Keeping it plugged in

Keeping it plugged in

Today I decided to use my "Killawatt" meter to measure how much my Model S draws while parked and full charged. I unplugged it from my NEMA 14-50 connector and put on the standard 120V connector.
To my surprise, the car started charging, even though it had been connected to 240V overnight. It drew 12A at between 122 - 123 V and took 1h 11 minutes to reach a full charge again. During that time, it drew about 1.73 kWh, confirming that the meter is relatively accurate.
Then, when the charge was complete, the draw dropped to my meter's minimum reading of 0.01A. The kWh read-out, after over 2 1/2 hours, is still at 1.73 kWh. It's about 78 F in my garage (25.5 C), and so, now at 124 V, it looks like I'm drawing about 1.24 W. That's about 0.03 kWh in 24 hours - i.e a negligible amount.
I assume it helped that my test happened under optimal temperature conditions, where the battery pack neither gets cooled nor heated, but it gives me a good measure of comfort to know that counteracting the dreaded "vampire drain" will not suck me dry unbeknownst to me. In fact it looks like it will cost me less than a cent per day.
This, of course does not account for the range loss that we do experience, when we leave it unplugged, but I'm also guessing that at 75 F that will be less. Perhaps the difference really is only in the heating and cooling cycles, which of course *have* to draw more energy than I measured.

Carefree | 14. mars 2013

My Tesla loses about 12 miles per 24 hours even in the best of conditions (Arizona, garage, 70 F).

jbunn | 14. mars 2013

That 1.24 watt you read is probably from the electronics in the charging cable.

Life has funny twists. Mu wife and I are moving from a house with garage to an appartment. Before we signed the lease, they promised a plug. Two days after move in, now the leasing agent needs education from his management. No outlet possible. So we need to get the vampire load squared away soon, before we move the car. Otherwisee, ill spend hours just driving to chargers to keep a parked car charged.

DonS | 15. mars 2013

The battery charges until full, and then charging stops until below a threshold. If you want any kind of accurate results from your watt meter, you would have to park it for week to get a reasonable average.

Logical_Thinker | 15. mars 2013

@Jbunn, dude you gotta get it in writing. If they promised a plug on the lease, then they have broken the contract if they don't provide it, and you are free to leave the lease.

hnashif | 15. mars 2013


I wonder if the threshold is known. I plugged my car Monday night and did not unplug til Friday. It charged to max Standard (rated 241 miles) then stopped charging as expected. Then for three days it kept loosing range and not replenishing leaving 219 rated miles on Fri morning. I live in Oakland hills with car in a garage with outside temperature dropped to high 40's at night and up to low 60's during day. Inside the garage the is about high 50's to low 60's.

Brian H | 15. mars 2013

shortly before leaving, switch it to Max Range, and let it charge for an hour or so. It will gain most or all of the lost range back, and will be warmed and ready to rumble (full regen).

hnashif | 15. mars 2013

@Brian H. I thought about doing that, but since my daily commute was 20 miles, I did not bother. I am more curious about the behavior of the car as I learn its idiosyncrasies.

jbunn | 15. mars 2013


I agree, but the mrs was down there taking care of the details. Anyway, its a great place on the water, 23rd floor, view, two blocks from muni and bart. Ive loved the location for years. There are some charge points one block over, so I can charge there if needed. Otherwise, work. Or perhaps I get storage nearby, and plug into the ceiling lightbulb circut... heh.

AMR | 16. mars 2013

What is the threshold as which the car will start charging again? Mine has been plugged to a 120v for a week. Charging stopped at 241 4-5 days ago. The charge is down to 185 with no sign of "threshold charging". I still haven't figured out how to start charging from the app. either.

jbunn | 16. mars 2013

I would suggest to tesla engineering that the upper and lower limits should be closer together on lower wattage sources since it takes longer to toggle on and off. X

DavidN | 19. mars 2013

When plugged in and fully charged, the Model S doesn't steadily trickle charge to keep the battery topped off. The battery gradually discharges for (in my experience) 18-24 hours. Then it will kick on and recharge in a short period. Then remain off and slowly discharge for another 18-24 hours.

According to my Tesla hotline guy, the threshold loss of charge where the car starts charging again is three percent.

However, I've lost as much as 3.5 kWh (measured by a kWh meter similar to the Kill-A-Watt) without kicking off the charging process. In my 60-kWh car, that amounts to almost six percent

Interesting Fact #1: You can kick-start the charging process by simply removing the plug and then reinserting it. Come back 15 minutes later, and you'll have a topped-off battery, warmer battery, and morel regen, which is normally restricted when the battery is cold.

Intersting fact #2: The Model S battery is not maintained at any particular temperature when the car's off. On a really cold night, the battery gets really cold. So vampire loss is essentially independent of temperature. It all goes to the electronics.

Mark Z | 19. mars 2013

Having the 85 kWh battery charging to 90%, my loss is about 10 miles of range before the charging restarts. Charges to 241 or 242 and then restarts about 231.

Version 4.0 and 4.1 had a problem where Model S would "disconnect" the charge station and prevent restart. Since I have a ChargePoint charge station, this was easy to verify as the status is remotely monitored by ChargePoint. Tesla Motors has solved the problem with version 4.2 and no more disconnects occur. IMHO, this may have had something to do with the battery saving feature added in 4.0 and removed from 4.2.

hnashif | 19. mars 2013

For me on 4.2 w/85kWh battery charging to 90%, the car is losing 9-10 miles per day. When left for 90 hours connected, it lost 32 miles (down to 209) w/o replenishing. My home PC scheduled start and stop charge commands every day at 12:05 am and 6:55am respectively during this period. Once the car reached max charge on the first night, it never replenished.