Tesla SDU capacitor size / precharge

Post Reply
ev340
Posts: 36
Joined: Sat Mar 20, 2021 12:06 pm

Tesla SDU capacitor size / precharge

Post by ev340 »

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.
User avatar
joromy
Posts: 367
Joined: Fri Jun 28, 2019 12:56 pm

Re: Tesla SDU capacitor size / precharge

Post by joromy »

If you use same as Tesla does, it should be properly? I my model S pack it was 23ohm.
Have used 50ohm myself (50W)
Thomas A. Edison “I have not failed. I've just found 10,000 ways that won't work"
Isaac96
Posts: 650
Joined: Sat Oct 05, 2019 6:50 pm
Location: Northern California, USA
Contact:

Re: Tesla SDU capacitor size / precharge

Post by Isaac96 »

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?
ev340
Posts: 36
Joined: Sat Mar 20, 2021 12:06 pm

Re: Tesla SDU capacitor size / precharge

Post by ev340 »

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 🤣
User avatar
joromy
Posts: 367
Joined: Fri Jun 28, 2019 12:56 pm

Re: Tesla SDU capacitor size / precharge

Post by joromy »

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.
Thomas A. Edison “I have not failed. I've just found 10,000 ways that won't work"
ev340
Posts: 36
Joined: Sat Mar 20, 2021 12:06 pm

Re: Tesla SDU capacitor size / precharge

Post by ev340 »

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?🤣
js1tr3
Posts: 39
Joined: Thu Jun 13, 2019 11:28 am
Location: A2 usa
Contact:

Re: Tesla SDU capacitor size / precharge

Post by js1tr3 »

The panasonic capacitors are marked 430V 150uF x3
Attachments
20220404_231535.jpg
20220404_231549.jpg
20220404_231518.jpg
1988 Porsche 911 targa with warp11 motor. Soon to be Tesla SDU.
alexbeatle
Posts: 60
Joined: Sun Jan 03, 2021 6:12 am

Re: Tesla SDU capacitor size / precharge

Post by alexbeatle »

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?
User avatar
joromy
Posts: 367
Joined: Fri Jun 28, 2019 12:56 pm

Re: Tesla SDU capacitor size / precharge

Post by joromy »

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?
Thomas A. Edison “I have not failed. I've just found 10,000 ways that won't work"
User avatar
catphish
Posts: 433
Joined: Fri Oct 08, 2021 11:02 pm
Location: Dorset, UK

Re: Tesla SDU capacitor size / precharge

Post by catphish »

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.
alexbeatle
Posts: 60
Joined: Sun Jan 03, 2021 6:12 am

Re: Tesla SDU capacitor size / precharge

Post by alexbeatle »

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
Post Reply