JoeS wrote:There are four ways that the car turns off charging -
1. When it is fully charged, the OBC sends a shutoff signal to the EVSE which stops the power flow
By this time, the charge current has reduced to a small fraction of rated current. But the principle is still the same (see below).
2. When timed using the Remote, presumably the Remote receiver sends a signal to the OBC which then sends the shutoff signal to the EVSE
My understanding is that the EVCU (or whatever controls charging) disconnects a relay (K1 below) that was putting a 1.3 kΩ (R2 below) load on the pilot signal, so the pilot signal jumps from a peak of +6 V (charging) to a peak of +9 V (not charging, vehicle ready). (The diode ensures that the pilot signal always troughs at -12 V).
From Wikipedia's SAE J1772
This presumably triggers the EVSE to open its contactor (not shown above), which is the equivalent of pulling the plug. But I also assume that before the EVCU drops the relay, it will ramp the charging power down, say over about 10 mains cycles (~170 ms, doesn't have to be a whole number of mains cycles). This allows the energy in the PFC inductor to fully flow to the battery, and the mains current is nearly zero by the end. So now after the relay is dropped and the EVSE drops the mains contactor, nothing bad happens.
3. Depressing the pushbutton on the J1772 connector sends the shutoff signal to the EVSE
The proximity signal (from the pushbutton) usually doesn't connect to the EVSE (it seems that some do, most don't). The pushing of the button changes some voltages at the vehicle, which is detected by the EVCU, which again ramps down the charge current and opens the relay.
4. Input power is removed from the EVSE, either by the use of a timer or simply pulling out the EVSE wallplug.
In this case, the EVCU gets no warning, and can't ramp the charging power down. The PFC inductor stores considerable energy, and now one end of it is effectively open circuit. The inductor tries to keep the current flowing, because that's what inductors do, and it will attempt to do this by creating a high (kilovolt) potential at the now open circuited end. Usually, there are parts that are designed to absorb this pulse.
There have been a few posts which condemn unplugging the EVSE.
I'm probably the major proponent of this condemnation, and my concern is based on experience with a completely different EV charger, so this should be taken with appropriate scepticism.
does the OBC signaling back to the EVSE either do a zero-crossing timing for power cutoff or some other scheme which gradually removes power and avoids the inevitable inductive voltage spike on the input circuitry?
As per the above, my understanding is that it gradually reduces charging power, though over a time scale that is fairly short by human perception.
As I wrote the above, I note:
[ Edit: "voltage" -> "pilot signal" ][ Edit: "±6 V" -> "peak of +6 V"; added sentence about the troughs always being -12 V. ]
- As I wrote above, there are usually parts that are designed to absorb the "pull the plug" transient. In the case of the Elcon/TC chargers I'm familiar with, these components appear to be rather under-sized, so they tend to fail in such a way that they are ineffective, but still allow the charger to operate. One would hope that the iMiEV's OBC would have more robust parts.
- Assuming these parts designed to absorb the transient have failed, then in order for the small blue capacitors to fail, the transient has to be transferred from the input stage through the IGBTs, transformer, and rectifier to the blue capacitors. I'm unsure how believable this transfer is.