Are 51 and 51R the Same Battery?

No—Group 51 and Group 51R batteries are the same size but have opposite terminal orientation. Both share near-identical dimensions and similar performance ranges, but the “R” indicates reversed polarity: the positive and negative posts are swapped left-to-right compared with the standard 51. Choosing the wrong one can make your cables not reach, force unsafe cable routing, or risk reverse-polarity hookup.

What the Designations Mean

“51” and “51R” are BCI (Battery Council International) group sizes. The group number specifies the physical footprint and general case height so the battery fits the tray and hold‑down. The letter “R” stands for reversed terminal orientation relative to the standard layout for that group.

Dimensions and Form Factor

Both 51 and 51R typically measure about 9.37 in (238 mm) long, 5.06 in (129 mm) wide, and about 8.7–8.9 in (≈223–226 mm) tall, depending on brand and terminal style. Because the case size and hold‑down points are the same, either will physically sit in the same tray; what differs is which side the positive post sits on.

Terminal Orientation: The Key Difference

Terminal orientation is defined with the posts closest to you. On a Group 51, the positive (+) post is usually on the left; on a Group 51R, the positive post is on the right. That reversal is critical because vehicle battery cables are cut to length and routed to match the OEM layout.

The points below summarize how 51 and 51R compare at a glance.

• Physical size: Same case footprint and height.
• Terminal layout: 51 = positive on left; 51R = positive on right (with posts facing you).
• Electrical performance: Similar ranges; “R” does not imply different CCA or capacity.
• Fitment impact: Cable reach and routing depend on getting the correct orientation.

In practice, this means most cars that call for 51R specifically need that orientation to avoid cable strain or crossing; the reverse is true for cars designed for 51.

Compatibility and Interchangeability

While a 51 and 51R will both fit the same tray, they are not universally interchangeable because of cable length, routing, and safety considerations.

Consider the following before swapping one for the other:

• Cable reach and routing: If swapping to the opposite orientation forces cables to stretch, cross, or contact metal parts, it’s a no-go.
• Clamp size: Some vehicles use smaller JIS-style terminal clamps; many aftermarket 51/51R batteries use standard SAE posts. Verify clamp size or use proper adapters.
• Hold-down and height: Ensure the top or bottom hold-down fits and the hood will close without contacting terminals.
• Venting: In trunks or cabins, use a vented (often AGM) battery with the correct vent fittings.
• Warranty and spec: Match or exceed OEM cold-cranking amps (CCA) and reserve capacity (RC).

If your vehicle specifies 51R, use 51R. A 51 may work only if cable length and routing remain safe—with no tension, crossing, or risk of shorting. When in doubt, follow the manufacturer’s specified group and polarity.

Typical Applications

Group 51R is common in many Honda models (e.g., Civic, Fit, CR‑V and others depending on year) and appears in some Toyota and Nissan applications. Group 51 (non‑R) shows up in vehicles whose cable routing places the positive lead on the opposite side. Always verify by checking your current battery’s label and the owner’s manual or fitment guide—some models also require JIS terminal sizes or AGM/vented designs.

How to Choose the Right One

Use this quick check to ensure you buy the correct battery without fitment surprises.

1. Check your owner’s manual or OEM label for the specified group (51 or 51R) and any AGM/vent requirements.
2. Confirm terminal orientation by looking at your existing battery with posts facing you (note which side is positive).
3. Measure the tray and hold-down; verify the case size and height will fit.
4. Inspect cable length and routing; ensure the specified orientation keeps cables relaxed and uncrossed.
5. Verify terminal clamp size (SAE vs JIS) and get adapters if needed.
6. Match or exceed CCA and RC ratings recommended for your climate and vehicle.

Completing these checks minimizes installation issues and helps you avoid polarity mistakes or cable strain.

Common Misconceptions

These notes clear up frequent points of confusion about 51 vs. 51R batteries.

• “R” does not mean “right-hand drive”; it means reversed terminal orientation.
• “R” does not indicate higher or lower performance; CCA and RC are brand/model specific.
• Physical fit does not guarantee safe electrical fit; cable routing and polarity matter.

Keeping these distinctions in mind will help you focus on the specs that actually affect compatibility and safety.

Summary

Group 51 and 51R batteries are the same size, but their terminals are mirror-imaged. Choose the group that matches your vehicle’s specified polarity layout—51 for positive on the left, 51R for positive on the right (with posts facing you). While performance ratings vary by brand, the “R” only denotes reversed orientation, and using the correct one ensures proper cable reach, safe routing, and trouble-free installation.

What size is a 51R battery?

A 51R battery has standard dimensions of approximately 9.4 inches long x 5.1 inches wide x 8.8 inches high, with the “R” indicating the terminal post orientation. Other specifications vary by brand and model, but generally include 12-volt power, a range of Cold Cranking Amps (CCA) from roughly 400 to 500, and a positive terminal on the right side. 
Typical Dimensions 

• Length: ~9.4 inches (9 3/8″)
• Width: ~5.1 inches (5″)
• Height: ~8.8 inches (8 3/4″)

Key Characteristics 

• BCI Group Size: 51R
• Voltage: 12V
• Terminal Type: Top Post
• Terminal Orientation: Right-hand positive terminal

Important Considerations

• “R” Designation: The “R” signifies that the positive terminal is on the right side when viewed from the front of the battery, which is crucial for correct installation in a vehicle. 
• Varying Specs: While dimensions are standardized, features like Cold Cranking Amps (CCA), Cranking Amps (CA), and Reserve Capacity (RC) will vary depending on the specific battery’s brand and technology (e.g., flooded, AGM). 
• Vehicle Compatibility: This battery is designed for a wide range of vehicles with limited space, but you should always verify the battery size against your vehicle’s specifications for a proper fit. 

What is the difference between Group 51 and 51R battery?

The primary difference between a Group 51 and a 51R battery lies in the terminal orientation (positive and negative terminals). Group 51R batteries have the terminals reversed compared to Group 51 batteries. Both battery types are otherwise similar in size and physical dimensions. 
Key Differences:

• Terminal Orientation: The “R” in 51R signifies “reverse,” meaning the positive and negative terminals are positioned opposite to those on a standard Group 51 battery. 
• Application: The choice between the two often depends on the specific vehicle or application. Some vehicles are designed for one type or the other. 
• Compatibility: While the dimensions are similar, the reversed terminals can make one battery incompatible with a setup designed for the other. 
• Examples: Canadian vehicles like certain Honda, Toyota, Mazda, Nissan, and Subaru models might use either Group 51 or 51R. 

In essence: If you’re replacing a battery, double-check your vehicle’s manual or the existing battery to ensure you get the correct terminal orientation.

What does the R mean in a 51R battery?

The “51R” designation refers to the battery’s size and terminal orientation, with the “R” indicating that the positive terminal is on the right side when the battery is viewed from the front.

What batteries are compatible with 51R?

A Group 51R battery, known for its compact size, primarily fits smaller, compact, and some mid-sized Japanese and American cars and trucks, including many Honda (like the Accord, Civic, CR-V, and Fit), Toyota (Corolla, Prius, Yaris), and Mazda models. It’s also used in some power sports applications and can be a popular upgrade for vehicles with smaller stock batteries, like the Honda Fit.
 
Common Vehicles that use Group 51R Batteries 

• Honda: Accord, Civic, CR-V, Fit
• Toyota: Corolla, Prius, Yaris
• Mazda: Mazda3, Mazda5
• Nissan: Sentra, Versa, Juke
• Subaru: Impreza

Why is it a common choice?

• Compact Size: The 51R battery’s small dimensions make it suitable for the smaller engine compartments found in compact cars. 
• Reliable Performance: It offers dependable power, even in cold weather, making it a reliable choice for various applications. 
• Power Sports: Its consistent power delivery makes it a preferred option for motorcycles and ATVs. 
• Upgrade Option: In some cases, a 51R battery can be used to replace smaller stock batteries (like a Group 151R in a Honda Fit) to provide more power capacity. 

How to be sure it fits your vehicle

• Check your car’s manual: . Opens in new tabYour owner’s manual will list the correct battery group size for your specific vehicle. 
• Use a fitment tool: . Opens in new tabMany auto parts stores offer online tools where you can enter your car’s year and model to find the correct battery.