Power draw from accessories? 12v battery vs the big battery

[mattack4000]

Hey guys,

I didn't get much info from the X side, but I want to see if you guys have the answer for this. Just how does the battery on the S/X work. Obviously any power to the motor comes from the traction battery, is the dash and all the electronics powered by the 12v or the big battery? If it is powered off the 12v, I am thinking the 12v must be constantly charging in order to stay ready.

 

[FlatSix911]

Here you go ... interesting reading

Syonyk's Project Blog: Tesla Model S 12V Battery Analysis

The Model S uses the 12V system to run pretty much everything. The computers run off the 12V system, the interior electronics (locks, windows, seats, etc.) run off 12V, and the main contactor for the big battery underneath requires a functioning 12V system. If the 12V battery is dead, so is the car. There are jump start terminals (which, by the way, you shouldn't use to jump another car), but the Model S is entirely reliant on this 12V lead acid battery to function.

 

[Chrisizzle]

What about items plugged into USB / 12v plug?

The 12V plug in my 2014 Model S used to power down when I left the car. However, it is constantly powered in my 2016 Model X.

Curious as to whether that is due to a difference in model years (eg. A 2016 MS would also power accessories full time) or a difference in car models (meaning it’s a Model X thing). I’ve noticed simply because I kept my dash camera when I switched cars.

Anyways, would loading up the USB and 12v plugs kill the 12v battery? Or will it draw near infinite power from my 90kw main battery?

 

[boaterva]

All (12V) accessories are powered from the 12V battery. It's the only 12V battery there is. That battery is recharged through the DC-DC converter from the 'large battery'. The issue with 12V batteries dying over the year(s) is that the 12V battery is relatively small, nowhere near what is in most ICE cars and can't take hundreds of charge/discharge cycles. In warrantee, it's replaced by Tesla if/when it dies.

The only limit in normal usage should be the fuse on a particular circuit (USB, power socket) and whether that connection is on when the car is off. Zac and Jesse went coast to coast with a small refrigerator in the back of their Model X Sparky. I don't know the load, but their videos might have a link to it on Now You Know.

 

[KArnold]

I read the article. Maybe I'm missing it but does diving the car affect the 12-volt charging differently from being plugged in? My car was idle but plugged in for 3 weeks - when I came back the 7-month old battery complained that it needed replacing - see Sitting Idle - Plugged In

Also, should rigging a smart charger help with 12-volt life? If so it would be easy to do.

 

[Uncle Paul]

Not an answer to your question, but my Mercedes also has two batteries. A big one on the trunk to provide starting power and lights, plus another smaller one to power the electronics in the car. It is a stablilzer battery that always provide the same current, and is not affected by the heavy starter draw which offen varies current.

When they change out a battery they must use a memory saver which plugs into the OBD port. This provides temporary steady current when one or the other batterys are disconnected for replacement. This prevents all the computer memory from being deleted and you needing to reprogram all the features.

 

[mattack4000]

Can we use a trickle charger on the car via the jump start location?

I guess the bottom line is that since the 12v is getting pulled all the time, it shouldn't matter if a dash cam is powered on all the time given it uses very little power in comparison.

 

[RandyS]

If you want to make your 12 volt cigarette lighter socket hot all the time on the Model S, it is easy. Courtesy of Ingineer...In the frunk area, remove the relay as pictured in fuse box #2. Make up a jumper using a short piece of 14 AWG wire and male 1/4" spade terminals. Pull out the relay, then insert the jumper into the exposed 1/4" female receptacles as shown below. You can change the socket back to switched power just by removing the jumper and re-inserting the relay.

Once the socket is hot all the time, you can use a CTEK battery tender to inject 12 volts and keep the 12 volt battery system from discharging / charging the 12 volt battery and putting cycles on it (avoiding the 4-5 cycles per day that happen normally)....

 

[lymex2018]

Several cycles per day? All the evidence supporting this I saw was in past tense. Since most of the related devices are programmable, Tesla may have already changed this by firmware update.

Two reasons I can think of why the 12V battery in existence are the surge of drain in short time which an ordinary DC-DC cannot provide, and the inefficiency of DC-DC to run all times. However, Tesla seems allow the DC-DC to run continuously at some circumstance in recent firmware update to lessen the small battery cycle.

I recently test my car for 12V voltage, there is less than one cycle in 24 hours if Energy Saving on and Always Connected off:

Apart from the drive train, my tests also show that there are others drawing power directly from the big battery such as cabin heater and the battery heater, although the circuit diagram of Model S shows otherwise.