While I'm making every effort to be as accurate and safe as possible, there are always unknowns. This is not professional advice, and you should use anything here only at your own discretion.
We need a 12V source for any aftermarket stereo. And it needs to have enough Watts to power the amps properly. There are three possible sources: the front 12V battery, the auxiliary plug in the console, and the 12V DC penthouse connector under the rear seat.
I wanted to collect all the information I found from searching and studying into one spot, because this information is all over the map with people talking about different approaches and what is best. It’s very hard to sift through the rubbish to get to solid conclusions. Writing all this down helps me to clarify my thinking, as well as to decide which information I can trust. I plan to keep this updated as we learn new things, so please contact me for anything you think is wrong.
12V battery under the front hood
This is the usual choice for adding aftermarket stereos, and it can work for a Model 3 as long as your stereo is on the small side. The battery is charged from the DC-DC converter under the rear seat, which acts as a sort of alternator for the EV.
However… current understanding is that the charging power available to the front battery is limited by some circuit that is monitoring the health of the 12V battery and trying to look for failing batteries. That circuit limits maximum charging to about 5 Amps, which is 5A*12VDC=72 Watts. That’s probably not sufficient for anything you’d want to add to the car, because a stereo can have a sustained power consumption of 100W without much problem. If you are cranking the tunes, you would thus be draining the 12V battery.
See this blog for details for using the penthouse connector with a large power draw: http://marc.merlins.org/perso/electronics/post_2019-12-01_Using-a-Tesla-Model-3-as-Emergency-Power-Source-In-Case-of-Grid-Failure.html
Here is Marc’s original forum post where he talks about testing the front 12V battery and finding it’s got lower amperage than even the console plug: https://teslamotorsclub.com/tmc/threads/how-many-amps-can-you-get-from-the-12v-system-for-emergency-situations.178079/
It’s also worth noting that there were a number of people connecting radar detectors and dash cams to the 12V battery who reported that they were getting errors about the front 12V battery failing. The Model 3 appears to be confusing that extra draw to be the battery failing. This was the original motivation to find a better 12V source, and lead people to the penthouse connector.
Here’s a thread regarding Tesla pushing back on replacing the 12V battery under warranty because of a stereo being connected.
12V auxiliary plug in center console
This is a possibility for any stereo that is not all that powerful, has the advantage of ultimate simplicity for wiring, and also being the only fully supported Tesla approach as a power source. It’s got a software based circuit breaker for overload handling. Being fully supported by Tesla is a huge advantage to this approach. If anything goes wrong with the car, Tesla won’t second-guess your use of this power source.
The socket is rated at 12 Amps continuous, and can provide 16A peak. That’s 12A*12VDC=144 Watts, with 16A*12VDC=192 Watts peak. Better than the front battery, but still pretty modest. Might work for even a 1000W RMS amplifier setup, because the power that a stereo uses is almost never at that high level. As long as your continuous consumption is below that 144 Watts it could work. To use this setup, it would probably make sense to add a large 2F stiffening capacitor to your power circuit, so that the cap can provide transient spikes that might otherwise overload the plug circuit.
Nice link from the user manual: https://teslamotorsclub.com/tmc/threads/model-3-12v-power-socket.162119/
You would of course want to avoid using anything else on that circuit. Here is a cheap adapter from Amazon with a 15A fuse: https://www.amazon.com/12Volt-Replacement-Cigarette-Lighter-Leads/dp/B08Q39MQRF/ref=sr_1_4
I haven’t seen any instances where people have used this, but I think it should work nicely for the NVX Boost kit on any SR Model 3. The VADM1 is rated at 180 Watts RMS when used at 4 ohms. A bit higher than the 144W, but bassline music isn’t a continuous draw like that. However, the Boost kit has the subwoofer wired for dual 4 ohm and thus 2 ohm load, so the amp will be running at 320W RMS by default.
12V penthouse connector under rear seat
The connector is under the rear seat, under a red protective hood, and is a bolt connection. That makes it easy to add a spade lug power line by just adding a nut to the bolt. The connector can provide 200A at 12VDC, as demonstrated by Marc when using his 2000W 120VAC inverter. There are other power draws for this 12V power, including the main computer and your driving lights, so you probably don’t want to push past 180A while driving.
This connector has been fairly well tested at this point. Earlier on in 2019, it was not well understood and people were getting errors from the car and having to fully power off the car for an hour to reset the errors.
Superb link for discussion from JWardell about whether using 12V battery is best or using penthouse is best:
Travis’ most excellent blog post regarding his ultra-premium stereo installation. Includes good info that he worked out regarding using penthouse power, grounding, and how to add a relay to the penthouse power to avoid faults.
Another blog, from Jared Bienz, regarding his experience adding a monster stereo.
Counter arguments against using penthouse connector
Counter argument that people should not use penthouse. I’m not convinced by apocryphal stories, but it’s worth consideration. Seems apocryphal to me, because the only way it could brick the car from shorting the amp is if the amp was not fused. Only a moron installs a stereo without having a fuse on input power.
There is some discussion in this thread that the ground is not the ground. That the penthouse connector is using a ground that is different from the normal chassis ground, and that you can thus get voltage differences or noise between them. This is also unconvicing to me and I think JWardell is correct here that the OP is confusing the 400V ground with the penthouse ground.
My provisional recommendation
For what it’s worth, my conclusion is that the penthouse is the optimal choice for powering aftermarket stereos.
The front 12V battery is more complicated than a normal car, because the Tesla is trying to detect early failures of that battery. When it goes bad it disables your entire car.
- Their monitoring system doesn’t work well if you add extra draws to the 12V battery, because it can make the car think the battery has an internal short that is draining the battery.
- Marc tested the front battery, and at least on his vintage of Model 3 the front charging is not providing enough amps to run a decent stereo.
- Tesla has argued about warranty concerns for people using the front battery for radar detectors, so it’s not an approved by Tesla approach.
- Lastly, we know the 12V battery is a weak spot in their design, and I want to avoid putting any additional load on it that can potentially shorten it’s already too short lifetime.
Proper power down procedure during installs.
Do not risk accidents by pretending it does not matter. If you drop a wrench on that 12V penthouse connector when it’s live, you get free 200A arc welding.
- Leave open the hood and doors
- Power off car from service screen. Wait for clunk of high voltage connectors releasing. 5 minutes.
- Disconnect the ground wire from the front 12V battery.
- Disconnect the connector right next to the penthouse bolt. It’s a bail connector, open bail to the right, pull straight up.