openinverter forum

I am wondering how other people deal with the situation that the difference between nearly flat and full charge on a 96s pack could be 100v, and that the precharge control is based upon the voltage reaching a certain voltage to engage the main contactor.

the current method of control means that in order to get the main contactor to work at the lower pack charge level the switch voltage for main contactor is say 297v, but a full pack could be 403v, resulting in a 100v surge.

Could it be that the main contactor control works by monitoring rising voltage, and fires when the voltage hasn't risen any more for a period of time ?

Or have i misunderstood how this works or how to configure it ?

FFMan wrote: ↑Fri Apr 14, 2023 4:07 pm ....

Could it be that the main contactor control works by monitoring rising voltage, and fires when the voltage hasn't risen any more for a period of time ?

Or have i misunderstood how this works or how to configure it ?

I simply use 2s delay between precharge and main contactor firing. This means my precharge resistor needs to be rather low resistance - 30R. But it is a short overload only.
OI uses voltage measurement up to 75% to confirm precharge was performed succesfully.

yeah i thought of a delay, and i know others do it manually using position 1 & 2 on the ignition switch, all of which is good, but is jsut a way of getting round the limited design in the inverter board. I have wired it just now to use the key, but if my wife drives it, i need to ensure she pauses between position 1 & 2 which i can't guarantee would always be remembered as it's counter intuitive to our other petrol car.

FFMan wrote: ↑Fri Apr 14, 2023 6:36 pm yeah i thought of a delay, and i know others do it manually using position 1 & 2 on the ignition switch, all of which is good, but is jsut a way of getting round the limited design in the inverter board. I have wired it just now to use the key, but if my wife drives it, i need to ensure she pauses between position 1 & 2 which i can't guarantee would always be remembered as it's counter intuitive to our other petrol car.

I use a PIC12F chip to cycle through its timer and releases darlington transistor which provides GND to precharge and DC contactor relays. That way i provide 12V from the key and starter trigger and GND from the time delay circuit.
That worked for me good for like 3 years now.

Indeed the current precharge method dates from the time where LFP batteries were more of a thing with their flat voltage profile. On Touran I've set udcsw to 300V and otherwise rely on precharge to happen so quickly via the 39R resistor that I can't turn the key fast enough to get the surge.

A dV/dt method would certainly be preferable and could even work without configuration.

On Audi I let the VCU send the start signal via CAN as soon as the difference between inverter and BMS voltage drops below a threshold.