Two Technical Charging Questions

Evbwcaer · Aug 25, 2015

1)From what I have read about NEC (electric codes), a continuous load is classified as thee hours or more. Why doesn't Tesla allow you to draw 50 amps from a 50 amp outlet for three hours and then ramp down, or maybe cycle something like five minutes at 50 amps, two minutes at 40 amps? Perhaps more importantly, when using a household outlet or a 20 amp outlet, an arrangement life this could make a meaningful difference as a percentage, and at 50 amps you are around six miles more per hour of charge.

2) Can someone explain to me why the motor's inverter cannot be used as the onboard charger? It can operate as an AC to DC converter, can it not? I can get up to 60kw of regen, sure would be nice to be able to charge 60kw AC and not have chargers. ( I believe during regen the AC motor is putting DC back into the battery).

Thanks.

Oba · Aug 25, 2015

Evbwcaer said:
1)From what I have read about NEC (electric codes), a continuous load is classified as thee hours or more. Why doesn't Tesla allow you to draw 50 amps from a 50 amp outlet for three hours and then ramp down, or maybe cycle something like five minutes at 50 amps, two minutes at 40 amps? Perhaps more importantly, when using a household outlet or a 20 amp outlet, an arrangement life this could make a meaningful difference as a percentage, and at 50 amps you are around six miles more per hour of charge.

Well, for folks who live by skirting technicalities, sure, you could do this. Your odds of popping the circuit breaker are very good. The whole idea is a continuous load will "heat soak" the wires and circuit breaker, and they were not designed to operate at maximum amps on a continuous basis. Some rule making genius came up with 3 hours... it could have been 30 minutes, or 8 hours for that matter. It's somewhat arbitrary as to the time; the issue, again, is heating.

2) Can someone explain to me why the motor's inverter cannot be used as the onboard charger? It can operate as an AC to DC converter, can it not? I can get up to 60kw of regen, sure would be nice to be able to charge 60kw AC and not have chargers. ( I believe during regen the AC motor is putting DC back into the battery).

That's the way the first generation (1.5) Tesla Roadster is, from the AC Propulsion patent. Renault also does this with their EVs, using 3 phase power at 22kW (32 amps) or 43kW (63 amps). France has oodles of three phase power outlets called "Chameleon".

FlasherZ · Aug 25, 2015

Evbwcaer said:
1)From what I have read about NEC (electric codes), a continuous load is classified as thee hours or more. Why doesn't Tesla allow you to draw 50 amps from a 50 amp outlet for three hours and then ramp down, or maybe cycle something like five minutes at 50 amps, two minutes at 40 amps? Perhaps more importantly, when using a household outlet or a 20 amp outlet, an arrangement life this could make a meaningful difference as a percentage, and at 50 amps you are around six miles more per hour of charge.

Because 3 hours is merely an arbitrary number that helped to differentiate those items that cycle their loads (e.g., heating appliances) and those that draw current constantly. NEC article 625 (EV charging equipment) specifies that all EV charging loads are continuous, so you don't get any room to play there.

2) Can someone explain to me why the motor's inverter cannot be used as the onboard charger? It can operate as an AC to DC converter, can it not? I can get up to 60kw of regen, sure would be nice to be able to charge 60kw AC and not have chargers. ( I believe during regen the AC motor is putting DC back into the battery).

Different voltage & current profiles, I'd imagine.

- - - Updated - - -

Oba said:
Well, for folks who live by skirting technicalities, sure, you could do this. Your odds of popping the circuit breaker are very good. The whole idea is a continuous load will "heat soak" the wires and circuit breaker, and they were not designed to operate at maximum amps on a continuous basis. Some rule making genius came up with 3 hours... it could have be 30 minutes, or 8 hours for that matter. It's somewhat arbitrary as to the time; the issue, again, is heating.

Actually, it's not skirting technicalities. Article 625 says all EV charging loads are continuous loads, so you can't even do that.

davewill · Aug 25, 2015

My understanding is that dropping the charging current for a few minutes wouldn't really change anything crucial. The reason for derating is that wires, connections, and breakers all get warm when subjected to high current. Most high amperage devices (like heaters, stoves, A/C) only use those amperages for a short periods, then drop down to almost nothing. Things get to cool off.

It IS possible to use the inverter for double duty. The Renault Zoe does this and can do 43kW charging on three phase AC. I don't know whether there are technical, economic, or patent issues that keep more companies from doing that.

miimura · Aug 25, 2015

davewill said:
My understanding is that dropping the charging current for a few minutes wouldn't really change anything crucial. The reason for derating is that wires, connections, and breakers all get warm when subjected to high current. Most high amperage devices (like heaters, stoves, A/C) only use those amperages for a short periods, then drop down to almost nothing. Things get to cool off.

It IS possible to use the inverter for double duty. The Renault Zoe does this and can do 43kW charging on three phase AC. I don't know whether there are technical, economic, or patent issues that keep more companies from doing that.

The Zoe charging efficiency is also terrible on single phase. Add that to the Technical reason column.

mknox · Aug 27, 2015

FlasherZ said:
Because 3 hours is merely an arbitrary number that helped to differentiate those items that cycle their loads

And probably to let things like electric heat and water heaters squeak through as intermittent loads. If there was a sustained power outage, water heaters and baseboard units may run continuously for that long getting things back up to temperature.

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Two Technical Charging Questions

Evbwcaer

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Oba

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FlasherZ

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davewill

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miimura

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mknox

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