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Discussion Starter · #1 · (Edited)
I am creating a 48v system, which needs some precharging to start up nicely.
Now I am using precharge resistors reused from EVs.

I see online available 25W resistors, size them around 90 Ohms, and it will work well.
But what I see from the EVs is for example resistors of 23 or 34 Ohm, rated from 25W to 50W.
However, especially the 23 Ohm one, rated at 50W, at a 400V pack in the Tesla, it would get something like 6000W briefly.
I looked up the datasheet and it says for maximum 5 seconds you can multiply the rating by 5, so 250W.
I know the 6000W will be very very briefly, but still I am a bit confused that they design it like that.

Probably something that I don't get.

The others are Mitsubishi and Volkswagen, both around 33 Ohm, and the Volkswagen one has the same package as online available 25W resistors.
Mitubishi one is labeled at 40W.
Anyway, wether it is 50W or 25W, they all briefly take a beating because current is still pretty high at 300V or 400V right?
 

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I usually use one of the HS series by Arcol (formerly several different names). They had a 25x rating for peak power; I don't see it mentioned in the latest "datasheet". Usually a HS100 (nominally 100 W) will come in under that limit. These are aluminium clad, bolt down types. They don't need a heatsink, because of the short duration of the power pulse.

A pre-charge resistor that explodes is not fun, and stinks for a week. Plus, there could be danger with wire ejected. So I would not exceed the rating.

On the old datasheets, there were graphs of peak energy(?) that I admit I don't understand. It may be that that's how Tesla get away with appearing to exceed the ratings.
 

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Discussion Starter · #3 · (Edited)
Yes it probably works something like this. I mean it obviously works because these resistors are coming out of EVs of which we know the higher voltage.

Anyway I will stay on the safe side, and couple 2 of them in series for my 48v setup.

There is also no economic reason to reuse them since a resistor of 25W in the amount of Ohm that you want is cheap.

Edit: Now I got the online ordered resistors in the mail. They are rated at 25W just like the VW resistor, but really a lot smaller. Almost a quarter of the size, regardless of Ohm rating.
So it feels better to use the EV ones. And they are not comparible because I am sure these would not last when their rating is exceeded so greatly in an EV.
 

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I use an old kettle element as a pre-charge resister - or in the USA you don't have kettles the heating element out of hot water tank
About the right resistance and VERY robust -
 
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