I am in the processes of designing / building my high voltage junction box for my 450h project. I managed to get three Gigavac 242MAB contactors for a decent price, so I'll be using those in the build. My question is, how do you go about sizing the HV wiring that goes with everything? What gauge wire should I use? Where can I buy it? Also, what size resistor should I use? It looks like folks generally use a 400A fuse as the main fuse, should I go with that?
I understand that the wire gauge is dependent on the expected current flow, but what kind of current should I expect from the build? ~350V battery from a Chevy Volt, the wiki for the 450h (https://openinverter.org/wiki/Lexus_GS450h_Inverter) mentions that the motor is capable of 250kW, so that would mean:
P = VI -> I = P/V => 250000 [W]/350 [V] = ~714A
So should I be disigning for 714 Amps? That's already more than the 400 A fuse.
Any thoughts?
Re: HVJB Build
Posted: Sat Dec 05, 2020 8:53 am
by rstevens81
Remember the 400a is the continuous rating, I would be surprised if any of us could draw 400a continuosly at 350v without either crashing or melting something.
Have a look at this data sheet and pay attention to the melt time curves.
Enjoy
Re: HVJB Build
Posted: Sat Dec 05, 2020 8:36 pm
by bobbyj_408
rstevens81 wrote: ↑Sat Dec 05, 2020 8:53 am
Remember the 400a is the continuous rating, I would be surprised if any of us could draw 400a continuosly at 350v without either crashing or melting something.
Have a look at this data sheet and pay attention to the melt time curves.
Enjoy
Nice! Thanks for the info.
Based on the datasheet, it looks like the fuse can handle 700A for quite some time without melting. I re-watched Damien's video with his Bexus drive (see below), and I notice that he does reach 400A+ for a few seconds when he's accelerating to highway speeds. Hypothetically speaking, if I were to build by box with this fuse, and I were to hypothetically have some fun and punch the accelerator, I would expect a high surge of current, right? At that point I should expect something like the full 700A, right?
All this is just to make sure I get the correct components, and even make sure I get the correct wire gauge for the power lines.
Thanks
Re: HVJB Build
Posted: Sun Dec 06, 2020 5:39 pm
by rstevens81
For reference a model s seems to use a 700amp fuse...
Can anyone give any advice on how to size a precharge resistor for this build?
Re: HVJB Build
Posted: Mon Dec 14, 2020 7:12 pm
by jnsaff
For the precharge you want to allow around 2A of current. So at 400V you'd want a 220 Ohm resistor rated for about 100W. Tesla people seem to be more sporty with a 100 Ohm resistor IIRC but this is the ballpark.
Also I too have a 450h sitting in my garage so happy to exchange notes. Mine is currently stuck 'cause of life' but should be unblocked over xmas.
Re: HVJB Build
Posted: Mon Dec 14, 2020 7:44 pm
by bobbyj_408
jnsaff wrote: ↑Mon Dec 14, 2020 7:12 pm
For the precharge you want to allow around 2A of current. So at 400V you'd want a 220 Ohm resistor rated for about 100W. Tesla people seem to be more sporty with a 100 Ohm resistor IIRC but this is the ballpark.
Also I too have a 450h sitting in my garage so happy to exchange notes. Mine is currently stuck 'cause of life' but should be unblocked over xmas.
Very nice! Thanks! This is exactly the advice I was looking for. Out of curiosity, where does that 2A spec come from? Also, this is a super noob question, but how would I know when precharging is done to close the main contactor?
Also, where could I but precharge resistors?
Re: HVJB Build
Posted: Tue Dec 15, 2020 1:58 pm
by jnsaff
The 2A is something I remember from JB's conversion course.
As for when to close the main contactor, in general you have three options.
1. If you have a current sensor (the ISA shunt for example) in your HVJB then you can close the main contactor when current draw has fallen to almost zero (take into account some consumption for the inverter itself etc).
2. If you have voltage sensors on the inverter side and the difference between the battery and inverter side is small (say within 10%).
3. The ghetto option: manually measure how fast either of the above happens in your setup, add a safety margin and use that timing as your precharge timer.
The 3rd option is obviously dangerous as you will not detect for example the precharge contactor not closing for whatever reason. Magic smoke with maybe a big bang.
It could be useful to think about disabling driving before main contactor has closed as even JB has started driving with only pre-charge contactor closed and some amount of magic smoke coming from the HVJB.
The resistors you can put on the wall of the HVJB or on the inside of the lid.
Re: HVJB Build
Posted: Tue Dec 15, 2020 5:00 pm
by bobbyj_408
Awesome! Ok, I'll take the 2A recommendation. That's the most solid spec I've seen, so I'll run with it.
1) Yeah, I think this might be the best option. I'll make sure to add in a shunt. I have noticed that not everyone has one. I assume they have another mechanism to measure current. For example, the Tesla LDU can measure current through the inverter.
Great idea to disable the drive!!
Also, that was a typo, but thanks for letting me know about the placement. I meant to write, where could I buy a precharge resistor?
Re: HVJB Build
Posted: Tue Dec 15, 2020 6:37 pm
by jnsaff
You can get the resistors any electronics reseller.
