A Peek Under the “Hood” of a Tesla Model S P85D


If you’re like us, you’re always peering into whatever open compartments of any car you see.  It’s a little safer to do that at auto shows and such, but when you can do it via the internet?  Ideal.

Courtesy of Reddit user “EatMoarToads” (?), we have the shot above, posted on imgur, here.  Go ahead, click on it for the big one.  We’ll wait.

The comments and discussions are kind of amusing, as is most of Reddit, but they all seem to focus on the orange, automotive-style deep-cycle 12V battery.  Like this:

– The fact that they moved the battery top center for the D chassis is pretty telling that they had no idea they’d be replacing these things every year on average.
On the RWD platform it’s hidden under all kinds of stuff.
There’s even a shortage of them. Mine popped the warning Monday and it goes in tomorrow for replacement. They couldn’t get me in sooner because they didn’t have one to give me. Presumably they have some in reserve for acute failures.
– Can someone explain to me why this car even needs a 12V battery? Why not power everything from the traction battery?
– Here’s my four guesses. Some are better reasons than others.
auto electronic components are commonly made for 12 volts.
to act as a backup, For when the main pack is either disconnected or charging.
to prevent accessories from continuing to drain the traction pack.
To keep dangerous power levels further away from the occupants of the vehicle.

…and more.  Much, much more.

Of interest, though, is a comparison photo posted of a non-dual motor car:

Non- "D" P85 under the "hood"

Non- “D” P85 under the “hood”

Really, all we’re seeing in the dual-motor car is the 12V battery (for whatever reason) and the windshield-washer reservoir. This led us on a quest to find out what all that other plumbing-looking stuff was, in both of the photos.  Then we found this video:

So, okay – AC, some suspension controls and reservoirs, braking stuff, the usual automotive systems.  Hold on for the rest of the car in the video, and you’ll see the motor/inverter system on the rear.  Presumably, there’s now one just like it in the front, requiring the displacement of aforementioned 12V battery.  And wishy-washy juice.

Gotta love those bracing struts that are as thick as your wrist, though…

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13 Comments on "A Peek Under the “Hood” of a Tesla Model S P85D"

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The 1.5 Roadster uses a component in the main pack for operating the 12v items. Tesla went to a separate 12v battery for the 2.0 and 2.5 but I’m not sure of the exact reasons. My 1.5’s 12V system REALLY hates cold weather. Not sure if that is one of the reasons or not.

the big one is coolant. the window washer fluid is the blue cap on top right, next to brake fluid.

That’s what I was thinking as well. It clearly has the large diameter coolant lines attached.

The 12V battery runs all the non-powertrain electrical loads and the 12V battery is charged by the main pack. I would assume the main reason for doing this is that it allows them to use COTS components.

Can’t find the 12V battery in the non-D version photo. Can someone point it out? It seems that the load of having the car always online for remote access using a phone App is one of the main reasons for having a buffer battery. And that load necessitates a large number of discharge/charge cycles on that battery, which takes a toll on it.

The right bottom corner of the photo I think.
It is not orange.

The 12V battery is for safety.

Tesla wanted to make things simple for first responders in crash situations. The logic is you pull 12V battery power, the contactor in the main pack cuts out and all the High Voltage is gone.

Not really. The 12V battery is not easily or quickly accessible in either D or non-D cars. There is a marked loop exposed in the frunk that first responders should cut to cut power to the car. There is also a pyrotechnic disconnect for the HV pack–in the case of an accident, it blows and isolates the pack.

Well, there goes the frunk.

I’ve been told, you still have the frunk, you just lose the “microwave oven” section in the back.

Yeah I don’t see the reason for the big 12 volt battery, and Tesla apparently didn’t give this much thought if they are burning out every year as several blogs here seem to indicate, besides going ‘dead’ while charging if there are problems with the supplied cord, at least as I was led to believe. Not sure how the Roadster 1.5 handles the problem but my 2.5 has a motorcycle battery in a separate box in the right front wheel well. The only time its needed is if the high voltage battery is broken for any period of time, Normally, it just runs the remote controls and so you could consider it normally unbelievably oversized. Since all power has to come from an ‘alternator replacement’ anyway, there is no point having the battery discharge while attempting to charge the high voltage battery, or anytime there is a human near the car. It is simply easier to turn on the converter and avoid needless charging/discharging of a 12 volt battery. The only thing the 12 volt battery needs to do is run remote controls or these days also remote telemetry until the human gets near the car. (And that stuff draws… Read more »

The 12v Battery has the same use in all other BEVs. The accessory control is just a bonus (its standard parts so it makes stuff cheap). The real reason is the traction battery – it is the same reason you cannot start any BEV without a 12v – a dead 12v makes it impossible to start a LEAF or any other BEV.

The reason is safety related to the traction battery. The 12v helps run the control chips, safety mechanisms, and the BMS for the battery. This allows independent control of the pack to insure the safety control is maintained right at startup. You wouldn’t want to start the traction battery up if it was failing in some way – this checks to make sure before the full battery is online.

They want to keep a separate 12 volt battery, OK, but why an old fashion lead one. Beside the fact they are cheap, they are still a heavy weight compared to what a lithium based 12 volt battery would be. They also don’t last long and are not small. They should test if putting a dozen lithium 18650 cells in series per four and in parallel per three would not be a better option than a lead battery. Being it only for the image of having no outdated lead battery in the car.