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Question about being plugged in at home

1. When I get home at 7pm, I plug in my 60.
2. I have the charge set to start at 3am.
3. Let's say the charge stops at 5am.
4. I unplug and leave at 8am.

2 questions:

From 7pm (when I plug in) to 3am (when the charge starts), is there any power being consumed? Assume nobody is in the car, it stays locked all night.

From 5am (when the charge is full) to 8am (when I leave for work), is there any power being consumed. Same assumption as above.

I'm asking these questions because my wife thinks there is some power being drawn while the car is plugged in, but not actually charging (whether before or after the charge). Unfortunately, I didn't have an answer for her.

Any thoughts?

Your wife gets the gold star. If you are using 4.5 and above, the car does maintain itself using outlet power, but will not charge the battery if you have set a schedule. The power draw is minimal.

Q1 - yes - the computers consume power. With S/W v4.5 the car will pull that power from the wall; with 4.4 and below it comes from the battery.

Q2 - ditto.

This is known as the 'vampire drain'. A future software update will reduce this to near zero.

-- near zero, at the cost of having screens blank for a bit during bootup. No effect on driving.

Plugged or not, my MS consumes about 12 miles (roughly 4kW) of power per day when the car is off and no one is using it. That's not a lot of $ but it certainly is not trivial. This is my case and its been confirmed by 100's of posts in this forums. Tesla is working on a solution for that. Look up " vampire drain" in other posts.
Hope this helps. Best. TT

She is right the car battery is not being used while plugged in also 12Volt system is trickle charged and any onboard usage such as A/C and battery maintenance does use a bit of your power. I just calculated what it cost to drive 190 miles a day based on our utility cost and it equals $6.45 . A real bargain Tesla S owners.

Every 3 miles, you blow another 10¢‼ The horror! >;(

Despite what it says in the Release Notes, no power is consumed from the wall unless it is charging or the heater or A/C or battery heater is on. So to answer the OP's questions directly, when plugged in, no power is consumed (from the wall socket) from 7pm to 3am. Similarly, no power is consumed (from the wall socket), after charging stops, from 5am to 8am.

During both of those time periods, if the 12V battery gets too low, it will draw some power from the main battery to recharge, but it will not draw any power from the wall socket, because the charger can't produce 12 volts.

This can be verified using a Kill-A-Watt meter and 110V charging, but you'll have to get up at 3 am to verify this.

Due for revision to enable shore-power for all accessories?

All the posts are correct. I would just like to add that keeping your car fully charged, even for 3 hours (from 5am to 8 am) may slightly affect battery longevity in the long run. The battery is happiest if maintained at approximately 50% charge, and if you start driving soon after a full standard or range charge (try not to keep the battery fully charged for too long before driving). Depending on your daily commute distance, you may want to set your autocharge to begin later, ie 6am and leave the house with a enough range so that you return at end of day with 50% charge ie 100 miles. You can fine tune the miles you start with in the morning by adjusting the charge amps on the charging screen.

I don't think shore-power can ever be used to power the 12V accessories directly without a major hardware modification.

The charge port seems to be wired to a large relay that switches the port power between three states:

  1. Disconnected
  2. Connected to on-board Chargers(s) (10 kW AC-to-DC high voltage rectifiers)
  3. Connected directly to main battery (for DC SuperCharging)

A/C, cabin heater, and battery heater are high voltage devices, so shore power can be used for those. They can be powered by the on-board chargers.

But the accessories are powered by the 12V battery, which isn't connected to the chargers. The 12V battery is connected to a DC-to-DC converter. It activates only when the 12V battery voltage gets too low. It doesn't run continuously.

I suppose Tesla could make a software change to activate the DC-to-DC converter all the time when plugged in, then activate the HV chargers but dial them down to 0.25A output to match the 100W or so needed to power the accessories, but the heat and energy lost by this process just wouldn't be worth it. If they can make the CPUs go to sleep properly, they can solve most of the vampire drain problems.

In theory you could use a 12V Battery Tender to maintain the 12V battery overnight, eliminating 99% of the vampire drain, but it would not be very convenient, as you would have to pop the nose to get to the 12V terminals. The 12V cigarette ligther adapter can't be used with the Model S because the 12V socket is disconnected when you power down the car.

See my post to this thread for more information.

I'm on 4.5 and still get vampire drain. My understanding is the wall socket, other than charging, draws power only for climate control. I'm not sure how much the battery needs conditioning in the summer months, so maybe the 4.5 upgrade will be a bigger deal in the colder months.

"battery is happiest at 50%" - can you provide a reference to where Tesla has officially stated this?

This statement is inconsistent with Tesla's recommendation in the Owners Guide to have the car plugged in whenever possible.

The vampire sleep fix is not in yet. Should cut losses by 90% or so. You'll be able to tell easily because the screens will take 30 sec or so to boot, though the car will start and drive fine immediately.

@ bp - with the new ability to charge anywhere between 50% and 100%, it is possible to keep the car plugged in whenever possible and still average 50% charge, so this is no longer inconsistent. But you are correct that no official statement has ever said that battery life is maximized by keeping SOC near 50%. Even if it did add some incremental extension to battery life, the price would be the need to delay an unexpected road trip, while you spend 3-4 hours topping off the carge to 90% (assuming no access to supercharger). I hope Tesla clarifies some of these details, although i am skeptical that the data is there yet.

@bp while rocky is right about 50%, the difference in battery degradation between 50% and 90% is small and so Tesla doesn't want to confuse owners. With the 4.5 slider Tesla has publicly stated that you should "charge to what you need" but even then they have been reluctant to get into the battery chemistry specifics.

The fact is that as long as you don't routinely max charge and then leave it there for long periods of time, your battery will last you for longer than you want to keep the car.

Some people want to baby their cars (look at the threads discussing detailing) and this translates to their batteries. If you are one of those people there is a lot you can learn about Lithium ion batteries and their care, if not you can just charge to 90% and not worry about it.

Great that Tesla is finally giving people the option though!

Thanks everyone, these are great replies. Keep them going if/when you have more info.

@bp - Elon said on video that "long term storage of the battery is best done at 50% charge and cold"

Note that is a very specific situation, and does not necessarily apply to the discussion here. I think Rocky maybe inferred a bit too much.

The battery management can get very complicated. Not only is having too much charge potentially harmful - but also fully depleting the batteries can also cause problems.

Ultimately, I think we have to have some faith in how the car is programmed to manage the batteries.

The advice in the Owners Guide is simple - charge when possible, and don't do max charge very often.

Anything more complicated than that will create the perception that managing the battery is complicated (trying to predict the next day's mileage) and that the battery pack is easily damaged by not charging it to the optimum level every time.

The best thing that Tesla could do to help out with this would be to keep the rules simple, continue to fine tune the software in the car to protect the batteries as much as possible, and revise the battery warranty to state that if recommended charging practices are followed (i.e., avoid frequent max charges or running out of charge), that the warranty guarantees a minimum charge level - for the term of the warranty.


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