Best Group 31 AGM Cranking Batteries for Cummins ISB 6.7 Diesel RV

What's Going On

For your 2017 RED with the 6.7 ISB Cummins engine, both Duracell and NAPA Group 31 cranking batteries are solid choices that will handle your diesel's high cranking demands. The Duracell Ultra batteries (typically made by East Penn/Deka) offer excellent cold cranking amps around 925-1000 CCA and come with a solid 3-year free replacement warranty. NAPA's Group 31 batteries are usually manufactured by Johnson Controls or East Penn and provide similar performance specs with competitive pricing, often running $20-40 less than premium brands while delivering 900-950 CCA.

Many RV owners with similar Cummins setups report great success with both brands, though the deciding factor often comes down to local availability and warranty service. Your 6.7 ISB requires substantial cranking power, especially in cold weather, so either choice will be a significant upgrade if you're replacing older batteries. The key is ensuring you get true cranking batteries designed for diesel engines, not marine/RV combo batteries that compromise on cranking performance.

Why This Happens

Group 31 cranking batteries in diesel motorhomes typically need replacement every 4-6 years due to the extreme demands of starting a large diesel engine. Your 2017 RED's original batteries are likely showing their age through slower cranking, especially in cold weather, or difficulty starting after the coach has sat for extended periods. The 6.7 ISB Cummins requires 850+ cold cranking amps to start reliably, and as batteries age, their internal resistance increases while their capacity decreases.

The constant cycling between charging from the alternator during driving and discharging during starting gradually breaks down the lead plates inside the batteries. Additionally, if your coach sits unused for months, sulfation builds up on the plates, further reducing capacity. Heat from engine compartment exposure also accelerates the chemical breakdown of the electrolyte and internal components.

Choosing quality batteries like Duracell or NAPA helps because they use thicker lead plates, better separators, and more robust construction compared to bargain brands. This translates to better vibration resistance, longer life, and more consistent performance in the demanding RV environment where batteries face constant movement and temperature extremes.

Getting Ready

Before purchasing your new Group 31 batteries, measure your current battery compartment dimensions to confirm fitit. Group 31 batteries are standardized at approximately 13 x 6.81 x 9.44 inches, but slight variations exist between manufacturers. Check your existing batteries for any custom mounting brackets or hold-down systems that might affect clearance. Also verify the terminal configuration - most Group 31 batteries have top-post terminals, but some coaches use side-post connections.

Gather the necessary tools including a 10mm and 13mm socket set, battery terminal cleaner or wire brush, dielectric grease, and safety equipment like gloves and eye protection. Purchase battery terminal protectant spray and consider getting new battery cables if your current ones show corrosion or damage. You'll also want a digital multimeter to test the new batteries after installation and verify proper charging system operation.

Plan for proper disposal of your old batteries - most auto parts stores accept them for recycling and may offer a core charge credit toward your new purchase. Schedule the replacement when you have at least 2-3 hours available, as you'll want to properly clean all connections and test the charging system. Consider doing this work when the coach is parked on level ground with easy access to the battery compartment.

Walking Through the Fix

Start by disconnecting shore power and turning off the battery disconnect switch if equipped. Always remove the negative terminal first, then the positive terminal to prevent sparking. Remove the old batteries and thoroughly clean the battery compartment, removing any corrosion buildup with a baking soda solution, then neutralizing and thoroughly rinsing the area. Clean all cable terminals with a wire brush until they're bright and shiny - corroded connections can cause voltage drop and starting problems.

Install the new batteries in the same positions as the old ones, ensuring they're properly secured with hold-down brackets. Connect the positive terminals first, then the negative terminals, applying dielectric grease to prevent future corrosion. Torque the terminal bolts to 6-8 ft-lbs - overtightening can crack the terminal posts. Verify all connections are tight and properly seated before proceeding.

Test the installation by starting the engine and checking that it cranks normally and starts quickly. Use your multimeter to verify the charging system is working properly - you should see 13.2-14.4 volts at the batteries with the engine running. Check that all electrical systems function normally, including house systems that may share power with the chassis batteries. Take photos of the final installation for future reference.

Apply terminal protectant spray to all connections and mark your calendar for the installation date. New batteries should be exercised regularly - if you store your RV for extended periods, plan to start it monthly and let it run for 15-20 minutes to keep the batteries properly charged and prevent sulfation.

Beyond DIY Territory

If you discover significant corrosion in the battery cables or wiring harness during your inspection, this indicates a more serious electrical issue that may require professional diagnosis. Corroded cables can create resistance that causes batteries to fail prematurely and may indicate problems with the charging system or inadequate ventilation in the battery compartment.

Any issues with the alternator or charging system should be addressed by a qualified diesel technician familiar with Cummins engines. The 6.7 ISB has specific charging requirements, and improper voltage regulation can damage expensive electrical components throughout the coach. Similarly, if you find evidence of battery compartment flooding or damage, this may indicate seal failures that need professional attention to prevent ongoing problems.

Consider having a professional load test your charging system after installing new batteries, especially if the old batteries failed prematurely. This can identify alternator problems, voltage regulator issues, or parasitic drains that could damage your new investment. A proper electrical system analysis costs $100-150 but can save you hundreds in premature battery replacement and electrical component damage.