GlmnAlyAirCar
Active Member
Although we are not privy to the Tesla charger's algorithm for current throttling, it is likely it works something like this:
First, a little electrical circuits 101. (I realize I am applying DC circuit equations to an AC circuit here but, due to the primarily resistive load of the batteries, this should be a reasonable conceptual approximation.)
V=IR (Voltage is current * resistance)
P=VI (Power is voltage * current)
Now, power dissipated in the branch circuit (circuit breaker, wire to the 14-50, the 14-50 outlet itself, and the UMC) is what needs to be monitored. Power = heat. Too much heat = fire. So, power must be controlled.
Combining the above gives us P=I^2*R. Since power goes up with the square of current, reducing current even a little can significantly reduce heat. Since the charger can control the resistance in the circuit, this is the only action it can take.
Of course, the charger can't measure power directly. It only knows input voltage (V) and the current (I) drawn from the circuit. It can calculate the power that the charger uses but not the power dissipated in the circuit. So, how does the charger know if any power is getting dissipated in the circuit itself?
Let's say the charger notices a voltage drop. It could be a result of heat in the circuit causing its resistance to go up. Or it could be caused by any number of other issues cited above (loads in the neighborhood, appliance kicking on, etc.). Well if the charger notices a voltage drop, it could respond by momentarily reducing its current draw. Since the resistance (R) in the branch circuit is constant over a short time period, a reduction in current should result in an increase in voltage. If this happens, it is likely there is a wiring issue in the branch circuit. However, if voltage does not increase, the issue is likely somewhere else in the grid.
So, if the charger detects the first situation (perhaps by performing multiple tests over an interval), it could then decide to permanently reduce current.
Again, this is just my speculation as to how this works but if I were designing such a system this is how I would do it.
First, a little electrical circuits 101. (I realize I am applying DC circuit equations to an AC circuit here but, due to the primarily resistive load of the batteries, this should be a reasonable conceptual approximation.)
V=IR (Voltage is current * resistance)
P=VI (Power is voltage * current)
Now, power dissipated in the branch circuit (circuit breaker, wire to the 14-50, the 14-50 outlet itself, and the UMC) is what needs to be monitored. Power = heat. Too much heat = fire. So, power must be controlled.
Combining the above gives us P=I^2*R. Since power goes up with the square of current, reducing current even a little can significantly reduce heat. Since the charger can control the resistance in the circuit, this is the only action it can take.
Of course, the charger can't measure power directly. It only knows input voltage (V) and the current (I) drawn from the circuit. It can calculate the power that the charger uses but not the power dissipated in the circuit. So, how does the charger know if any power is getting dissipated in the circuit itself?
Let's say the charger notices a voltage drop. It could be a result of heat in the circuit causing its resistance to go up. Or it could be caused by any number of other issues cited above (loads in the neighborhood, appliance kicking on, etc.). Well if the charger notices a voltage drop, it could respond by momentarily reducing its current draw. Since the resistance (R) in the branch circuit is constant over a short time period, a reduction in current should result in an increase in voltage. If this happens, it is likely there is a wiring issue in the branch circuit. However, if voltage does not increase, the issue is likely somewhere else in the grid.
So, if the charger detects the first situation (perhaps by performing multiple tests over an interval), it could then decide to permanently reduce current.
Again, this is just my speculation as to how this works but if I were designing such a system this is how I would do it.