Mysterious Slow Charge Times

My house is solar powered and I have a “smart” time-of-use meter, so I charge my Roadster automatically at 10pm. The rates are the cheapest all year after 10pm to 10am, although depending upon the time of year and day of the week, there are other time windows where the rates also fall into the lowest “off-peak” category.

Still Charging  Eleven Hours Later

Still Charging Eleven Hours Later

I have noticed on three occasions so far when I have gone to my car around 9am and have seen the touch screen displaying “charging”. I expected after almost a half day charging, the Roadster would have been fully recharged. After over five months of driving the Roadster, the amount of charge in the battery has not been a problem. I was just a little suspicious and decided to look into this further.

The charge time depends upon your connection. In the manual they are listed in the following table.

Roadster Charging Times

Roadster Charging Times

I am using the Tesla Home Charger described in my earlier post on Charging Components. Although the first High Power Connector (HPC) did not work, Tesla did replace the first one and it passed their test. Tesla has a small device that plugs into the charger that mimics a car being hooked up into it to test the connections.

The slow charging time could be attributed to several things:  not enough amps from the power company, a low amperage home electric service panel, bad wiring to the HPC, a faulty HPC or a problem with the car itself?

The electrician who installed my High Power Connector brought a dedicated line from the utility directly into the garage. He chose to separate it from the solar and all other connections in order to get the maximum amps. The electrician was the same one who wired the house when it was built, so he was intimately familiar with the wiring. So I doubt the problem is with the connection between the public utility and the Telsa connector.

One clue to the problem is in the charging screen itself. In the lower right hand corner the last piece of information is “241V 12/12A”. The car is recording that is using a 240Volt connection but only 12 amps. So looking back at the recharge times from a completely empty battery, my Roadster would take 20 hours to fully recharge at 12 amps. This table seems in line with the data I have collected unsuccessfully trying to charge from a fairly empty battery to a full “standard mode” battery charge in less than 12 hours.

First I checked with the utility company to see how much power they provide.  This data is unavailable on their web site.  The amount of amps to any location is dependent upon your local transformer. After a couple of phone calls, I was informed my house could receive up to 600amps.

Then I checked with my electrician.  He says my main electrical panel has a capacity of 400amps, which is 4x that of the average home.  Most homes have 100 or 150amp electrical panels.  The electrician also installed a dedicated line for the HPC with a 90amp rating.

User Limit - 70 amps

70 amp current Limit

The only two remaining culprits could be the HPC or the car.  In the roadster, you can control the amount of amps at each location.  Many houses could not charge a car at 70amps.  I looked into this menu and found mine at the default 70amp setting.

After a phone call to Tesla, two service guys came to the house.  The first thing they did was plug in their car to my charger and got the same 12amp charging rate eliminating my car as the problem.  The only remaining culprit was the HPC unit.

Turns out inside the HPC unit is a small dial allowing an electrician to set the amp output.  Mine had never been set off the default 10amp limit.

Mystery solved!

One thing I did notice that at the faster charging rates, the front fans blow quite loudly.  It sounds like there is a leaf blower in the garage.


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