2021 Subaru Forester Battery Replacement Guide

Quick Answer

• BCI Group Size: 25
• CCA Rating: 600-640 CCA (stock)
• Reserve Capacity: 100-110 RC
• AGM vs. Flooded: Flooded (AGM upgrade recommended for enhanced performance and longevity)
• Terminal Layout: Top Post (positive on the right)
• Physical Dimensions: (Approximate) 9.06" L x 6.81" W x 8.86" H
• Dual-Battery Setup: Not applicable (gasoline vehicle)

Replacement Battery Options

Finding the right battery for your 2021 Subaru Forester is crucial for reliable starting and powering its numerous electronic systems. While exact part numbers can vary by retailer and specific models, here are some popular options you might consider:

• Interstate: MTZ-25 (AGM), MTP-25 (Flooded)
• Optima: DS46B24R (YellowTop, Group 35, often used as an upgrade with minor tray modification), D35 (YellowTop, Group 35)
• DieHard: Platinum AGM Group 25, Gold Group 25 (Flooded)
• Odyssey: 25-660 (AGM)
• ACDelco: 25AGM (AGM), 25APG (Flooded)

Note: Always double-check the BCI group size, dimensions, and terminal layout to ensure proper fitment and compatibility with your vehicle before purchasing.

AGM vs. Flooded – Which Does This Vehicle Need?

Your 2021 Subaru Forester originally came equipped with a flooded lead-acid battery. These are the traditional, less expensive option and perform adequately under normal conditions. However, newer vehicles like the Forester, with their advanced infotainment systems, safety features (like EyeSight), and numerous electronic modules, place a higher demand on the electrical system.

AGM (Absorbed Glass Mat) batteries are a superior choice for the 2021 Forester for several reasons:

• Improved Durability: AGM batteries are more resistant to vibration and impacts, making them ideal for varied driving conditions.
• Longer Lifespan: On average, AGM batteries last longer than flooded batteries, especially in demanding applications.
• Better Performance: They offer a more consistent power delivery, which is beneficial for vehicles with high electrical loads. They also have a lower self-discharge rate, meaning they hold a charge longer when the vehicle is not in use.
• Maintenance-Free: Unlike flooded batteries, AGMs are sealed and do not require water top-offs.
• Safer: They are spill-proof and produce less hydrogen gas during charging.

While an AGM battery represents a higher upfront cost, its benefits in terms of reliability, performance, and longevity often outweigh the initial expense, making it a highly recommended upgrade for your Forester.

Post-Installation Relearn / Reset Procedure

After replacing the battery in your 2021 Subaru Forester, it's common to experience some minor issues or warning lights due to the temporary loss of power to the vehicle's computer systems. Follow these general steps for a proper post-installation reset:

1. Secure the Battery: Ensure the new battery is securely mounted and the terminals are clean and tightly connected.
2. Ignition Cycle: Turn the ignition key to the ON position (without starting the engine) for about 10-15 seconds. Then turn it OFF. Repeat this once or twice.
3. Start the Engine: Start the engine and let it idle for 10-15 minutes without touching the accelerator. This allows the engine control unit (ECU) to re-learn idle parameters.
4. Drive Cycle: Drive the vehicle for at least 15-20 minutes, including some varied driving conditions (city and highway) if possible. This helps various systems, including the transmission and emissions monitors, to recalibrate.
5. Address Warning Lights:
   • EyeSight System: The EyeSight system may temporarily disable or show a warning light. It often re-calibrates itself after a short drive. If not, consult your owner's manual for specific EyeSight reset procedures or visit a Subaru dealership.
   • Power Window Auto Up/Down: You might need to reset the auto up/down function for your power windows. To do this, fully lower each window, then fully raise it and hold the button for a few seconds after it's closed.
   • Radio/Navigation: You may need to re-enter your radio presets and navigation settings.

If any persistent warning lights remain after these steps, it's advisable to have your vehicle inspected by a qualified technician.

Signs of a Failing Battery

Don't wait for a complete battery failure to replace it. Here are common signs that your 2021 Subaru Forester's battery might be on its way out:

• Slow Engine Crank: The most common sign. The engine turns over slowly, or "grinds" before starting.
• Dim Headlights/Interior Lights: When the engine is off, or during cranking, lights appear dim.
• Warning Lights: A battery warning light on your dashboard, or other unusual electrical system warnings.
• Corrosion on Terminals: White or blue powdery substance around the battery terminals can indicate a poor connection or a leaking battery.
• Swollen Battery Case: If the battery case appears bloated or swollen, especially on the sides, it's a sign of internal damage and overheating.
• Electrical Gremlins: Unexplained issues with electronics, such as intermittent radio power, erratic dashboard lights, or problematic power windows.
• Constant Jump-Starts: Needing frequent jump-starts is a clear indicator of a battery that can no longer hold a charge.

Common Battery Issues for This Generation

While the 2021 Subaru Forester is a reliable vehicle, its battery system can present some specific considerations:

• High Electrical Demand: Modern Foresters are packed with electronics, from the Starlink infotainment system to the EyeSight safety suite. These systems draw significant power, especially when the vehicle is off (parasitic drain), which can put a strain on the battery, particularly in colder climates or if the vehicle is frequently driven short distances.
• OEM Battery Lifespan: The original equipment manufacturer (OEM) batteries often have a lifespan of 3-5 years, depending on climate and driving habits. Some owners report needing replacement sooner.
• Cold Weather Performance: Like all batteries, the Forester's battery performance degrades in cold weather, reducing available CCA. If you live in a region with harsh winters, a higher CCA rated battery or an AGM battery is highly recommended.
• i-Stop (Auto Start-Stop) System: Vehicles equipped with the i-Stop system (which shuts off the engine at idle to save fuel) place an even greater demand on the battery. These vehicles often come with Enhanced Flooded Batteries (EFBs) or AGMs from the factory and absolutely require a compatible EFB or AGM replacement to function correctly and prevent premature battery failure.

Regular battery testing, especially as your vehicle approaches its third year, can help prevent unexpected breakdowns.

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