Optima batteries come in a variety of sizes; there isn't a single Optima size. They are built to standard BCI group sizes that align with automotive, marine, and deep-cycle applications.
In practice, Optima offers several lines—RedTop for starting power, YellowTop for deep-cycle reliability, and BlueTop for marine use—spanning multiple group sizes such as 34/78, 75, 24/24F, 31, and 24M, among others. To find the right fit, you must match the BCI group size printed on the battery you replace or consult Optima's fitment guides. Here’s a guide to sizes and how to identify them.
Optima sizing landscape
Optima uses BCI group numbers to identify fit, rather than a single universal size. Below are some of the common group sizes used across Optima's automotive, deep-cycle, and marine lines.
Key sizing terms you should know
BCI group size is a standardized code that encodes approximate dimensions, terminal locations, and fitment. Most Optima batteries state their group size on the label alongside voltage and cranking/amperage specs.
- Group 34/78: a popular automotive starting option often found in RedTop batteries; fits many mid-size cars and some SUVs.
- Group 75: a larger option used for heavier vehicles or where a bit more reserve is desired; available in some RedTop and YellowTop models.
- Group 24/24F: an older but still common choice for certain cars and some deep-cycle configurations; used in some YellowTop or BlueTop variants.
- Group 31: a larger marine and heavy-duty group; seen in BlueTop marine batteries and some trucks.
In short, you should verify the exact group size indicated on your battery or consult the fitment guide to ensure compatibility.
How to identify the right size for your vehicle
To ensure you pick the correct Optima size for your car, boat, or equipment, follow these steps to verify the fit and compatibility.
- Check the BCI group size printed on the current battery label or in the owner's manual.
- Measure the available space (length, width, height) and compare with the dimensions of the Optima group you’re considering.
- Match terminal configuration (top-post or side-post) and terminal orientation to your vehicle’s cables and mounting.
- Use Optima's official product finder by entering your make, model, and year to retrieve a recommended group size.
By confirming the BCI group and physical fit, you reduce the chance of ordering a battery that won’t fit or connect properly.
Summary
Optima does not have a single size. The brand offers multiple sizes across its starting, deep-cycle, and marine lines, all mapped to standard BCI group numbers. To ensure you get the right fit, identify the current battery’s BCI group size and match it to Optima’s catalog or use the official fitment tools.
What group size are Optima batteries?
Optima Group 34M Starting Battery
See if you qualify at checkout.
Are group 34 and 78 the same?
However, BCI group sizes can be very confusing, including the Group 34/78 battery size. The "34" number refers to a battery of specific dimensions with top terminals, while a "78" number also refers to a battery of specific dimensions with side terminals.
Is a group 27 battery bigger than a group 24?
Group 27 batteries are typical in size, but larger than Group 24 batteries. The dimensions are 12.5" L x 6.75" W x 9.37" H. These batteries are commonly used in RVs, boats, off-grid systems, and backup power systems.
What kills batteries faster?
Than the wire. Charging. As we saw before heat accelerates your battery aging the latest independent tests show wireless charging can degrade your battery up to 30% faster than the wire charging.