Outside Tesla: News

    Today (Saturday 8 May) this notice appeared on the TESS dashboard: “Replace 12V battery soon. SW completed only after maint.”

    Outside Tesla: News

    The car is still in perfect working order and today I used it without problems for quite a long journey.

    In the warning log there is the same indication, with the addition of the code GTW_w174 (the indication about not loading is a separate and normal issue: I had simply unplugged the magazine):

    Outside Tesla: News

    Electric cars, like fuel-powered cars, also have a 12 V battery, separate from the primary traction battery: it is used to start the computer that manages the primary battery and for other essential vehicle systems. However, this is not an identical 12V battery to those used by fuel-powered cars.

    A user of Teslari.it, Stefano, explained to me that the one mounted on Teslas is an AGM (Absorbent Glass Mat) whose use is very different from what occurs in a traditional car, in which the starter needs of a lot of power for a short time, with peaks of a few seconds even of 600-700 A, and therefore must have internal plates sized accordingly.

    On Teslas, on the other hand, the 12 V battery must not give large current peaks but must constantly feed the on-board electronics even when the car is off. This means that even when the car is stationary, the 12 V battery gradually discharges. When it drops below a certain voltage, the computer draws energy from the primary battery (the traction battery) and recharges the 12 V battery. It is for this reason that the charge of the primary battery also decreases when the car is not being used (this is the vampire drain). If the primary battery is completely discharged, the 12V battery cannot recharge and therefore the car stalls, requiring a complex intervention. It is therefore advisable to prevent the primary battery from discharging.

    The 12 V battery is designed to allow even deep discharges, but the charging process must be precise in order not to reduce its useful life. Furthermore, thanks to a software update, a few years ago Tesla modified the charging process in order to extend its life.

    Replacing this battery is not easy: for example, you have to disconnect the primary traction battery and remember that its poles are reversed compared to traditional batteries. More information on the subject is in this post by Teslari.it.


    Unlike many previous cars I've owned that left me stranded without warning due to battery problems, I'm pleased to note that TESS warns me before the battery goes into crisis. The specific problem, which is a rather rapid deterioration of the battery, seems well documented by Tesla users.

    The GTW_w174 code is explained thus in the Model S manual:

    The 12V battery is too old or deteriorated and needs to be replaced. Until you replace the battery, software updates will not occur.

    It is recommended to replace the battery as soon as possible. In the meantime, continue to use the vehicle normally.

    The vehicle can still be driven when this warning has appeared, for a certain period of time. However, if you continue to put off replacing the 12V battery, your vehicle may not have enough energy to start or restart.

    If the 12V battery level is too low to start the vehicle or open the doors, see Battery Care for recommended actions. For more information on the battery system, see Battery Information.

    I immediately used the Tesla app to book the battery replacement, going to Service - Schedule a service appointment, where I found a copy of the notice, which I selected to indicate the type of service required. I loaded the photo of the dashboard into the app and chose home assistance, since the Tesla service centers are quite far away and I don't have any urgency.

    Outside Tesla: News

    I chose the first date proposed, namely May 26, which by a curious coincidence is the anniversary of my last refuelling, a year ago. I received a few minutes after a booking confirmation SMS.

    I don't know yet if the intervention will be covered by the guarantee.

    The indication "SW update only completed after maintenance" is also interesting, as if the car's software updates are blocked in the event of problems with the 12V battery. This makes quite sense, since after an update the rebooting the car depends on the 12V battery, so it is wise not to upgrade if there is any doubt about this battery. It would also explain the fact that TESS is stuck at update 2020.48.37.1.


    2021 / 05 / 09. The estimate appeared in the app, which has an estimated cost of 355,94 CHF (rather high for a simple battery change, even considering the convenience of the home delivery service). Fortunately, the quote specifies "You will not be charged on this occasion". I accepted.


    2021 / 05 / 26. Today, as per the appointment scheduled in the app, the ranger, i.e. the Tesla technician, came by and replaced the 12V TESS battery at home. The battery was still the original one with which the car was delivered to the first owner in 2016. I updated the car's software: everything is fine. The intervention did not cost me anything, being covered by the guarantee.

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