openinverter forum

I have put this question in the getting started section as I didn’t want to clutter the Tesla board.

Despite extensive googling and searching this forum and others I cannot find what Value capacitors are in the SDU I’m trying to calculate what value precharge resistor I need with a volt battery to get a precharge around 4.5 seconds.

From my searches it appears you can throw in roughly 30 - 100 ohm and it won’t cause any issues?

But I’d like to try to do things properly and get as longer charge time as possible if I can.

If you use same as Tesla does, it should be properly? I my model S pack it was 23ohm.
Have used 50ohm myself (50W)

I'm running a Leaf precharge resistor with my SDU - 40 ohm IIRC, precharges pretty darn fast.
Why do you prefer a long charge time?

joromy wrote: ↑Thu May 20, 2021 1:41 pm If you use same as Tesla does, it should be properly? I my model S pack it was 23ohm.
Have used 50ohm myself (50W)

Thank you for the info
I think it will be more or less correct, but Tesla batteries are a slightly lower peak voltage than the Vauxhall’s so the delta across the contactor would also be bigger, however I don’t know how much of an effect that will have on the current hitting the capacitors .

Isaac96 wrote: ↑Thu May 20, 2021 3:36 pm I'm running a Leaf precharge resistor with my SDU - 40 ohm IIRC, precharges pretty darn fast.
Why do you prefer a long charge time?

Thanks for the help
I gather that it’s the inrush of current which can damage the Capacitors, by extending the time I think I also lower the current? Therefore extend the lifetime or mitigate the risk to the capacitors.

I may tune it down if the 5 seconds annoys me, but certainly to start with I am happy being cautious if it stops me popping a motor.

But I actually know so little it’s scary I’m reading and learning as much as I can but there are a lot of gaps

ev340 wrote: ↑Thu May 20, 2021 6:49 pm
Thanks for the help
I gather that it’s the inrush of current which can damage the Capacitors, by extending the time I think I also lower the current? Therefore extend the lifetime or mitigate the risk to the capacitors.

The main reason for precharge is to save the main contactor, and the resistor have to be big enough to prevent precharge relay arcing/burn.

joromy wrote: ↑Thu May 20, 2021 7:24 pm

ev340 wrote: ↑Thu May 20, 2021 6:49 pm
Thanks for the help
I gather that it’s the inrush of current which can damage the Capacitors, by extending the time I think I also lower the current? Therefore extend the lifetime or mitigate the risk to the capacitors.

The main reason for precharge is to save the main contactor, and the resistor have to be big enough to prevent precharge relay arcing/burn.

Thank you yea that makes sense.
I have read about inrush current to capacitors being a problem? But i only ever hear it mentioned in relation to inverters? Am I hunting capacitor values which don’t exist?

The panasonic capacitors are marked 430V 150uF x3

joromy wrote: ↑Thu May 20, 2021 1:41 pm If you use same as Tesla does, it should be properly? I my model S pack it was 23ohm.
Have used 50ohm myself (50W)

So in the 75kWh pack =>
14modules * 25.2Vmax per module = 352.8Vmax per pack
Imax = 352.8V / 23ohm = 15.34A
So the the pre-charge contactor would just need to handle 15.3A peak. Correct?

alexbeatle wrote: ↑Tue Jun 28, 2022 7:15 am
So in the 75kWh pack =>
14modules * 25.2Vmax per module = 352.8Vmax per pack
Imax = 352.8V / 23ohm = 15.34A
So the the pre-charge contactor would just need to handle 15.3A peak. Correct?

Yes, looks sane to me. If the capasitors is seen as full short?

I haven't measured the capacitance, but I am precharging my SDU through a 25R resistor. That's a 12A peak current. I'm charging through a contactor rated for 10A continuous, 30A peak.

joromy wrote: ↑Wed Jul 06, 2022 6:02 pm

alexbeatle wrote: ↑Tue Jun 28, 2022 7:15 am
So in the 75kWh pack =>
14modules * 25.2Vmax per module = 352.8Vmax per pack
Imax = 352.8V / 23ohm = 15.34A
So the the pre-charge contactor would just need to handle 15.3A peak. Correct?

Yes, looks sane to me. If the capasitors is seen as full short?

That would be the nature of the capacitor under DC - short when fully discharged, open when fully charged

Hi,
I am learning about this topic, i wanted to understand if it is an electrolytic capacitor?
Thanks

Only film caps are used nowadays for better reliability. Capacity is not important, ripple current rating is. And that is always quite poor for elcaps.