Taupilz wrote: Maybe the capacitor has a decreased capacitance and is not 800 microfarad due to aging?
If it has, it will only make the pre-charge process faster. It may have excess leakage, but I don't expect that from a film capacitor (if that's what it is).
if they are the same, and still the EV-ECU receives only 342, when the battery voltage is 354 at the moment, maybe i can conclude that there is something wrong with the measuring circuit.
I note that there is always some leakage / power drawn by the high voltage circuit, so the capacitor/condensor voltage will never reach exactlt the battery voltage. I believe that the pre-charge resistor is 24Ω, which is pretty low, so it should reach very close to the battery voltage. I don't know how close it typically gets, or how close it has to be before the condenser charge timeout trouble code is avoided. I would expect at least a 5% gap; it takes some 3 time constants for a capacitor to charge to 95% of the input voltage via a resistor, and that's neglecting any loads/leakage. Though the time constant here is small, around 25 ms (assuming a total bus capacitance of just over 1000 μF, or 1 mF).
I imagine that something similar is going on in the measuring circuit, the op-amp comparator receives about 4VDC (reduced by a factor of 90) from the capacitor, compares it with the 4VDC from the battery, and if the resulting voltage is different from 0V the output is set to "1" and then throw the P1A15 code.
I would think that it's just a buffer circuit, so that other parts of the circuit don't load the high value resistors. Often, both the positive and negative from the thing being measured (the capacitors in this case) are subtracted, so it's a differential amplifier. So the output is proportional to the difference between the inputs, which means proportional to the capacitor voltage. The comparison between battery and capacitor voltage would then be digitally inside the ECU (possibly the BMU but I don't know). But you may be right, it might be measuring the voltage difference between capacitor and battery (so across the main positive contactor, I think).
is it a good idea, or waste of time??
I really don't know; you'll quite possibly learn something, but be very careful with safety. You probably want a multimeter that can record maximum and minimum voltages, because pre-charge is over in well under one second. It might still not be fast enough to catch the true maximum. You will probably need special high voltage probes to use an oscilloscope safely.
i also see that user czerodk has posted a graph from an oscilloscope with exactly the same values that i have, 342V when charging, and 2V after discharging, strange or?
As Kiev has stated, the interesting question is why it goes to 2 V after the charge. Is it because the ECU decided the capacitor voltage wasn't high enough, so the pre-charge is aborted, or because the capacitor voltage was fine (maybe 342 V is high enough) and the voltage collapses because the main contactor doesn't work? [ Edit: duh, see next post. ] The latter could be because of burned contacts, failed coil, no drive to the coil, or wiring to the coil. Or it could be a failed measurement. I would not expect the capacitor voltage to collapse very quickly if the main contactor fails, unless some HV load comes on. The bleed resistors probably take at around 30 seconds to bleed the capacitor voltage to a safe level, and minutes to drop down to 2 V,