240v 40amp charging slow (22mi/hr)

When placing charge on my car with 240V 40AMP it charges at a rate of 22mi/hr. suppose to be 32mi/hr. any ideas why?

The quoted 31mi/hr is in ideal miles, not rated miles. There are many other threads about this.

1 hour at 240/40A gives you 9600Wh.
9600Wh / (85000Wh/300miles) = 33,9 miles (ideal)
9600Wh / (85000Wh/265miles) = 29,9 miles (EPA rated)

Given that it charges at ~80% of available amperage (I believe, I'm not saying it's accurate), it turns 29,9 miles into 23,9.
If the charger is ~90% efficient, you end up at 22.

@glaserud - the 80% factor has already been taken into account at 40A rather than 50A. However, the charging efficiency seems to be around 86-87% (energy from the wall to energy stored in the battery), and the rate of charge varies depending on the battery's current state of charge (it is slower when nearly empty or nearly full) and temperature.

Personally, I usually see around 23-24 rated mi/hr, but it varies somewhat. What is the voltage the car shows while charging?

It generally charges more quickly than it shows. It might say 21 miles, but in fact it's charging more.

I have only had my car for a week or so and I noticed the same slower than expected charge rate (around 24mi/hr). But most days during the week I was only using about 20 miles of range, so my battery was already close to a standard charge. I took a longer trip over the weekend so that my battery was about half empty, then I noticed I was getting about a 31mi/h charge rate. It seems to be kind of like a balloon where it gets harder/slower to fill up when it gets closer to the limit.

If my memory serves me correctly: Standard LiIon charging profile is to apply a steady current until top cell voltage is reached. Then you decrease the current to maintain the voltage until you're done charging.

So yes, the balloon analogy is about right. :)

As diego sez, watch the actual total rather than the rate number. There's a bug that under-reports the rate.

@glaserud - I don't know if this applies to the chemistry used in the Model S, but li-poly batteries charge slowly when they are almost fully depleted, then after a bit of charge they stay constant until the constant voltage phase when it the charge rate drops off again. The Model S may actually prevent you from discharging the battery that low, so it might be linear all the way down to what the car calls "zero charge".

Have you checked which voltage you actually get while charging? It usually isn't the nominal 240V, because of cable losses in your house wiring. Not unusal to see 220 - 230 V. The charging screen shows the actual voltage received by the car. This reduces the miles per hour in proportion to the voltage loss.

A few things to consider:

* 40A rated circuit provides 32A continuous for charging.

* 240 VAC nominal can end up at 220 or less at the charger

* actual charging efficiency averages about 85% (92% max at optimum temperature, full empty...can be less than 85% when nearing full, cold, etc.)

combining the 2 results in charge rate of 21 "rated" mph (308 Wh/mi)...well within expected (if your circuit is 40A rated - if you have 50A rated circuit, then the charge will be up to 40A)

Spreadsheets/tables at

As others have mentioned, variation in actual voltage may be the difference. I also get 22-24mi/hr of charging with 14-50, but the voltage coming in to my house is only 204-210v. I didn't know that until my electrician installed 14-50.

I had my electrician install #6 wire and change the breaker to 50Amp so I can pull 40Amp charge. The house has 200Amp circuit.

I find I get about 26 to 29 MPH @ 40 Amps / 240 volts.

I have noticed an odd thing very recently with my Model S 60 when charging. For some reason the car sets itself to an amp of 5, even though I am using my home NEMA 14-50 outlet. When I manually dial it up to 40, it charges at a nice clip again without any trouble. Until recently, it properly set the amps at 40 automatically. Does anyone have ideas why all of a sudden the car is setting itself at 5 amps?

Also depends on additional loads on your 240v circuit causing the charging voltage to dip to the point that the charger circuits retest with lower currents.


Many threads on 5A charging problem here and TMC, e.g.

Check your voltage drop (same check applies to OP) and heating at UMC connectors.


I had my electrician install #6 wire and change the breaker to 50Amp so I can pull 40Amp charge. The house has 200Amp circuit.

I am an electrical contractor and I'll let you know some details. Changing the wire and breaker will not change the current going to the charging station. Changing the wire and breaker is only changing what your capabilities are. You can put in larger wire and a 200A breaker and it won't charge any faster.

Wire size - larger will allow more current potentially without heating up
Breaker - The amount of current it will allow before it will trip

Wire and breakers do not "push" current as many think. It is dependant on the load "pulling" the current. The charger will only pull set max amount of current (will possibly pull less if trickle charging). It will never go above set amount even if you change the breaker and wire. Having a larger breaker and wire will help with preventing accidental tripping. That's all.

Having a good steady 208v or 240v system will create the best possible charge.

NOTE: Don't get me wrong; having undersized breakers and wire will create problems! Having oversized will not make a difference.

Correction to above.

This is quoting his message:
"I had my electrician install #6 wire and change the breaker to 50Amp so I can pull 40Amp charge. The house has 200Amp circuit."

3 months, 7500 miles charges consistently at 29 mph.

@Musterion, many thanks for the threads on this topic. Still a mystery I guess, but at least I am not alone.

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