How to replace the 12V battery in a Tesla Model 3: the steps from TeslaHolic's video, checked against Tesla's manual

A Tesla has two batteries. The big high-voltage pack moves the car. A small 12V battery keeps the computers, screen, locks and accessories alive. When the small one starts to fail, the car can act strangely even with a full main pack. On October 4, Romanian channel TeslaHolic posted a video of almost 9 minutes in which they replace the 12V battery of a 2019 Tesla Model 3 Dual Motor at home, in a yard.

Video

Watch on YouTube

The video is in Romanian. For this article we worked from YouTube's automatic captions, so some details are paraphrased.

The symptoms

The car had received a new Varta battery one and a half to two months earlier (the video says a month and a half, the description says two months). Even so, the problems came back:

• about three alerts kept cycling on the screen, one of them plainly asking for the 12V battery to be replaced;
• every now and then, the accessories stopped working;
• in Service Mode, the power distribution diagram showed the small battery disconnected from the system. The car was running everything off the high-voltage pack.

One detail from the video matters. After the previous swap, the battery type in the car had been set to "Clarios", the company behind Varta batteries. This time they went with Tesla's original type of battery, an Atlas BX, bought for 950 lei (about €190) from Boom Car Service. The price comes from the video and its description; we have not verified it independently. Boom Car Service, in Popești-Leordeni near Bucharest, is one of the Tesla-approved body shops in Romania we listed in our Ottimo Iași article.

The steps in the video

The presenter says they follow Tesla's own procedure. In short, here is what the video shows:

1. Preparation: both front seats moved fully forward, all windows lowered, doors open.
2. Rear seat: the lower cushion pops out of its clips. Underneath, the connectors on both sides are unplugged.
3. The car is powered off from the menu. From now on the doors only open mechanically, not electrically. That is why they stay open.
4. The negative terminal comes off first, at the battery in the frunk.
5. The link between the small and the big battery is unplugged, under a cover by the right rear door.
6. Then the positive terminal and the hold-down clamp come off, and the battery comes out.
7. Installation: the clamp, the rear connection, the plugs under the seat, then positive first, negative last.

Tools needed: a 10 mm wrench. After reconnecting, the car woke up on its own, with headlights and climate control working and no errors.

What Tesla's manual says

We compared the steps with Tesla's Model 3 service manual, published on Tesla's service website. The basic order matches:

• Tesla says not to operate the windows for at least one minute before disconnecting, to keep the rear doors from latching (the manual suggests a towel through the strikers) and to remove the rear lower seat cushion.
• Disconnect the 12V negative before the "first responder loop", located next to the right-hand side of the 12V battery. On reassembly the manual is explicit: don't reconnect the negative terminal until the loop is reconnected.

In the video, the link between the two batteries is unplugged after the negative terminal and reconnected before the terminals. That matches the rule above. For exact details, torque values and the right variant for your car, follow the official manual, not just the video.

The step many people forget: battery type in Service Mode

After the install, the presenter went into Service Mode and switched the battery type from "Clarios" back to the original "Atlas BX". The car asked for a confirmation (brake, turn signal), then ran a routine lasting a few seconds.

Tesla's manual confirms the step. If you replace a Clarios (Varta) battery with an AtlasBX one or vice versa, the vehicle configuration must be updated under Low Voltage > Power Distribution > Change Battery Type. The feature is only available on software 2023.32 or later. Our inference: if the car is set to a different battery type than the one fitted, the small battery may be charged with the wrong parameters. That may be where the previous battery's trouble began, but the video does not prove it.

Two warnings from the video, confirmed by Tesla

• Don't jump-start another car from a Tesla. The presenter warns that the small battery would drain immediately. The Model 3 manual is even more direct: the car cannot be used to jump-start another vehicle, and doing so can damage it.
• Don't let the 12V battery run flat. According to Tesla's tech note, a lead-acid 12V battery discharged below 10.5V loses capacity. Tesla recommends replacing it if that happens.

At the end, they say that if errors return even with the new battery, the problem would be the system that charges the small battery. The captions say "alternator", but on a Tesla that job is done by the DC-DC converter inside the power conversion system (PCS), which also shows up in Service Mode (our interpretation). The description says everything has worked perfectly since filming, and that they will update in the comments if anything changes.

Does this apply to every Tesla?

Not quite. The procedure in the video is for a Model 3 with a lead-acid 12V battery. According to the manual, Model 3 cars built in Shanghai after about October 2021 and in Fremont after about December 2021 have a lithium-ion low-voltage battery, with a different procedure. For that battery, Tesla says to try recovering it before deciding it needs replacing. You can see your battery type in the car under Controls > Software > Additional Vehicle Information.

What it means for owners

• It's a doable job with a 10 mm wrench and a little patience, as long as you keep the order of the steps. The presenter's verdict: the procedure is "not complicated at all".
• Don't skip Service Mode. If you switch between Varta/Clarios and Atlas BX, set the correct battery type.
• If you're not sure, Tesla Service or a specialist shop does the same job. At least for a car under warranty, it's worth asking them first.

The video and images belong to the TeslaHolic channel. We have featured TeslaHolic before with their Cupra Raval long-distance test.