Charging completed, battery drain

[scfreed]

Newbie question here. If I have the car plugged in but I am not charging, should the battery still drain, or should it pull power from the wall for things such as the cabin overheat protection?

I plugged in at work this morning and set the charge limit to 60% (for the purposes of this test). It charged up to 186 miles and said charging complete (it finished by 10am). By the time I came out to the car at the end of the day, the car range was at 182 miles, but the car was still plugged in. So it 1) didn't seem to pull power from the wall, and 2) didn't start charging again after it drained the battery a few miles.

Is this normal?

(Side note: a few days ago I was connected to a charger that caused it to keep starting and stopping charging, so I wanted to make sure the behavior above wasn't caused by some type of damage from the apparently intermittent charger).

 

[Uncle Paul]

Tesla will run accessories off the battery pack. It will not recharge the pack until a certain amount of power is needed. Don't want to be charging and discarging all the time as there are only a finite number of cycles the batteries will endure.

No worries though. From time to time your batteries will recharge to the specified level. Pretty good system.

 

[eprosenx]

I am pretty sure my Tesla Model 3 is using "shore power" for whatever it can rather than the battery.

When I fire up the AC before I leave the house in the AM I can see it draw varying amounts of current from my house using my Sense Monitor app. It is not enough current for battery charging, and it clearly looks like a variable speed unit ramping up and down as needed.

So I think while the car is in standby it is on battery, but it will kick the EVSE back on when it needs to boost charge or to run something like AC. Not sure about cabin overheat protection, etc... But I assume that would all be shore power.

 

[Tozz]

I am pretty sure my Tesla Model 3 is using "shore power" for whatever it can rather than the battery.

Not completely true, but not completely false either. Tesla's charge to the set percentage (or range) and then the charger is disconnected. When the SoC drops to a certain level (i believe it was 5% below set level) the car starts charging again.

So it is completely normal that the car will have 1 or 2 percent lower charge after a day or two after it has charged, even while power is still connected to the car. If you leave it for a week or so it will charge back up to your set level.

Exceptions are AC and heating. Those run off shore power when it is available. I don't know about cabin overheat protection.

So: Onboard systems such as the displays/computer are running off the battery. Big power users such as AC and heating use shore power.

 

[Tozz]

Don't want to be charging and discarging all the time as there are only a finite number of cycles the batteries will endure.

This is not true.

It is true that a battery has a finite number of cycles. However, it is not true that every switch between charge and discharge counts as a cycle. If you charge from eg. 89% to 90% that does not count as a full cycle. Cycles are usually seen as going from 0% to 100% or somewhere close to that.

Trickle charge isn't using up a cycle every "trickle". If you drive from 55% SoC to 50% SoC and then charge back to 55% you did not waste a charge cycle.

Li-Ion cells are perfectly fine with a float charge, but Tesla doesn't do that for reasons unknown to me. Could be power consumption, as the on-board computers remain on while charging, and thus a float charge could very well be using more energy than required, as the computers keep running.

 

[srijey]

This is not true.

It is true that a battery has a finite number of cycles. However, it is not true that every switch between charge and discharge counts as a cycle. If you charge from eg. 89% to 90% that does not count as a full cycle. Cycles are usually seen as going from 0% to 100% or somewhere close to that.

Trickle charge isn't using up a cycle every "trickle". If you drive from 55% SoC to 50% SoC and then charge back to 55% you did not waste a charge cycle.

Li-Ion cells are perfectly fine with a float charge, but Tesla doesn't do that for reasons unknown to me. Could be power consumption, as the on-board computers remain on while charging, and thus a float charge could very well be using more energy than required, as the computers keep running.

Thank you Tozz. I'm yet to take the delivery of my X (in May). I wonder whether the display (in phone or in the car) displays charging cycles or is there a better app to monitor the charging cycle (cycle count just like in the phone)? What are the tips to reduce the number of charging cycles? I am planning to keep charged between min 30% and max 75% while inside the city.

 

[Tozz]

Thank you Tozz. I'm yet to take the delivery of my X (in May). I wonder whether the display (in phone or in the car) displays charging cycles or is there a better app to monitor the charging cycle (cycle count just like in the phone)? What are the tips to reduce the number of charging cycles? I am planning to keep charged between min 30% and max 75% while inside the city.

Well, you can't really see the charge cycles but you can do some basic calculations. eg (metric system) I use on average 250 Wh/km. So each 400km is 100 kWh equals 1 full/complete cycle. So 4000km would be 10 cycles.

But again, you should not worry about how often you plug it in. If you want to worry at all, you should keep the battery around 50% SoC. So from 60% to 40% is better than to go from 90% to 70% every day.

 

[srijey]

You're right. Li ion battery life depends chiefly on lifetime average temperature and time spent at high states of charge. Because there's a difference between the actual and the indicated % charge in the car and it varies between the models, Cycling in mid-state-of-charge as per your advice would have best longevity: 40 to 60 is ideal; I would say 20 to 40 at the minimum and 60 to 75% at the max is safe to minimize the loss of capacity. I agree there's no standard definition of cycle count. Smaller depth of discharge really matters. Every 70mV drop in charge voltage lowers the usable capacity by about 10%. Based on the Coulombic efficiency, watt hour/ mile, keeping the battery at 4.05V/cell and offering a SoC of about 80 % (actual battery- Tesla factory set?) should be done at the factory level. I was told that one hour before traveling, the user requests the “Full Capacity” mode to bring the charge to 4.20V/cell (does this # show up anywhere?). How to avoid dwelling above 4.1V/cell by an owner? Hope there exists some app which monitors this function?

 

[Tozz]

Hope there exists some app which monitors this function?

There is. You can connect a 'dongle' to the diagnostic port and show those details on your phone. Google 'scan my tesla'.