2021 GMC Sierra 2500HD Battery: Group Size, CCA, & Replacements

Quick answer

• BCI Group Size: Group 48 (H6) or Group 94R (H7) for gas engines; Group 31 for diesel engines (dual battery setup).
• CCA Rating (Gas): 730 CCA minimum; 800 CCA or higher recommended.
• CCA Rating (Diesel): 900 CCA minimum per battery; 1000 CCA or higher recommended.
• Reserve Capacity (Gas): 120 minutes minimum.
• Reserve Capacity (Diesel): 160 minutes minimum per battery.
• AGM vs. Flooded: Gas models typically come with flooded lead-acid, but AGM is a popular upgrade. Diesel models often come with AGM from the factory, and AGM is strongly recommended for replacement due to higher demands.
• Terminal Layout: Top post, typically SAE (standard).
• Physical Dimensions (Group 48/H6): (L) 10.94 in x (W) 6.88 in x (H) 7.56 in.
• Physical Dimensions (Group 94R/H7): (L) 12.44 in x (W) 6.88 in x (H) 7.56 in.
• Physical Dimensions (Group 31): (L) 13.00 in x (W) 6.88 in x (H) 9.44 in.
• Dual-Battery Setup: All 2021 GMC Sierra 2500HD models with the 6.6L Duramax diesel engine utilize a dual-battery setup (two Group 31 batteries).

Replacement battery options

When it's time to replace the battery in your 2021 GMC Sierra 2500HD, you have several excellent options from reputable brands. Always double-check the BCI group size and CCA rating to ensure compatibility with your specific engine (gas or diesel).

• Interstate (Gas): MTX-48 / MTX-94R (Flooded), MTZ-48 / MTZ-94R (AGM)
• Interstate (Diesel): M-31S (Flooded), M-AGM-31 (AGM)
• Optima (Gas): YellowTop DH6 (AGM), YellowTop DH7 (AGM)
• Odyssey (Gas): ODP-AGM48 H6 (AGM), ODP-AGM94R H7 (AGM)
• Odyssey (Diesel): ODP-AGM31 (AGM)
• DieHard (Gas): Platinum AGM Group 48 / Group 94R (AGM)
• DieHard (Diesel): Platinum AGM Group 31 (AGM)
• ACDelco (Gas): 48AGM / 94RAGM (AGM)
• ACDelco (Diesel): 31AGMS (AGM)

AGM vs. flooded — which does this vehicle need?

For the 2021 GMC Sierra 2500HD, the choice between AGM (Absorbent Glass Mat) and traditional flooded lead-acid batteries depends largely on your engine and usage.

Gasoline Engines (6.6L L8T V8): These models typically come with a flooded lead-acid battery from the factory. A flooded battery is perfectly adequate for most drivers. However, upgrading to an AGM battery offers several advantages: higher resistance to vibration, longer lifespan, better deep-cycle capabilities (useful for accessories or prolonged off-engine power draw), and a lower self-discharge rate. If you live in extreme climates, regularly use accessories with the engine off, or want maximum reliability, an AGM is a worthwhile upgrade.

Diesel Engines (6.6L L5P Duramax V8): The 2021 Sierra 2500HD diesel is known to come equipped with a dual-battery AGM setup from the factory. This isn't just for marketing; the Duramax diesel engine, especially in cold weather, places immense demands on the batteries during starting. The higher cranking power, superior cold-weather performance, and vibration resistance of AGM batteries are crucial for reliable operation. It is highly recommended to stick with AGM batteries for your diesel Sierra 2500HD as a replacement. Using flooded batteries in a diesel application designed for AGM may result in premature failure and unreliable starting, particularly in harsh conditions.

Post-installation relearn / reset procedure

After disconnecting and replacing the battery in your 2021 GMC Sierra 2500HD, it's important to perform a basic relearn or reset procedure. While modern vehicles are quite adaptive, this can help prevent minor issues and ensure all systems function optimally.

1. Open the Driver's Side Window: Before disconnecting power, roll down the driver's side window. This can help if the "express up/down" feature needs to be reset.
2. Connect New Battery: Ensure the terminals are clean and securely fastened (positive first, then negative).
3. Idle Relearn: Start the engine and let it idle for 5–10 minutes without touching the accelerator. This allows the engine control module (ECM) to re-learn idle parameters.
4. Drive Cycle: Drive the vehicle for at least 10–15 minutes under varying conditions (city and highway driving) to allow the transmission control module (TCM) and other systems to re-learn operating parameters.
5. Window Express Up/Down Reset (if needed): If your express up/down windows don