[noonespecial]

120v @ 15A is 1800 watts. As I said, I was able to measure the wattage being delivered at the end of my 100 foot 16 gauge "home depot" orange extension cord connected to a space heater and found less than 900 watts being delivered to a load that normally pulls 1750. The voltage had also dropped below 100 volts.

Its possible that the Tesla simply won't charge at all if the current or voltage on the line becomes too low. In this case, it would be "plugged in" but not charging.

Edit: Lets figure it out: Range=244mi. Battery capacity=58kWh. So: 5 miles takes 1.086kWh. 10% is lost in chemical conversion so we really need 1.207kWh for those miles. We pull at a rate of 1.8kWh/h from the plug. So 600 watts or so is lost elsewhere. We know the "always on" battery cooling system alone takes around 150 watts. Its not that far fetched.

Edit: I deleted my comment below where I calculated the cost of keeping the car on standby because I don't think I made clear enough (and didn't want to type it all twice) the difference between the actual discharge rate of the battery and the amount of power required at the charger to stop it. It just seems to take a lot of power at the charge port before any gets to the battery. The battery seems to discharge at an average rate of just 30 watts, but it seems to take a much, much greater amount of power input to prevent this and failing to provide this power has dire consequences. Why?

[jws]

The pack is 53kWh according to wikipedia and discharges in 11 weeks from full according to the article. Unless I botched a decimal place, that is a 29 watt idle load. (53000 watt hours / (11 weeks * 7 days/week * 24 hours/day) ). In perspective, that is about the same as 4 night lights.

Clearly there is about 20 times that much power available at the end of a 100 foot extension cord in a standard household outlet.

Failure modes available:

• Was not actually plugged in.

• Was unplugged then plugged back in much later, possibly unbeknownst to owner. (Someone in my household unplugs my DVR to use an outlet. Grr.)

• Was on a switched outlet. (I once bought a new drill over that. Who knew one of the duplex outlets was switched and the other not? But the new drill wasn't $40k.)

• The Tesla charger could have a cutoff where it stops charging if unable to pull X amps where X is somewhere between 7 and 15. This could be either intentional or unintentional. Brown out protection circuitry, if needed by the design, could do this. They also might fear they are setting your house on fire by dropping 800+ watts somewhere in a wall and shut off.

• Broken charger.

• Broken extension cord.

[nash]

You forgot "Owner made up story"

[Vixter]

A roadster that is plugged in, is not really "off". It will provide power for heating or cooling of the battery pack if the temps get too high or too cold. This would be much more than a small idle load.

[bradleyland]

That's because a 16 gauge extension cord isn't supposed to supply 120v @ 15A. A 16 gauge cord is only good for around 10 amps.

Which means you have a point. If the owner in question plugged the Tesla supplied 120v cord in to an additional 100-foot, 16 gauge cord, they might have a net loss situation on their hands.

[HeyLaughingBoy]

A little OT, but I've found that it's better to make your own extension cables. I have a 1kW tank heater that is about 100' from the closest outlet. The cost of 100' of 12gauge house wiring and a plug, socket and outdoor outlet was much less than even a cheap 100' extension cord and has much less voltage drop.

[noonespecial]

That depends on your application. Extension cords are more expensive because they are made of stranded wire that remains flexible and resists breaking. Household wire is solid and not meant to be rolled and unrolled repeatedly. It will get kinks and weak spots. But if your application is for a relatively static run, left in place for a long time, the household wire is better.

[tlb]

The charge circuitry probably turns on at a high charge rate, senses the line voltage drop below 100v, and shuts down for a few seconds. Stuck in this loop, it might never deliver much charge.

[D_Alex]

Er, no, this is probably wrong. The voltage drop is due to resistive losses in the long (and skinny) cable, and the resistive losses depend on current flowing. So, unless there are some weird effects due to the car's electronics, you cannot stop the charging, only slow it down.

[marshray]

So the charger sees the 120 V, decides to begin charging, current flows, and the voltage drops. If the voltage drops out of range, the charger aborts because it's out of design spec and/or there's a significant amount of power being dissipated, probably by resistive heating (i.e., risk of fire).

This is probably necessary to achieve UL rating.