# Precharge resistor: which value should I take?



## DIYguy (Sep 18, 2008)

I think you are right. 750 to 1 kohm.


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## major (Apr 4, 2008)

Tyn245GL said:


> The reason I ask is that my supplier says I need to use a 30 - 50 Ohms, 25W resistor. But that would result in something like 7 Amps peak precharge current and 2450 Watts peak power !


Hi Martijn,

Does the controller command the closure of the contactor? If so, it may be timed so it needs a faster charge rate. The supplier may know what he is talking about.

For instance, I have used controllers (350V) needing 1 Ohm precharge. Used a 90 watt resistor.

Regards,

major


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## Tesseract (Sep 27, 2008)

Tyn245GL said:


> ...
> Which value in Ohms and Watts should I choose for my precharge resistor?


Whatever the controller manufacturer recommends. Seriously. Which is...




Tyn245GL said:


> The reason I ask is that my supplier says I need to use a 30 - 50 Ohms, 25W resistor. But that would result in something like 7 Amps peak precharge current and 2450 Watts peak power !


Maybe (ie - I'm not checking your math) but it is for a very brief period and many power resistors have a pulse rating so you might be perfectly fine. Typically, for precharging a capacitor, it is acceptable to overload them by 10x to as much as 100x their continuous rating. You could get away with a much lower power resistor if you went with 750 ohms rather than somewhere between 30 and 50, sure, but you can also overload more if the duration is shorter (i^2t rating is what is important at this point).




Tyn245GL said:


> My feeling is that I am doing something wrong in these calculations... But what...?


You aren't integrating the power over time.


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## madderscience (Jun 28, 2008)

I believe curtis recommends a precharge resistor of 750 ohms at 10 watts. However the specific resistance isn't terribly critical; anything anywhere near 1K ohms would do the job; as long as the wattage was in the range of 10. 

Alternatively, a 15 watt, 110V light bulb apparently works well too.


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## Tesseract (Sep 27, 2008)

madderscience said:


> I believe curtis recommends a precharge resistor of 750 ohms at 10 watts. However the specific resistance isn't terribly critical; anything anywhere near 1K ohms would do the job...


Sure, that's what Curtis recommends, and for their controllers it isn't at all critical because precharge time is left entirely up to the end user... but the OP, here, is running a 350VDC pack so, probably not a Curtis 

EDIT - I finally got around to clicking on the link in the first post - http://liionbms.com/php/precharge.php - and I give it a thumb's up for clarity and accuracy. Nice article.


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## Tyn245GL (Oct 12, 2009)

major said:


> Does the controller command the closure of the contactor?


No the controller (DC-AC inverter in my case) does not control contactor closure. That will be managed by the Lithiumate BMS system, which monitors the precharge curve and will only close the contactors after the precharge cycle has finished. So timing is not directly dependant on the resistor value. The precharge time is fully controlled by the Lithiumate's safety control. (Which is very good of course - it also monitors if something goes wrong in which case it will not close the contactors and generate an alarm signal).



Tesseract said:


> but the OP, here, is running a 350VDC pack so, probably not a Curtis


I have a MES-DEA TIM-600 inverter. MES-DEA only provides info related to their own Zebra battery system which has an integrated precharge system. Therefore MES-DEA can not advice a precharge resistor value to me, like Curtis will always be able to do for their controllers. However, I will not use Zebra; I will use LiFePo4, so I have to find out myself what the best resistor value would be. 
I think I'll just try several values: first take a 750 Ohm / 25W (with high peak Wattage cpability) and see how long precharge will take. If that takes too long I will try a lower Ohm value. Would that be a good approach?


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## kittydog42 (Sep 18, 2007)

Tesseract said:


> Sure, that's what Curtis recommends, and for their controllers it isn't at all critical because precharge time is left entirely up to the end user...


I once worked on a Corbin Sparrow for a customer which was factory fitted with a custom 156V rated Curtis 1231C. Not many Sparrows came with that controller. Anyway, they wired the system with only one contactor and with the precharge resistor connected across all the time, so the duration really must not be critical at all for their hardware.


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