Normal voltage drop while charging

[NeilErdwien]

When charging using the UMC in my garage, and monitoring the voltage and current with the phone app, I've noticed that the voltage drops 5-6 volts between the first measurement, often at 0 or 1 amps, and when the current draw hits the expected 40 amps.

I usually start at 249 volts, and it drops to 244 or 243 by the time I start sucking 40 amps. Is this normal? What sort of drop do others see?

5 volts at 40 amps is 200 watts, so there is some heat being dissipated somewhere. I've felt around the outlet and the breaker box and can detect no extra heat.

I've never seen the amps drop to 30, which is what the car is supposed to do if it senses a wiring problem.

 

[Ande]

Some voltage drop is normal, if it's a long cable of adequate (but not big) gauge.
I have a 50m cable myself.
Then, you must be verify that all the drop is on this cable only, not including the drop between your house intake and the transformer.
I would inspect all connections using a thermal camera if in doubt.

 

[murphyS90D]

What you are seeing is normal.
The wire in your walls has resistance. The power lost to heating of the wire is given by the formula P = I * I * R or in English I squared R where I is the current in amperes and R is the resistance of the wire in ohms. Resistance increases with the length of the wire. Resistance is less for thicker wire (smaller wire gauge number).

 

[widodh]

I see the same. I charge with 3-phase 230V, but my "idle" voltage is around 247V. When my Model S starts to charge with 32A on each phase the voltage drops to about 241V.

So I my case the loss is: 6*32*3 = 576W

From my main panel it's about 25m of cabling to the inlet of my Model S.

 

[gavine]

My amps drop because my voltage is too HIGH. My voltage goes-up to 246 so the amps automatically drop from 48A to 47A, presumably because the charger is limited in the total amount of watts it can support (V x A = W). So 240V x 48A = 11,520W and 246V x 47A = 11,562W.

 

[robby]

I would check voltage on your main line (per Ande's advice) before trying to track down a loss between your walls. We have a 16V drop, all between the transformer and our house.