Um. On various Teslas capable of charging at 48A, it turns out that there's three actual rectifier blocks, each capable of 16A.
When one first plugs the car into a 48A circuit (I happen to have that, here) one will see the charge current go from 0A to 16A; stay there a bit, then pop up to 18A, then drop down to 10A, ramp up to 32A, stay there a bit, pop a bit, then drop to 28A or so, then ramp from 28A up to 48A and stay there.
Clearly, the software is turning on one rectifier at a time.
On TMC, there has been the occasional case where one (or more!) of the rectifiers in the car fail; in which case, if one has a 48A circuit, suddenly the car's charging only at 32A. Of, if more than one of these rectifiers has died, 16A. Or, I guess if all three have died, Supercharging works fine (that bypasses the rectifiers) by AC charging is kaput.
Standard Range Teslas that have smaller batteries have on-board AC chargers that are sized for that smaller battery, saving the user money on both the battery and rectifiers. Those cars max out at 32A, having but two 16A rectifiers.
In any case: As an old-timey general electronics and telecommunications hardware troubleshooter, my Main Mantra is, "I hate intermittents."
Teslas do keep fault logs; so it's possible that if one of the rectifiers died, even intermittently, that may have been logged. In which case opening a service ticket, describing the problem, and asking plaintively, "Does the car keep a log of such events?" may get a tech to remote into the car and Look.
In terms of other problems: Both the Tesla, Ye Typical Wall Connector, and the Tesla Mobile Connector all have thermal sensors. Like, all over. If something goes overtemperature, the Car Gets Informed. It's well known that the car will reduce current draw if:
- The AC voltage droops.
- The temperature sensor in the plug of the Tesla Mobile Connector gets too hot.
OK: In the above cases, what we're talking about is Resistance, as in Too Much Of It. Say.. for example.. that the NEMA14-50 socket you're using has developed a case of the worn-out contacts. The resistance goes up. This can potentially cause two things:
- Vdrop = Amps * Resistance. The the resistance of ye typical connector (plug + socket) is on the order of 10 milliOhms, at 32A this would be a voltage drop of 0.32V. So, in stead of getting (say) 240V, you'd get 239.68, which nobody would care about. But suppose it gets up to, I dunno, 0.5 Ohms? Then you'd have a V drop of 32A * 0.5 = 16V - and believe you me, the car would notice that and either stop charging or drop the current, a lot.
- Power dissipation. As a first order handwaving, power dissipation in a resistor at (more or less) constant current goes as Amps*Amps*Resistance. Take the 10 milliOhm case. At 32A, Pdissipation = 32A*32A*0.010 = 10.24W. Um. Ten Watts isn't huge, but the contacts in this case will get Warm and the plug and socket will both warm up. If it gets to the point where it gets dangerous (fires, anybody, and, no, I'm not kidding), the TMC will detect it, tell the car, and the car's answer will be to drop the current. Which, given that were talking (A^2)*R, is actually pretty effective: Going to 16A will drop the power dissipation by a factor of four.
So.. things to try.
- With the car charging at 32A, carefully put one hand on the plug and around the socket. If you have to pull your hand away, fast, lest it get burnt, STOP CHARGING IMMEDIATELY. This is life-or-death stuff here, you do not want an electrical fire, which is what you're going to get if the situation isn't rectified.
- If you don't have an immediate heat issue, then get into the driver's seat and stop the car from charging using the control screen. Note the displayed voltage. Start charging and watch the voltage. In my case, the breaker box is in the garage, there a TWC on the wall with a 60A circuit, and there's no appreciable droop. If you see more than a volt of droop as the car works its way up to 32A, STOP CHARGING IMMEDIATELY. You may have a bad breaker, loose wire, or some other nasty.
- If everything is fine and dandy.. consider how hot your garage is getting. We had a case last summer where some yahoo discovered that aiming a house fan at the NEMA14-50 socket did the trick. Is it warm in your garage?
- As I said initially, I Hate Intermittents. If you can reliably trick the car into doing its low-current trick somehow, take yourself and the car to some J1772-based public charger at the local parking lot. Most of those do 32A. Try it there, using the J1772 adapter that came with the car. If, after a few tries, you're still getting 16A - it's not the TMC, it's the car. Service ticket time. If you take the car to Tesla (with their permission) to try one of their Wall Connectors, you may get some additional help. If they're not too busy.
Finally: NEMA14-50 sockets that one tends to find in Home Depot and the like are designed for the likes of clothes driers and electric stoves. In both of those cases, a plug gets inserted and
left there for years, if not for the life of the house. There's not a lot of physical wear and tear.
However, when using a TMC, where it gets unplugged on a regular basis and thrown into the trunk, then reinserted later when one gets home, Wear Out Is A Real Thing. I'm too lazy to go looking right now, but there's been some
spectacular pictures of melted/burned/shorted NEMA14-50 sockets around these parts over the years..
There are industrial grade NEMA14-50 sockets - and they're a
lot more expensive. But not melting down in the middle of the night causing house fires, well, there's a certain cachet to that kind of thing. The industrial grade sockets have MUCH bigger hunks of metal in them with large contacts that, while they do wear out eventually, it's sure a lot longer than the cheapie Home Depot specials. (And, yeah, recently Home Depot's been stocking better sockets. Explicitly for BEV charging.)
There's a bit of another thing. You implied that the NEMA14-50 in your place was put in there by a previous owner. Hm. My understanding from reading these forums (and being an electronics engineer/techie) was that there was a change in the National Electric Code that stated that NEMA14-50 sockets in
garages in particular needed to have GFIs (Ground Fault Interrupters) attached. That's roughly the same technology one finds in the 120 VAC sockets in one's bathroom or kitchen (for obvious reasons), but the 240 VAC circuits for BEV charging require that the breaker on that circuit be of a GFCI type. And those, I've heard, are expensive.
For the above reasons, given that a new installation requires both an expensive GFCI breaker as well as an industrial grade socket, the additional cost to go to Wall Connector, hardwired to the breaker panel, doesn't require a GFCI breaker, and Doesn't Have Problems with wear-out gets very attractive. Around here, we all usually recommend going with a Wall Connector - it's cheaper and safer.
Good luck!
