# Precharge, what is it, why do I need it, how do I do it.



## TX_Dj (Jul 25, 2008)

Nice! Thanks for the contribution!


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## ClintK (Apr 27, 2008)

TX_Dj said:


> Nice! Thanks for the contribution!


Yes, something I definitely needed help with! Thanks!

Looks like I got a second contactor for Christmas.  Is there anything wrong with using your 2nd to last schematic, and having my second contactor be the "precharge switch"? When I turn the car to "on" it will engage the precharge switch. When I press the pedal my PotBox will engage the 2nd resistance free contactor? As long as I wait a few seconds before hitting the "gas" (what most people probably do anyways), then I am precharging correctly?


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## rfengineers (Jun 2, 2008)

ClintK said:


> Yes, something I definitely needed help with! Thanks!
> 
> Looks like I got a second contactor for Christmas.  Is there anything wrong with using your 2nd to last schematic, and having my second contactor be the "precharge switch"? When I turn the car to "on" it will engage the precharge switch. When I press the pedal my PotBox will engage the 2nd resistance free contactor? As long as I wait a few seconds before hitting the "gas" (what most people probably do anyways), then I am precharging correctly?


The only thing wrong is you are using a very nice, heavy duty contactor to do a job that a much smaller (less expensive) relay could perform. There is not much current flowing in the precharge leg of the schematic. 
The precharge resistor will do a good job of limiting that current to a very small amount, so you don't need a big contactor there.
I am planning on addressing the question of: "Contactors, how many and where" in one of my next "lessons."
In answer to your other question, With a small enough (in resistance value) precharge resistor, a few seconds CAN be enough time. It is all a matter of how low should the voltage across the contacts be before they close (to reduce arcing and contact damage).

Joe


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## stormconnors (Aug 19, 2008)

I am using a 60 watt 120V household lightbulb for my precharge resistor on my 144v EV. The key switch turns on the contactor on the negative side of the pack. The precharge circuit is connected to the hot side of the contactor at the positive end of the pack. Thus, turning on the keyswitch enables the precharge and the lightbulb lights. On the Raptor controller, once precharge is complete the controller turns on the positive contactor. With a 60 watt bulb, this takes about 5 seconds.


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## RIPPERTON (Jan 26, 2010)

Can I precharge my caps with the 12v system during bootup.
If my main contactor low volt (coil) circuit is on a 4 second delay, I switch on the 12v system that parallels though the high voltage circuit via a diode instead of a resistor, or would I still need a resistor.
Then when the main contactor brings on the high voltage the diode stops the 12v system from nuking.


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## Electric Car-Nut (Jul 5, 2009)

The "Pre-charge" is to charge up the capacitors in the controller input circuit to full pack voltage (144 V.?) At a controlled rate and not as an "Inrush" Surge. that is what the resistor or household 120 V. light bulb id there to do. It limits inrush or initial charge-up of the input capacitors as otherwise the initial surge will shorten the life of the power contactor it is connected across. To start charging with only the 12 volt is unnecessary, and potentially a problem because the 12 v. auxiliary supply,(Battery and/or dc/dc) is preferably ISOLATED from the high voltage of the 144 v. traction battery pack. If you have more questions ask them (I just LIVE to give answers.)


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## cloudy9 (Aug 19, 2010)

In case they didn't know Precharge is very important because when current-limits the power source such that a controlled rise time of the system voltage during power up is achieved.When high voltage systems are designed appropriately to handle the flow of maximum rated power through its distribution system, the components within the system can still undergo considerable stress upon the system power up.


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## Woodsmith (Jun 5, 2008)

To get around the correct sequencing of the precharge ono my tractor I have a main isolator and a conventional ignition switch.

The isolator has to go on first or nothing else works.
Then the ignition key is turned to the accessories position which allows power to the instruments and other low current circuits.
Then the key is turned to the running position where the controller main isolator has feed, but is not energised, and the precharge resistors are powered.
This allows the caps to charge up for a while before I flick the key to the momentary 'starter' position. That sets the main relay to latch closed powering the controller and all the high current circuits.

Any loss of power in any part of the circuit will cause the main controller contactor to unlatch but leave the precharge resistors on. A quick flick of the key to the starter position will restart the controller if all is well.

I don't know how useful this would be in a road car but it works well on the tractor.


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## sjc (Oct 28, 2008)

I can't see the schematics in the original posting. Are they still visible to others?


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## RoughRider (Aug 14, 2008)

sjc said:


> I can't see the schematics in the original posting. Are they still visible to others?


i cant see them 

would someone upload them again??


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## surgy (May 15, 2009)

yeah, i cant see any schematics either and i need to learn about precharging.


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## david85 (Nov 12, 2007)

From what I can tell, the photos have been taken down at the source. Since they were only linked to and not hosted directly by the forum server, there isn't much we can do from this end.

It also seems the original contributor hasn't been posting since 2009


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## surgy (May 15, 2009)

Fair enough. Could someone please explain in detail how you would wire the pre-charge resistor? is it just another power cable going to the controller with a resistor connected in-line?


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## major (Apr 4, 2008)

surgy said:


> Fair enough. Could someone please explain in detail how you would wire the pre-charge resistor? is it just another power cable going to the controller with a resistor connected in-line?


surgy,

The manual for your controller will have a wiring diagram. And you can go to controller company websites and download manuals which will show you. And you can use the search feature on this web site to find threads and posts about it. And this thread, a sticky, I think talks about it. http://www.diyelectriccar.com/forums/showthread.php/ev-high-voltage-turning-and-off-25318.html

But I do think some basic diagrams should be here in the EV Information section. 

Regards,

major


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## CaptConan (Jul 12, 2010)

I'm trying to find a relay for the precharge circuit that can "block" the pack voltage (144) but work (close) off of the 12V system. I was hoping to just use an automotive relay for this but from what I can find these are only rating up to about 50 VDC or so. I don't want to have to pay to get another ev200 just to switch 5 amps at 144volts. What is everyone else using for parts?


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## major (Apr 4, 2008)

CaptConan said:


> I'm trying to find a relay for the precharge circuit that can "block" the pack voltage (144) but work (close) off of the 12V system. I was hoping to just use an automotive relay for this but from what I can find these are only rating up to about 50 VDC or so. I don't want to have to pay to get another ev200 just to switch 5 amps at 144volts. What is everyone else using for parts?


Hi Cap,

I don't have a part for you, but a suggestion. Put a contactor like the EV200 in the negative (or mid pack) battery line turned on by the keyswitch. Then when it is turned on, it precharges thru the resistor across the contacts of the positive line contactor. After a few seconds, or when a precharge indicator lamp goes out, have a switch (like on the drive position of the gearshifter) which closes the positive contactor. That way the precharge is off when the keyswitch is off. And the high voltage accessories (DC/DC) can come on ahead of the precharge.

Different controllers can require different precharge circuits. There is no one-size-fits-all diagram.

major


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## CaptConan (Jul 12, 2010)

Right Major, thats pretty much the route I was planning to go, just acquiring parts right now as I'm doing a custom controller and all... what I wanted to get away from is buying ANOTHER ev200 which appear to run about 70-80 bux. There has to be a non-solid state relay out there that can handle 5 amps, block high voltage, and only require 12V to energize. I imagine this fictitious part costing me about $15 or $20. It's just a matter of finding it.


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## CaptConan (Jul 12, 2010)

Well I found two that fit spec, but neither one seems to be for sale anywhere...

www.fujitsu.com/downloads/MICRO/fcai/relays/ftr-j2.pdf

http://relays.tycoelectronics.com/datasheets/ev41a.pdf

I think I should be okay with a relay rated at 125vdc. The current I am "making" is only for a short time anyway and then it's done after a few seconds. Any objections from the field?


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## bluefxstc (Dec 29, 2007)

I
I am using a Mercury switch like these for my heater and am thinking about using one to enable my precharge resistor. I am thinking about using my interior lights to turn on the relay. That way anytime I open the door the precharge is enabled. I would just have to leave the door open for a couple of seconds to make sure it is charged which shouldn't be much of a problem. I tend to fasten my seatbelt before I shut the door so it should be enough time. Given the current the mercury switch will handle (16 amps at 125 VDC) I will probably use a lower resistor value which will make it faster. Also due to the construction of the switch (dipping contacts into liquid mercury which freezes at about -38C) I don't think arcing is as big a problem as it is with a standard contactor.
 
http://www.surplussales.com/Relays/mercury_con.html


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## rfengineers (Jun 2, 2008)

Sorry Guys, I was busy elsewhere and did not notice the graphics were gone.

I will fix it ASAP.

Joe


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## RoughRider (Aug 14, 2008)

Thanks...


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## rfengineers (Jun 2, 2008)

The images have been restored. Sorry for the problem, I'm not sure what happened to the hosting site, but it's gone!

I've uploaded the original images as attachments, so they are no longer dependent on an outside host.

I still have circuit boards and can order more parts if anyone is interested in a step-start controller. 

Also, I would love to hear feedback, positive or negative, from anyone who bought a controller from me and has been using it for a while. I am always interested in improving the design. My original prototype is still working in my Pontiac Sunfire (over 6500 miles under electric power and still running.)

Thanks,

Joe


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## david85 (Nov 12, 2007)

Thanks Joe!


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## cvanliew (Mar 1, 2012)

Hello everyone, so please bear with me as I'm trying to figure out what a precharge circuit looks like physically. I have been reading around and am seeing what I feel are a couple different things.

One suggests drawing the 12v needed for precharge from the 12v battery, but I believe I saw someone advise against this on a page about a Kelly that got fried.

The other, which I'm seeing here, according to the schematics provided, involves rerouting power from the main battery array around the contactor. 

The main questions I have are which is preferable, and if indeed you are drawing off of the main array, surely you cant be using 2/0 wire for the precharge route, so why doesn't the high gauge wire go up in flames from the high voltage instantly? (In my auto class my prof made a point of showing why we need fuses by running 12v through a piece of wire and letting us watch it catch on fire.)

And now the question that's going to show my utter lack of knowledge on the subject of contactors: are the closed manually with a pull string or something or does the inflow of current close the conactor automatically?

Thanks all!


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## Ziggythewiz (May 16, 2010)

A precharge can be as simple as a resistor across the contactor. Or it can be switched, by a simple switch or a timed or measuring circuit.

Mine is a resistor hooked to the contactor and 20 GA running to a switch on the dash. High voltage doesn't burn wire, high current does. High current is not possible when running through a large resistor, which is the whole point of the precharge.

The contactor is closed by a low voltage coil feed (usually 12V, maybe 24) tied to something like a key switch. It takes the low voltage and uses it to close high voltage contacts.


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## cvanliew (Mar 1, 2012)

Thanks for that reply! It helped clear up a lot of the questions I had of what a precharge actually looks like. I'm hoping to get to the electrical work on my car pretty soon so I'm sure there will be plenty more to follow.

Collins


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## Ziggythewiz (May 16, 2010)

Be sure to check your controller for the proper resistor size. For mine it's something like 750 ohm and 20 watts. I think my resistor is about 2" x .5" x .5"


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## argeas (Aug 29, 2012)

Hi Everyone ! i am busy trying to configure a motor, motor controller, throtle and conactor config....... the problem im getting is that the microcontrolerr doesnt like getting from zero to 24v directly... i.e. when i turn on the switch (24v) the controller goes on but is not happy( throtle doesnt work.) but if i gradually increase from zero to 24v around 14 v the controleer comes on! and when i set it to 24v it then works correclty. 

the motor im using is http://www.electricmotorsport.com/store/ems_ev_parts_motors_me0909.php

throtle
http://www.electricmotorsport.com/store/ems_ev_parts_throttles_ezgo_potbox.php

controller
http://www.electricmotorsport.com/store/ems_ev_parts_controllers_pgdrives_4825.php

contactor
http://www.electricmotorsport.com/store/ems_ev_parts_contactors_albright_24.php

and a 10k 5% 5W pre-charge resistor....

ive conected everything as the image above ( first image) 
does anyone know of this problem? 
is this problem the same as the one that the above solution was given for?


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## Ziggythewiz (May 16, 2010)

10K ohm, assuming that's what you meant, is a huge resistor. 24V / 10,000 = eternity. My Curtis specs a very conservative resistor, at 750 ohm. That is too high for a quick precharge, and with lower voltage you would need less resistance.


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## argeas (Aug 29, 2012)

I see what you are saying! i just went with the data sheet specs tho! what i did to make it work is i put a cap in parallel so th voltage to the controller is inputed gradually.... 25v 1mF .. and it to worked (slighlty since the motor would turn and stop , i believe cause the voltage from the batteries drop a bit and theat causes the cap discharge slighlty and hence not operate corectly..) i bypassed that by puting a resistor that stops it at 16V .. and then a switch to bypass the resistor..... the comfortable switching (on) point of the controler is at 16v if it passes to quickly from that point.. i.e. no cap.or limiting resistor and just str8 24v then the controller gives ERROR 12 (12 flashes) something to do with the footswitch... i am really finding hard to understand this manual of the controller... 

oh well i made it work now(kinda) .. but i am going to try what you said! yes it is a 10Kohm resistor i will try the one you recommend and send back some feedback! thanks for the reply!!


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## Electric Car-Nut (Jul 5, 2009)

For your set-up I would expect a One thousand ohm at five watts (1.0 K Ohm 5 W.) resistor not a ten thousand ohm and it needs to supply a limited current to the power input connection on the motor controller for 10 seconds or so before the main power Relay provides full power to that controller connection so that the input filtering capacitors have a "Pre-Charge" (similar to "Inrush" limiting In other applications.) before a large surge charges the capacitors like a bolt of lightning...


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## Ziggythewiz (May 16, 2010)

Even a 1K ohm resistor will make your precharge incredibly slow. Do you happen to know the size of the cap bank in the controller? I would expect you need something on the order of 100 ohm 5W


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## Electric Car-Nut (Jul 5, 2009)

Perhaps two 12 volt automotive light bulbs in series for 24 volts similar to side marker light bulbs (Less than one amp) and they would glow brightly as you apply the power thru them, and when they appear to have dimmed out bypass them for full operating power. That gives a visual indicator of the Pre-Charge taking place. Around here the truck stops sell replacement side marker assemblies with two lamps for reliability. rewire them into series configuration and buy the yellow or amber colored unit and put it where you can see it but not in your line of sight when driving.


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## iruraz (Sep 4, 2012)

> Here is how I did it. I have a Step-Start device that turns on the precharge relay when the start signal is received (the ignition key is turned to the START position). After a time delay the contactor is turned on.
> {see StepStart}


@rfengineers. Is it right to connect pre-charge relay to "start position"? According to this design when driver press throttle pedal with maximum capacity instantly inrush current would appear and it may damage the load. Should pre-charge relay work for once only starting(1)? Or should it work depend to throttle pedal everytime(2) against to inrush current. If case (2) is true pre-charge relay shouldn't wire to "start position". There is a high voltage circuit in the link below. In this circuit pre-charge relay works depends to "run position". Please would you enlighten me about pre-charge relay function? Best regards...

http://www.advanced-ev.com/Circuits/MainHighVoltage.jpg


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## major (Apr 4, 2008)

iruraz said:


> @rfengineers. Is it right to connect pre-charge relay to "start position"? According to this design when driver press throttle pedal with maximum capacity instantly inrush current would appear and it may damage the load. Should pre-charge relay work for once only starting(1)? Or should it work depend to throttle pedal everytime(2) against to inrush current. If case (2) is true pre-charge relay shouldn't wire to "start position". There is a high voltage circuit in the link below. In this circuit pre-charge relay works depends to "run position". Please would you enlighten me about pre-charge relay function? Best regards...
> 
> http://www.advanced-ev.com/Circuits/MainHighVoltage.jpg


The pre-charge circuit should just operate when the system is first turned on, not every time the throttle is depressed. The motor controller stays on during periods when the throttle is not depressed during the drive cycle and therefore stays charged. It is only when the key is turned off that the motor controller will bleed down the capacitor voltage and then need to be re-pre-charged when restarted with the keyswitch.

The pre-charge function is to limit inrush on initial energizing of the motor controller. The controller itself will limit current in the normal operation of controlling the motor including rapid depression of the throttle pedal. 

Number (1) is correct method.

Here is a good paper on it: http://liionbms.com/php/precharge.php


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## iruraz (Sep 4, 2012)

@major thanks for your response. Link is very helpful. But I have a question about after switch turned off.



> It is only when the key is turned off that the motor controller will bleed down the capacitor voltage and then need to be re-pre-charged when restarted with the keyswitch.


Does controller's fully capacitors discharge over itself after turned off? I thought Big load accumulate in capacitors after switch off. May it damage controller's circuit when it discharges?

Best regards...


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## major (Apr 4, 2008)

iruraz said:


> Does controller's fully capacitors discharge over itself after turned off? I thought Big load accumulate in capacitors after switch off. May it damage controller's circuit when it discharges?


There is no surge when a capacitor is disconnected however it may remain charged for a long period, perhaps even after the device has been disconnected. So, such devices as controllers which have large capacitors are designed with bleed down resistors to discharge those internal capacitors in a reasonable period, usually a minute or so. A few watts of power is sacrificed during operation for safety's sake.

Note: When working with such devices it is prudent to check the terminal voltage before applying tools. Most equipment is properly designed with bleed down but there is no law for it in the DIY parts market.


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## iruraz (Sep 4, 2012)

@major thanks for your explanation.


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## wessss77 (Jan 4, 2013)

Okay so this may sound stupid but that's how I feel sometimes around all you guys  but hey I am trying...would use of the new PulsaR (when it is finally ready for primetime) negate much of this since it is supposed to come with recharge and contractors built in as well as the automated switching of for input for charging and a DC-DC also? Not saying I would go with that as I feel when you put all your eggs in one basket, it can take a lot longer to get a full basket back instead of just picking up a couple eggs at the local market.


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## Ziggythewiz (May 16, 2010)

Yes, any controller that uses internal contactors will control its own precharge. In the case of the PulsaR it likely runs on Unicorn blood to make pigs fly as well.


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## Caps18 (Jun 8, 2008)

I would hope that this is the type of thing that the PulsaR will do, so the amateur builder won't need to worry about it anymore.


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## neuweiler (Nov 6, 2012)

Thanks a lot for making things clearer.

One thing I was not able to find yet is a guide or formula on selecting the right resistor for pre-charging. I'm having a hard time finding something appropriate. 

I'm looking for something like this:
*Input*: Traction Pack Voltage, Controller capacitor rating, desired pre-charge time (or a reasonable default)
*Output*: Resistance (Ohm) and power rating (W) of resistor

example (in my specific case) :
Input: 450V, 380uF, ?? sec
Output: ?? Ohm, ?? kW

As you see, I'm not even sure about how long it should take. Are 500ms enough? Or 3 sec ?


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## Electric Car-Nut (Jul 5, 2009)

neuweiler, The formula for "Pre-charge " is the R C Time constant Formula. That is Capacitance of the input capacitor bank totalized in Farads multiplied by the resistance in ohms is the time in seconds to reach 63.7% of the full applied voltage across the capacitors. We want the full voltage so the time needed is at least three times that Time calculated up to five times that Time calculated. Then the full pack voltage without the resistance is connected. But, there is no input surge because the capacitors have been slowly Pre-charged eliminating a major voltage difference when the main contactor is closed. Unless the vehicle is switched off for 10 minutes or longer there is generally no reason to Pre-charge again during a drive. The use of sufficiently rated tungsten filament light bulbs (Preferably heavy duty vibration resistant like "Garage closer, Ceiling fan, or oven and appliance rated bulbs)which offer a regulation of current magnitude during Pre-charge and therefore Pre-charge more quickly, but, at a lower peak current rate, are preferable and also less expensive than a large physical size resistor. You cannot use the "Start" position on the automotive ignition switch because the start is after "ON" and pre-charge will already have been missed. (In quality modern equipment like the controllers from EVnetics, the pre-charge is internal...) Good luck if you need more explanation I am always available for advice thru EVTI.ORG contact page.


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## neuweiler (Nov 6, 2012)

EC nut, thanks a lot! Do you happen to have the formula at hand?
To be honest, the light bulb approach would be a bit too geeky for my taste. I'd prefer a resistor in a cool looking case.
So I guess that the output of this formula would be the resistance and from there you could calculate the power dissipation at full load. But then the next question arises: the time to pre-charge is quite short. If you get 1000W maximum input current, do your really need a 1000w resistor or would it be overkill ? (as it won't be a continuous current).


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## major (Apr 4, 2008)

neuweiler said:


> So I guess that the output of this formula would be the resistance and from there you could calculate the power dissipation at full load. But then the next question arises: the time to pre-charge is quite short. If you get 1000W maximum input current, do your really need a 1000w resistor or would it be overkill ? (as it won't be a continuous current).


This may help. http://liionbms.com/php/precharge.php


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## Electric Car-Nut (Jul 5, 2009)

neuweiler said:


> EC nut, thanks a lot! Do you happen to have the formula at hand?
> To be honest, the light bulb approach would be a bit too geeky for my taste. I'd prefer a resistor in a cool looking case.
> So I guess that the output of this formula would be the resistance and from there you could calculate the power dissipation at full load. But then the next question arises: the time to pre-charge is quite short. If you get 1000W maximum input current, do your really need a 1000w resistor or would it be overkill ? (as it won't be a continuous current).


Neuweiler, I gave you the formula but you didn't recognize it in text. "the R C Time constant Formula. That is Capacitance of the input capacitor bank totalized in Farads multiplied by the resistance in ohms is the time in seconds to reach 63.7% of the full applied voltage across the capacitors. We want the full voltage so the time needed is at least three times that Time calculated up to five times that Time calculated." In algebraic form that is RC=Tc or resistance in Ohms times the Sum of the input capacitors (Because there are usually several you just add them together.) equals the "Time Constant for that amount of resistance and capacitance. and the "TC" in seconds is the time to charge the capacitors to 63.7% of the applied voltage then in the same number of additional seconds it charges up 63.7% of the remaining voltage and in five times the charge reaches 99.5% so we consider the capacitors fully charged. As a rule of "Thumb" we usually use about 470 to 750 ohms. Also the actual average dissipated power is only a fraction of a watt however we use a physically substantially larger resister for physical strength and vibration resistance from driving on cobblestone roads and such. Typical is a five watt resistor supported with a nylon clamp and bolted to a strong surface. For the same reason I usually connect it up with (American Wire Gauge) # AWG-12 stranded wire. The voltage does not affect the resistance for a particular time ... nor does the amperage capability of the system as that current does not pass thru the resistor. we just allow the capacitors to charge up slowly for five to ten seconds to minimize the surge of charging up discharged capacitors when first applying the full battery pack voltage. (It saves the relay some wear and tear upon switching on...) I'm here if you have more questions.


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## Electric Car-Nut (Jul 5, 2009)

major said:


> This may help. http://liionbms.com/php/precharge.php


Davide, only one problem, the current does not flow into the positive terminal of the relay, the Electrons do. but current flows the other direction (Especially for those of us educated in "Classic Electric Engineering" I could treat you to the 30 minute lecture on current flow but.....and the fee is $50 and you won't have to feel ignorant again. 8^)


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## major (Apr 4, 2008)

Electric Car-Nut said:


> Davide, only one problem, the current does not flow into the positive terminal of the relay, the Electrons do. but current flows the other direction (Especially for those of us educated in "Classic Electric Engineering" I could treat you to the 30 minute lecture on current flow but.....and the fee is $50 and you won't have to feel ignorant again. 8^)


I assume you refer to Davide's quote here:


> In that case, the contacts of the contactors are polarized (one terminal is labeled '+'); connect the contactor so that normally (that is, while discharging) the current flows into the '+' terminal.


 He is correct and consistent with EE convention with regards to current flow. You should follow his advice if you wish for your contactor to work properly.

You can teach how you wish, but I, and I suspect Davide, do not feel ignorant about it 

For easy reference see: http://en.wikipedia.org/wiki/Electric_current


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## Electric Car-Nut (Jul 5, 2009)

Major, and Davide, The discrepancy comes from engineers speaking to technicians. 
As Engineers tend to use, Conventional Current Flow (From Positive to Negative). 
Alternatively most Technicians use Electron Current Flow (From Negative to Positive) thus the simple statement, "the current flows into the terminal marked (+)" is inadequate to describe the direction of current flow. The relay is not a source, it is marked similarly to a "Load" and the (+) of the pack is connected to the (+) of the contactor.


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## major (Apr 4, 2008)

Electric Car-Nut said:


> Major, and Davide, The discrepancy comes from engineers speaking to technicians.
> As Engineers tend to use, Conventional Current Flow (From Positive to Negative).
> Alternatively most Technicians use Electron Current Flow (From Negative to Positive) thus the simple statement, "the current flows into the terminal marked (+)" is inadequate to describe the direction of current flow. The relay is not a source, it is marked similarly to a "Load" and the (+) of the pack is connected to the (+) of the contactor.


Davide stated it correctly. Why do you choose to insult him (and me) and confuse the issue for the reader?


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## neuweiler (Nov 6, 2012)

major, ec-nut: Thanks a lot. It's much clearer to me now. On http://liionbms.com/php/precharge.php in the lower section (Resistor), it's well explained.


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## Nabla_Operator (Aug 5, 2011)

*MOSFET) Re: Precharge, what is it, why do I need it, how do I do it.*



In this DIY-page I describe a $5 mosfet relay for use in a 600V precharge circuit. I am asking for everybodies suggestions. Please take a look.


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## jose luis rm (Oct 1, 2014)

thank you a lot man, i have a question:
¿for a curtis 1239-8501 controller and a 144 volt battery, wich are the correct values for the precharge resistor and the the precharge relay?

havea nice day


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## major (Apr 4, 2008)

The Curtis AC controllers handle the precharge internally meaning there is no need for an external precharge resistor.


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## bodo-ev (Mar 22, 2018)

Hi guys,

does anyone know the ratings of the Curtis 1238-7501 precharge resistor? By a mistake I've burnt it, and the value is not readable any more.


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## eram111 (Apr 5, 2018)

Can I use an inrush current limiter in place of a resistor?


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## brstina (Oct 3, 2018)

Did anyone tried to put contactor coil parallel with controller (connected to B+ and B-). That way when start switch is on, power goes through resistor until voltage in capacitors raises enough to activate contactor. Time needed to start the car will depend on how drained capacitors are and in case that they are full, car will start immediately. Switch would need to be double throw switch and will need to disconnect resistor and contactor coil, so that car could be turned "off" before capacitors deplete.

Sorry not able to upload picture at this time.


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## kennybobby (Aug 10, 2012)

Here's a scope trace showing the effect of the precharge resistor to limit the inrush current into the big capacitor in the motor controller. 

Using a 1mV/Amp current probe after the resistor on the positive lead coming into the capacitor with 2mV per division scale factor and 20msec per division time base. The current spikes up initially, then tapers off as the capacitor is being filled and it's voltage comes up.

The resistor is a JRM wire-wound SVM, R = 24 Ohms rated at 40W. The capacitor, C= 1020uF rated at 420VDC. The Mitsubishi iMiev pack voltage is 360VDC.

The resistor has to dissipate the energy stored in the capacitor over the precharge time, which is considered to be 5 time constants, where one time constant is R*C = 24R*1020uF =~25msec, so Tpc = 5 time constants is 0.125 sec. This can be seen in the scope trace where the current drops back down to nearly zero.

The energy stored in the cap charging up to 360 Volts is E = 0.5 *C * V^2 =~ 66 joules.

So the resistor power during precharge is E/Tpc = 66j/0.125s =~ 530 Watts , which is a short-time overload factor of about 13x, but for only 0.125 seconds. 

The JRM datasheet indicates they are designed to handle 10x overload for 5 seconds, so appears to be some margin.

The initial current spike is 14 amps in about 2 msec, which is a high power load of 5400 W. The average power over the first 20 msec is about 2900 W, so the resistor device must be very rugged and able to handle these sort of massive power overloads and survive.


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## Rayco (Jul 31, 2021)

surgy said:


> Fair enough. Could someone please explain in detail how you would wire the pre-charge resistor? is it just another power cable going to the controller with a resistor connected in-line?


It is simple ,wire the recharge resistor Across the Main terminals of the main contactor, there is a great video on you tube ,look up DIY pre charge resistor ,he explains it in layman's terms, installed on go cart!


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