How do you determine the state of charge?
Good question, simple answer: Tap the icon that shows how many miles you got; it changes to a percent symbol with a number.
The closer one is to full charge, the slower the battery will charge.
So, for a random example:
- One is at 90% of full charge
- It's 0F out there. Brr!
- The battery heater is out to lunch or the car's been Just Sitting There, freezing
Let all those things be true and you'll be lucky to get 20 kW.
Interestingly, if one is using a Wall Connector (i.e., L2, 240 VAC charging) and one is prepping for a trip, then charging to 100% is an OK thing to do. But the charge rate at 98% or thereabouts gets really, really small, like 1 kW or something. Note that on a M3 (yes, I know, you got a Model S) that normally can chug along at 11 kW up to 90% charge falls off at 98% or some such. And... So does a Supercharger. A Supercharger trying to charge a battery to 100% falls to the same 1 kW numbers that a L2 charger does.
Now, show up at a Supercharger with 3% State of Charge on a warm day: That's when one discovers (at least, on a M3, anyway) that said Supercharger can whoompf 250 kW into the battery. That keeps on until one gets to 30% state of charge, at which point the rate starts falling off; at around 90%, it's 30 kW or something and keeps on going down.
The charging system, as others have noted repetitively, restricts the charging rate when the battery is cold. This is why Tesla likes people to
navigate to an SC; when the car sees that one is doing that, it dissipates a ton of heat into the battery, warming it up so it'll charge faster when it Gets There. On the older Teslas, this was done by purposely dumping excessive current into the motor so the cooling fluid would get warm and heat the battery; the newer ones use the heat pump (and maybe the motor) for the same purpose.
Thing is: Even with all that, 24 kW with 120 miles on the gauge (maybe 50% SOC?) is a bit on the low side. Even if it
is cold out there. In my experience, anything warmer than 45F or so isn't all
that cold. And, yeah, I live in NJ and make trips to Boston, both places known for being much colder than 45F in the winter.
My suggestion: Run an experiment. Get the car down to 120 miles or so, then drive to an SC, or maybe two. If your charge rate is still 24 kW, it's probably a car issue. If, after running the car around the landscape a bit, the charge rate is more like 100 kW, then it might have been the SC. It's not clear, so a little troubleshooting like this might be in order.