2020 Tesla Model 3 Battery: Size, Replacement, and Resets

As an electric vehicle (EV), the 2020 Tesla Model 3 does not have a traditional starting battery like a gasoline or diesel car. Instead of cold cranking amps (CCA) and BCI group sizes for turning over an engine, it uses a high-voltage lithium-ion battery pack to power the motors and a separate, smaller 12-volt battery to run accessories and onboard computers.

This guide clarifies the unique dual-battery system in the 2020 Model 3, focusing on the 12V battery that requires periodic replacement.

Quick answer: 2020 Tesla Model 3 12V Battery

• BCI Group Size: The 2020 Model 3 does not use a standard BCI group size. The original equipment (OEM) battery is a JIS size 85B24LS. Some aftermarket suppliers list Group 51R as a potential fit, but modifications may be necessary. Using the OEM part is highly recommended.
• Function, Not CCA: This is not a starting battery, so Cold Cranking Amps (CCA) is not a primary specification. The battery's purpose is to power the car's electronics, computers, and safety systems when the high-voltage pack is offline. Its capacity is rated in Amp-hours (Ah), typically around 45 Ah.
• Battery Type: The OEM 12V battery in a 2020 Model 3 is a lead-acid type. It is NOT an AGM or lithium-ion battery from the factory (Tesla transitioned to a 12V lithium-ion battery in later model years).
• Terminal Layout: The positive terminal is on the right when viewing the battery from the front (Left Hand Positive).
• Physical Dimensions: Approximately 9.0" x 5.0" x 8.5" (238mm x 129mm x 227mm).
• Dual-Battery Setup: The Model 3 has a high-voltage (approx. 400V) lithium-ion pack for propulsion and a low-voltage (12V) lead-acid battery for accessories. This article focuses on the replaceable 12V battery.

Replacement 12V Battery Options

Due to the specific size and requirements, options are more limited than for traditional cars. The OEM Tesla part is the most common and recommended replacement.

• Tesla OEM: Part #1129182-00-B. This is the most direct and reliable replacement.
• Aftermarket: Some brands offer compatible batteries, but always confirm the fit and terminal configuration. Look for direct-fit 85B24LS batteries or check forums for user experiences with Group 51R batteries, which may require shims or hold-down adjustments.
  • Ohmmu: A popular aftermarket brand specializing in 12V lithium-ion upgrades for Teslas.
  • Mountain Pass Performance: Offers kits and advice for 12V battery replacements.

The Role of the 12V Battery in a Tesla

Unlike in an internal combustion engine (ICE) vehicle, the 12V battery in a Model 3 does not provide the massive power burst needed to crank an engine. Instead, its primary jobs are:

1. Booting Up: It powers the computers and contactors that "turn on" the main high-voltage battery pack when you get in the car.
2. Powering Accessories: It runs all standard 12V systems like lights, infotainment screen, window motors, wipers, and speakers.
3. Safety Backup: It powers critical safety systems, such as airbags and hazard lights, ensuring they function even if the high-voltage pack is disconnected in an emergency.

The 12V battery is charged by the main high-voltage pack via a DC-to-DC converter. It does not have a traditional alternator.

Post-Installation Relearn / Reset Procedure

Replacing the 12V battery is more involved than in a typical car and requires powering down the vehicle correctly. Failure to follow the procedure can damage sensitive electronics.

1. Power Down: Before disconnecting the old battery, you must power off the car completely through the touchscreen menu: Safety & Security > Power Off.
2. Disconnect & Replace: Disconnect the negative terminal first, then the positive. Remove the hold-down and swap the battery.
3. Reconnect: Reconnect the positive terminal first, then the negative.

After replacement, a couple of simple resets are required:

• Window Reset: The auto up/down function for the windows will be disabled. To reset, lower each window completely, then raise it and hold the button for a few seconds until you hear a faint click. Repeat for all four windows.
• Steering Wheel Reset: A warning about the stability control system may appear. To resolve this, turn the steering wheel all the way to the left, then all the way to the right. Center the wheel and the warning should disappear.

Signs of a Failing 12V Battery

A failing 12V battery in a Model 3 will trigger a specific warning on the touchscreen, typically "12V battery needs service" or "Replace 12V battery soon." You should heed this warning immediately. Other symptoms are rare but can include:

• Erratic screen behavior or system reboots.
• Inability to "wake" the car from sleep.
• Complete failure to power on, even with the high-voltage pack charged.

The car gives ample warning, and ignoring it can result in being unable to power on the vehicle.

Common Battery Issues for This Generation

The primary issue is simply the limited lifespan of the lead-acid 12V battery, which Tesla recommends replacing every 3–5 years as a preventative measure. Because it is critical for booting up the entire car, its failure is more immobilizing than a weak battery in a gas-powered car that might still be jump-started. The most common "issue" is owners being unaware of the 12V battery's existence and its critical role, leading them to ignore the replacement warning until the car fails to power on.

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