openinverter forum

Hello,
I am converting an old Mini using the following parts:
- Outlander rear motor
- Outlander rear inverter
- Zombieverter V2.20A
- BMW 540e battery 12 kWh with S-Box
I have taken a big step forward now, vehicle is running and I could drive the first 100 meters or so. Really happy. But of course still a lot to do.

Now for my question:
If I got it right, precharge needs some time to charge the capacitors of the inverter, and if the voltage behind the precharge circuit reaches the given setting, the positive contactor pulls in and bypasses the resistor. In my case the precharge goes VERY fast, I'm uncertain if this is correct.
I turn the ignition on, UDC reads 332 V and UDC2 something around 0. Then I turn the key to the start position and UDC2 immediately goes up to 332 V. I can hear two clicks from the S-Box, but they're within a fraction of a second. I was recording UDC2, there's no ramp-up visible, the graph shows a step. Is this correct? I was expecting a much longer time for the process.

Thanks,
Stefan

Grab some CAN logs. The precharge resistor's resistance in the BMW are very low so it can be very very fast. The Outlander rear inverter is a small capacitance really.

Hi I am running almost the same setup s-box seems to switch very quickly to, not really gave it much thought.
so looking to see what others say

Thanks for your fast reply. Unfortunately I'm not able to do CAN logs at the moment. I think I have to deal with this topic sooner or later for bad or for worse... I was afraid that I'll weld my contactors. So probably the fast precharge is normal and no problem I hope.
Another question: what is the correct value for udcsw? It should be lower than the minimum voltage of the pack I assume. Voltage range of the BMW pack is Min. 269 V – Max. 403 V, so would 260 V be the correct value?

snelly wrote: ↑Tue Feb 04, 2025 7:00 pm Hi I am running almost the same setup s-box seems to switch very quickly to, not really gave it much thought.
so looking to see what others say

In my Mitsu components installation i use 2s to precharge when main contactor comes online and 500ms pause after which precharge relay is turned off. I see a full precharge in my setup in 1s to about 300Vdc. I use Golf hybrid precharge resistor and relay. I think i measured about 300R resistance.
I intended to use 1s but i found out sometimes on EVSE PP charge signal after 1s precharge and 500ms delay charge could not start. I had to increase the time so charger electronics would be able to turn completely on.

In Audi A2 I run the contactors similar to Zombie. It's Nissans 39 Ohm precharge resistor and it takes only a few 100 ms. Click, click.

Which sort of makes sense: https://www.digikey.se/en/resources/con ... ass-filter

If you go with 700 uF (a typical capacity in most inverters) and 39 Ohm resistor the time constant is about 200 ms (1/5 Hz)

johu wrote: ↑Wed Feb 05, 2025 6:04 am In Audi A2 I run the contactors similar to Zombie. It's Nissans 39 Ohm precharge resistor and it takes only a few 100 ms. Click, click.

Which sort of makes sense: https://www.digikey.se/en/resources/con ... ass-filter

If you go with 700 uF (a typical capacity in most inverters) and 39 Ohm resistor the time constant is about 200 ms (1/5 Hz)

Like you say, however even though precharge is fully completed in 200ms it seems some components do not agree and i had to increase this time for 100% performance when charging.