# Precharge resistor size



## baerfoot (Jan 2, 2011)

My contactor (EV200) has been sticking recently. I even replaced it, and now have problems with the new one.
I've read some of the threads on precharge resistors but still don't have a good idea if I am using the right size. The altrax documentation says 1000 ohms for 72 Volts. This is what I have, but I don't leave mine on all the the time. I have a relay set up to turn it on when I go to the first key position. 
Any ideas on the best size resistor for a quick precharge that won't damage the relay?


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## Siwastaja (Aug 1, 2012)

The problem is elsewhere. Either your connections somehow skip the precharge resistor entirely, or you have a defective contactor. For 72V, even 1 ohm is enough to limit the current to something the contactor can handle. 

One possible explanation is that 1000 ohm is too much and precharge is too slow to complete, but a decent controller shouldn't enable the contactor before the precharge is really completed.


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## TheSGC (Nov 15, 2007)

What controller are you using? Some controllers have built in precharge controls while others don't. I know the Curtis DC controllers don't have a precharge control, and neither do the Kelly's. 

1000 ohm seems a bit high for a precharge, my 96 volt system had a 500 ohm for precharge and it would take few seconds to charge up the capacitors, but it was also enabled so once the power was applied the first time I was all set.


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## Ziggythewiz (May 16, 2010)

A higher ohm precharge may be better for the controller, but is often too much and worse for the contactor.

A lower ohm and higher watt resistor will make the precharge faster and fuller which is better for the contactor. I'd shoot for ~3 seconds.


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## baerfoot (Jan 2, 2011)

Ziggythewiz said:


> A higher ohm precharge may be better for the controller, but is often too much and worse for the contactor.
> 
> A lower ohm and higher watt resistor will make the precharge faster and fuller which is better for the contactor. I'd shoot for ~3 seconds.


BTW my controller is: Alltrax 7245 (72 vdc/450 amp)
I had a look at the voltage as I was precharging. I have not done this before, as my in dash voltmeter starts at 50 volts. The voltage climbs in a curve to about 10 volts in 3 seconds and then levels off at 16.2 volts at about 8 seconds. Even leaving it on for a couple of minutes in the precharge position the voltage does not climb further. And by the way, my system is 75 volts, 25 calb cells. So when I go to the next key position I go to 83 volts. 
When it gets to 16.2 volts are the caps fully charged, or still taking current?


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## major (Apr 4, 2008)

baerfoot said:


> BTW my controller is: Alltrax 7245 (72 vdc/450 amp)
> I had a look at the voltage as I was precharging. I have not done this before, as my in dash voltmeter starts at 50 volts. The voltage climbs in a curve to about 10 volts in 3 seconds and then levels off at 16.2 volts at about 8 seconds. Even leaving it on for a couple of minutes in the precharge position the voltage does not climb further. And by the way, my system is 75 volts, 25 calb cells. So when I go to the next key position I go to 83 volts.
> When it gets to 16.2 volts are the caps fully charged, or still taking current?


Hey baer,

Nope, fully charged is when the caps are at battery voltage or 83V. That's why your contactors are failing, like a 70V difference between the caps and battery. If the caps stop charging at 16V, then the precharge circuit (resistor, relay and wire) is dropping the 67V. For 1K, that is like 67 mA which seems high for a stand-by current draw on the controller. Try a much smaller Ohmic value resistor, like 50Ω. See if that get you up to 60 or 70V in a timely fashion.

major


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## baerfoot (Jan 2, 2011)

major said:


> Hey baer,
> 
> Nope, fully charged is when the caps are at battery voltage or 83V. That's why your contactors are failing, like a 70V difference between the caps and battery. If the caps stop charging at 16V, then the precharge circuit (resistor, relay and wire) is dropping the 67V. For 1K, that is like 67 mA which seems high for a stand-by current draw on the controller. Try a much smaller Ohmic value resistor, like 50Ω. See if that get you up to 60 or 70V in a timely fashion.
> 
> major


Would the relay or wire size make a difference?


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## major (Apr 4, 2008)

baerfoot said:


> Would the relay or wire size make a difference?


Sure, if it adds significant resistance to the circuit across the main contacts. Got a photo?


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## baerfoot (Jan 2, 2011)

major said:


> Sure, if it adds significant resistance to the circuit across the main contacts. Got a photo?












I think the wire is 14 awg
The relay is a fairly standard automotive relay


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## major (Apr 4, 2008)

baerfoot said:


> I think the wire is 14 awg
> The relay is a fairly standard automotive relay


The resistance of several feet of #14 wire and the contacts of the typical automotive relay should present a negligible voltage drop below one amp. It may be something about your implementation. Got a photo? Got a schematic? 

A typical automotive relay may have contact rating of only 32VDC. Even though closing the relay with the 1K resistor will conduct 83 mA to start, it is still closing 83V capacitive. Damaged contacts in the relay could result in high resistance.

Try a precharge without the relay. Just short it with a test lead jumper. See what voltage the controller settles to.

Take care when experimenting with the controller. Maybe even jack up the wheels so an unexpected event doesn't result in injury or damage.


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## baerfoot (Jan 2, 2011)

major said:


> The resistance of several feet of #14 wire and the contacts of the typical automotive relay should present a negligible voltage drop below one amp. It may be something about your implementation. Got a photo? Got a schematic?
> 
> A typical automotive relay may have contact rating of only 32VDC. Even though closing the relay with the 1K resistor will conduct 83 mA to start, it is still closing 83V capacitive. Damaged contacts in the relay could result in high resistance.
> 
> ...


Interesting, before your post came in I had the same thought and bypassed the relay with a jumper. Slightly higher reading at 16.4 volts.
Tomorrow I'll pick up a different resistor and try that.
I attached an image to an earlier post, not sure why it is not showing up.
Check the image from this page in my blog.
http://www.electricmetro.blogspot.com/2008/02/what.html
BTW: I have been remiss in updating my blog. The battery setup now is totally different, with all cells under the hood in the front.


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## major (Apr 4, 2008)

baerfoot said:


> Interesting, before your post came in I had the same thought and bypassed the relay with a jumper. Slightly higher reading at 16.4 volts.
> Tomorrow I'll pick up a different resistor and try that.
> I attached an image to an earlier post, not sure why it is not showing up.
> Check the image from this page in my blog.
> http://www.electricmetro.blogspot.com/2008/02/what.html


Just a thought. Have you verified the resistor is in fact 1000Ω? It kinda is acting like 1000KΩ. This actually happened to me once. I stopped by to help a guy with a step-up in Kansas City. It was really acting weird on precharge actually cycling on and off. It was an AC drive. Turns out he had used KΩ resistors in place of the recommended Ω values. Check it out forthehellofit


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## baerfoot (Jan 2, 2011)

major said:


> Just a thought. Have you verified the resistor is in fact 1000Ω? It kinda is acting like 1000KΩ. This actually happened to me once. I stopped by to help a guy with a step-up in Kansas City. It was really acting weird on precharge actually cycling on and off. It was an AC drive. Turns out he had used KΩ resistors in place of the recommended Ω values. Check it out forthehellofit


Had a look, it is a 1000Ω resistor, but worth checking.


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## baerfoot (Jan 2, 2011)

Something interesting I found here:

http://www.evdl.org/archive/index.html#nabble-td438538

There is not a whole lot of info to go on here but i have an idea... I have an altrax 7245 controller in my motorcycle and on the altrax website they recommend a 1k ohm resistor for the precharger when run at 72 volts. So i used a 1k ohm resistor out of a electronics hobby kit and watched the voltage (on the altrax monitoring program) rise to only 14 volts across the controller and it stayed there. This would be very harmful to you controller over time because when you close the main contactor about 60 volts of electricity would come rushing in which can fry your controller. I then tried a 40 watt light bulb and i saw the voltage rise to the full pack voltage over the course of about 3 seconds. I measured the resistance across the bulb and it was only 0.02 k ohms which i believe is 20 ohms right? So i installed the bulb as my precharge resistor. The only reason i could think of why the 1k ohm didn't work is that altrax's recommendation was just wrong? Maybe it was just me b! 
ut i don't think so... So check the voltage as it rises in the altrax monitoring program if you can... 

I also power the contactor in my motorcycle by a separate 12 volt battery i did not follow altrax's wiring diagram exactly. Do you power your contactor though the controller? if so you really have to worry about that diode across the contactor. I do not know how to check to see if a diode is still working correctly so i couldn't help you there.... 

See ya 
Tim Gamber


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## major (Apr 4, 2008)

baerfoot said:


> Something interesting I found here:
> 
> http://www.evdl.org/archive/index.html#nabble-td438538


I'm beginning to think the same thing. The 1000Ω is just plain wrong. I looked at the Alltrax spec and it is <35mA stand-by. But even that at 1000Ω is a 35V drop. You and Tim there are seeing more like >60mA and little voltage left to charge the caps. Get a lower Ω resistor.


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## Ziggythewiz (May 16, 2010)

These high values are creating a voltage divider with the controller, so unless it has much more resistance than the precharge, it can't get close to full.

By spec (750ohm IIRC) the curtis only charges to 70%. I think I went with 2 500s instead to get the precharge down ~3 sec and up to 90%.


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## baerfoot (Jan 2, 2011)

I went with 75 Ohms, the shop did not have a 25 OHM. 
With that in place I go up to 75 v in about 3 seconds, and I can see it on my in dash volt meter. 
Hopefully this will resolve my contactor issues. I actually put back in my old one which has stuck a couple of times, but I'll see how it goes. I have a manual disconnect in place if needed.
Thanks for everyone's input.


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## major (Apr 4, 2008)

baerfoot said:


> I went with 75 Ohms, the shop did not have a 25 OHM.
> With that in place I go up to 75 v in about 3 seconds, and I can see it on my in dash volt meter.
> Hopefully this will resolve my contactor issues. I actually put back in my old one which has stuck a couple of times, but I'll see how it goes. I have a manual disconnect in place if needed.
> Thanks for everyone's input.


That's great you have this resolved. As an Alltrax customer you could inform Alltrax about this and perhaps save future users the pain  It seems like a possible safety issue to me.


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## brainzel (Jun 15, 2009)

I wonder if Alltrax doesn't have such a table like Curtis.

75 Ohms sounds small to me, but if it works fine, than it's all good


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## major (Apr 4, 2008)

brainzel said:


> I wonder if Alltrax doesn't have such a table like Curtis.
> 
> 75 Ohms sounds small to me, but if it works fine, than it's all good


Alltrax has a table here: http://www.alltraxinc.com/files/Doc100-045-A_DWG-AXE-No-Reverse-wire-dia.pdf 

That is what we're saying is incorrect 

The Ohmic value of the precharge resistor has to be low enough to allow the voltage drop across the resistor to be small enough to get the controller caps charged to maybe 80% or higher of the supply (battery) voltage. If you wait 3 time constants or longer, the voltage drop across the precharge resistor will be the Ohmic value of the precharge resistor times the current thru the controller (assuming the controller is the only load after the contactor). The current through the controller is what I call stand-by current. The controller is energized but not commanded to start the motor. This stand-by current is comprised by the current needed by the controller logic and probably to a larger extent, the leakage and/or bleed down current of the bus caps. Most controllers will have a bleed down resistor internally so that the DC bus caps do not remain charged for long periods after the controller is turned off or disconnected. Industrial VFDs require this. I've seen plenty of EV drives have this feature also.

The Alltrax controller had a specification for this stand-by current of <35mA. From baerfoot's observations, it would seem to be about double that. 

Perhaps Curtis has a much lower stand-by current draw on their controllers and can therefore use a higher Ohmic precharge. For his application 75Ω does not sound too low. In fact I thought he should go 50Ω and even 25Ω was suggested. I've used precharge resistors as low as one Ohm (for certain drives; that is not a general recommendation).


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## Ziggythewiz (May 16, 2010)

I'll haven't looked at the standby current for my curtis in a while, but it's enough to drain the caps in 3 s. Most of the recommended values are grossly overinflated, which at first glance seems conservative but is probably ultimately worse.

My ideal solution would be a thermistor that starts near the recommended value and ramps down to near 0 within seconds. I was trying to source a suitable part but just throwing in lower valued ceramics was too easy to let me waste further effort on the ideal solution.


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## poprock (Apr 29, 2010)

Hi, I have this thread as my landing page and re-read it today. I have a similar problem with my AXE 7245 with 7x12 FLA. With the 1000 resistor as supplied with the controller it is showing 10.5-11.5v at rest; 23v with power to first contactor through the 1000 and 86v with open throttle through second contactor activated by footswitch in pot box. Adding a second 300 resistor in parallel, rest is the same,with 73v through first contactor and 86v through second. (10 seconds for each result). Should I connect the 300 permanently, leaving the 1000 as a backup? ( 90v is max current for 72/45).


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## major (Apr 4, 2008)

poprock1 said:


> Hi, I have this thread as my landing page and re-read it today. I have a similar problem with my AXE 7245 with 7x12 FLA. With the 1000 resistor as supplied with the controller it is showing 10.5-11.5v at rest; 23v with power to first contactor through the 1000 and 86v with open throttle through second contactor activated by footswitch in pot box. Adding a second 300 resistor in parallel, rest is the same,with 73v through first contactor and 86v through second. (10 seconds for each result). Should I connect the 300 permanently, leaving the 1000 as a backup? ( 90v is max current for 72/45).


It sounds like you're precharging around 2 contactors.  Got a wiring diagram of your circuit?


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## poprock (Apr 29, 2010)

major said:


> It sounds like you're precharging around 2 contactors.  Got a wiring diagram of your circuit?


 No, but I will have to make one. A pic may not show clearly. For the moment, the resistor is across pack + to common of both contactors. First contactor closes by accessory switch, precharge rises to low figure with standard resistor, then with ignition on pot box switch activates second contactor with full power. Diagram & pics following.


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## poprock (Apr 29, 2010)

If you can make sense of this, perhaps the resistor should go to the + of the second contactor?


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## Ziggythewiz (May 16, 2010)

it would work the way you have it, provided the top contactor is energized first, or you could just have the resistor go across the bottom contactor instead of over to the other.

But...you have one contactor backwards. you can't have the - connected to each other or the polarity will be wrong for 1 of them, which may cause a problem if disconnecting under load.


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## poprock (Apr 29, 2010)

Bottom contactor is first from accessory switch. You're right ; will reverse the + & - on the top one then re-visit the resistor issue. Thanks. My lack of expertise is exposed yet again.


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## Ziggythewiz (May 16, 2010)

in that case move the resistor to the top contactor. It has to bridge the one that's turned on last.


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## poprock (Apr 29, 2010)

Hi, Still having issues with my AXE7245. A new problem, it only works intermittently. Have tried three 0-5k pots, all second hand but it is unlikely to have three fail.I replaced the 1K resistor with a .4K and it now goes to 49v in 15s. The green light blinks once then stays strong green. When the potbox is opened fully it goes to strong red until eased back into range. Sometimes the motor will spin and next time nothing. It has full power when it does run. I have hooked up a 7" motor on the ground and it span immediately to high revs. But after turning off the contactors it would not spin again. The contactors are s/h Allbright 12v. Suggestions?


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