2015 Tesla Model S — Battery Size & Group (12V Auxiliary)

Quick answer

• BCI group size: Group 51R (JIS 51R) AGM auxiliary battery
• Cold Cranking Amps (CCA): About 450 CCA minimum; 450–500 CCA is typical
• Reserve capacity (RC): Typically 60–75 minutes for quality AGM 51R batteries
• Type required: AGM (Absorbent Glass Mat); do not use flooded lead‑acid
• Terminal layout: Top posts, positive on right (when posts are closest to you) = “R” polarity
• Physical dimensions (approx.): 9.4" L × 5.1" W × 8.7–8.9" H (238 × 129 × 222–227 mm)
• Dual‑battery setup (diesel): Not applicable — single 12V auxiliary; high‑voltage traction battery is separate

Note: Tesla used an AGM 12V battery in the frunk. It does not crank an engine but powers computers, contactors, pumps, and locks. Specs above are the common/accepted fit for 2012–2016 Model S; always confirm by VIN or by reading the label on your existing battery and bracket.

Replacement battery options

• Interstate MTX‑51R (AGM) — Direct‑fit 51R AGM with ventless design and ~450 CCA
• Optima YellowTop D51R (AGM) — Deep‑cycle AGM, popular upgrade in EV/hybrid applications
• DieHard Platinum AGM — Group 51R (verify fitment by VIN/trim)
• ACDelco Gold AGM — Group 51R (verify fitment; AGM only)
• Odyssey Performance AGM — Group 51R (check dimensions/terminals against your tray and cables)

If your car has been retrofitted with an alternate tray/bracket, some owners use H5/Group 47 AGM batteries. Unless you’ve specifically converted hardware, stick with the OE‑style 51R AGM for proper cable reach and hold‑down fit.

AGM vs flooded — which does this vehicle need?

Use AGM only. Tesla engineered the Model S charging profile and packaging for an AGM auxiliary battery. AGM is sealed, spill‑resistant, and handles frequent cycling better than a flooded battery — important in an EV where the DC‑DC converter tops up the 12V repeatedly. A flooded battery can vent acid mist, doesn’t tolerate cycling as well, and may fail prematurely or leak in the frunk environment. Lithium 12V conversions exist, but Tesla did not equip 2015 Model S with a lithium 12V; if you choose aftermarket lithium, ensure it is Tesla‑compatible and provides correct low‑temperature protection and charge acceptance.

Post-installation relearn / reset procedure

After replacing the 12V battery:

1. Wake and power down correctly
   • With doors open and frunk accessible, go to Controls > Safety > Power Off. Wait 2–3 minutes with the car undisturbed (no door handles, pedals, or screens touched).
2. Install the battery
   • Replace the 12V battery, ensure tight/clean terminals (positive first on install, negative last), and secure the hold‑down. Reconnect any frunk service covers.
3. Reboot the car
   • Perform a soft reboot: Press and hold both steering‑wheel scroll wheels until the center screen goes black and the Tesla “T” reappears (about 10–20 seconds). If equipped with a separate instrument cluster, you can also do a top‑button reboot.
4. Let systems initialize
   • Allow a few minutes for the car’s modules to re‑establish communication. Most warnings clear on their own after a short drive.
5. Window/charge‑port calibration (if needed)
   • If auto‑up windows misbehave, fully lower and raise each window and hold the switch up for 2–3 seconds to relearn limits. The charge port door and mirrors typically self‑calibrate.
6. Drive to finalize
   • A short, straight drive will complete steering angle/ABS calibrations. Any “12V battery” or “System powering up” messages should clear.

Signs of a failing battery

• “12V battery needs service/replacement” message on the screen or app
• Slow boot of the center display after opening doors; repeated reboots
• Contactors clicking repeatedly on wake‑up, or HVAC/fans momentarily starting and stopping
• Intermittent errors for systems that rely on 12V (suspension, ABS, power steering) that vanish after driving
• Visible swelling/corrosion at terminals or a battery older than 3–4 years

Common battery issues for this generation

• High cycling load: Early Model S vehicles cycled the 12V frequently due to always‑on telematics and accessory loads. Firmware updates improved this, but many early cars saw 12V replacement every 2–4 years.
• DC‑DC converter or wiring faults: A weak DC‑DC converter, poor grounds, or corroded connections can undercharge the 12V and cause repeated failures. If you replace the 12V and warnings return quickly, diagnose charging voltage and grounds.
• Storage and infrequent use: Extended parking with low state of charge on the high‑voltage pack can leave the 12V under‑supported. Keep the car plugged in when parked long‑term so the DC‑DC can maintain the 12V.
• Water intrusion: Damaged frunk seals or incorrect cover placement after service can allow moisture near the battery. Ensure covers and seals are seated to protect the battery and electronics.
• Non‑AGM installs: Using a flooded battery often leads to early failure and potential venting in the frunk. Stick with AGM 51R or a vetted lithium replacement.

Find a trusted shop on Repairius.