batteries: Many modern cars (especially with Start-Stop technology) have a second, smaller battery that may also need replacing.
- Don't Downgrade: If your car came with an AGM battery, you must replace it with an AGM battery. Switching to a standard flooded battery can cause early failure.
- Verify Before You Buy: Always confirm fitment using your owner's manual or an online fitment finder, even if you think you know the size.
Every car owner knows that sinking feeling when you turn the key and hear… nothing. When it's time to replace your battery, the options can be overwhelming. Is a Group 24 the same as a 24F? Do you need an AGM battery? And wait, does your car have two batteries?
Picking the wrong battery can lead to poor performance, frequent replacements, or even damage to your car's electrical system. This guide will walk you through exactly how to find the perfect fit for your vehicle.
Understanding Car Battery Group Sizes
Car batteries are not universal. A powerful V8 truck and a compact 4-cylinder hatchback have very different power needs and under-the-hood space.
Batteries are categorized by the Battery Council International (BCI) group size. This alphanumeric code (like Group 35 or Group 65) ensures that the battery will physically fit in your battery tray and that the positive and negative terminals are in the correct position for your cables to reach.
Car Battery Group Size Chart
Note: This chart is for reference only. Manufacturers change specifications often. Always confirm the correct battery size with our experts or use our fitment finder.
| Group Size | Commonly Seen In |
|---|---|
| 24/24F (Top Terminal) | Acura, Honda, Infiniti, Lexus, Nissan, Toyota |
| 35 (Top Terminal) | Honda, Nissan, Subaru, Toyota |
| 65 (Top Terminal) | Ford, Lincoln, Mercury |
| 48 (Top Terminal) | Acura, BMW, Cadillac, Chevy, Ford |
| 75 (Side Terminal) | Chrysler, Dodge, GM |
| 34/78 (Dual Terminal) | Chrysler, Dodge, GM |
How to Determine Which Car Battery Fits Your Vehicle
Finding the right battery is easier than you think if you know where to look. Here are three reliable ways to identify the correct fit:
- Check the Owner's Manual: The most accurate source. The battery section will list the recommended BCI Group Size and minimum Cold Cranking Amps (CCA).
- Look at the Existing Battery: The Group Size is almost always printed on the top or front label of your current battery. (e.g., you might see "Group 24F" or "H6/Group 48").
- Use Online Tools: You don't have to guess. The Batteries Plus Fitment Finder allows you to select your Year, Make, Model, and Trim to see every compatible option instantly.
What About Vehicle Auxiliary Batteries?
If you drive a modern vehicle—especially one made in the last 10 years—you might be surprised to learn you may have two batteries.
What is an Auxiliary Battery?
An auxiliary battery is a secondary 12-volt battery that works alongside your main starter battery. While the main battery cranks the engine, the auxiliary battery powers safety systems, onboard computers, and vehicle accessories.
Does My Car Have One?
Auxiliary batteries are most common in:
- Vehicles with Start-Stop Technology: When your engine shuts off at a red light, the auxiliary battery keeps your radio, AC, and lights running until the engine restarts.
- European Vehicles: Brands like Mercedes-Benz, BMW, and Volvo frequently use them.
- Hybrids & EVs: A 12V auxiliary battery is used to power the dashboard and safety systems (distinct from the massive high-voltage battery that drives the wheels).
- Modern Domestic SUVs: Newer Jeep, Chrysler, and Ford models often utilize them to manage high electrical loads.
Signs Your Auxiliary Battery is Failing
Because this battery doesn't start the engine, a "slow crank" isn't usually the symptom. Instead, look for:
- Dashboard Warning Lights: Messages like "Auxiliary Battery Malfunction" or a battery icon with a seemingly healthy main battery.
- Start-Stop Not Working: If your car no longer shuts off at stoplights, the system may have disabled the feature because the aux battery is weak.
- Electrical Glitches: Non-essential electronics acting strangely.
Pro Tip: If you replace your main battery but still have warning lights, check your auxiliary battery. They often have a similar lifespan (3-5 years) and are frequently hidden in the trunk, under a seat, or behind a fender panel.
Cold Cranking Amps (CCA): Why They Matter
Size isn't the only factor; power matters too. Cold Cranking Amps (CCA) measure a battery's ability to start an engine in freezing temperatures.
It is important to note that one Group Size can have multiple CCA options. For example, looking at a shelf of Group 24F batteries, you might see one rated at 650 CCA and another at 750 CCA.
Why the difference? While they share the same physical dimensions, the internal chemistry and design can vary significantly. Higher CCA options generally provide more reliable starts in extreme winter weather and handle heavy electronic loads better.
So, how do you choose between the 600 CCA and the 750 CCA option? That usually comes down to the Battery Design…
Types of Car Battery Designs: Flooded vs. AGM
Once you know the size, you must choose the technology.
- Flooded (Standard) Batteries: The traditional choice. Reliable and affordable, but they require regular maintenance and have shorter lifespans in extreme heat or high-accessory vehicles.
- Absorbed Glass Mat (AGM) Batteries: Designed for modern cars with heated seats, navigation, and Start-Stop tech. They are spill-proof, vibrate less, and last longer.
- Thin Plate Pure Lead (TPPL) AGM: The premium choice (like our X2Power line). These offer the highest CCAs and fastest recharging for enthusiasts or vehicles with heavy electrical demands (winches, erratic driving schedules).
Crucial Rule: If your car came from the factory with an AGM battery, you must replace it with an AGM battery. Downgrading to a flooded battery will confuse your car's charging system and lead to rapid battery failure.
Get the Best Car Battery at Batteries Plus
Still not sure what you need? Visit your local Batteries Plus store. Our experts can test your main and auxiliary batteries for free, find the exact fit, and install it for you.
Shop Car & Truck Batteries Find your nearest store
Frequently Asked Questions (FAQ)
Q: Will using the wrong size battery void my warranty?
A: It can. If you install an incorrect battery size that causes damage (like frying the alternator or shorting the electrical system), the vehicle manufacturer can deny warranty claims for those specific repairs. Additionally, using a battery in an application it wasn't designed for often voids the battery's own warranty. It is always safer to stick to the manufacturer's recommended Group Size.
Q: How long does a car battery typically last?
A: Most car batteries last between 3 to 5 years. Extreme heat actually damages batteries more than cold, so if you live in a hot climate, you might be closer to the 3-year mark.
Q: Can I install a different Group Size if it fits?
A: Generally, no. Even if a battery fits in the tray, it might be too tall (hitting the hood) or have terminals in the wrong spot, stretching your cables. Always stick to the Group Size recommended for your vehicle.
Q: Do I need to register my new battery?
A: For many newer vehicles (BMW, Audi, Volkswagen, etc.), yes. Battery Registration tells the car's computer that a new battery has been installed so it can reset the charging strategy. Skipping this can shorten the life of your new battery.
Q: My car starts fine, but my Start-Stop feature stopped working. Is it the battery?
A: It is very likely the battery. The vehicle's computer will automatically disable Start-Stop to save power if it detects the battery (main or auxiliary) is weak. A simple battery test can confirm this.