# Precharging controller



## ElectriCar (Jun 15, 2008)

I use 4 of the 750's to precharge my 1231 along with 10000Uf more I added. Takes about 10 seconds I believe but I use a voltage monitor to precharge it automatically to idiot proof it. If you don't do this and like me forget to or don't wait long enough, you cause premature contact failure. I've welded one and blew another one out, just replaced them and added that feature. 

See my conversion thread below, lots of things I've done in there, many that aren't "mainstream" but I think are most helpful & may interest you as well. I update it regularly as I progress. And I have NO bms on my lithium pack, instead devising or adopting others ideas to protect my pack.


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## EVfun (Mar 14, 2010)

I used a 7.5 watt light bulb, the little round top Edison base bulbs where the glass is barely larger in diameter than the base, as a precharge resistor for my Curtis 1221B. I usually left the precharge on all the time with the main contactor and ksi relay both open when the vehicle was off. If I needed to kill everything I just unscrewed the bulb.


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## gojo (Feb 1, 2011)

ElectriCar said:


> I use 4 of the 750's to precharge my 1231 along with 10000Uf more I added. Takes about 10 seconds I believe but I use a voltage monitor to precharge it automatically to idiot proof it. If you don't do this and like me forget to or don't wait long enough, you cause premature contact failure. I've welded one and blew another one out, just replaced them and added that feature.
> 
> See my conversion thread below, lots of things I've done in there, many that aren't "mainstream" but I think are most helpful & may interest you as well. I update it regularly as I progress. And I have NO bms on my lithium pack, instead devising or adopting others ideas to protect my pack.


I love simplicity. 
The bulb tells you when your caps are charged, and you can use it as a switch by just unscrewing it. 
Why did you leave it on all the time?
I have been leaving my 750 ohm resistor on all the time, because I have no way of know how long it takes to charge the caps in the controller, but that gives me problems with my Iota DC DC because the resistor will also charge those caps all the time. 
One solution may be to just shut everything down, and use the 7watt bulb to charge up both the controller and DC DC at the same time, whenever I use the car. My main concern in doing it this way is to do no harm to my controller. It only appears to take a few seconds to load up my Iota capacitors, so even if it took a minute to charge up both the controller, and DC DC, it would be the most simple way of dealing with this problem. The light bulb would let me know when things were charged. 
The nice thing about the light bulb is one can see it working. The 750 ohm I just have to believe or wait till the contacts go bad in the controller. 
I choose a 7 watt just because it was there. Why did you choose a 7.5 watt? Curtis warns customers to only use a 750 ohm resistor. I have saw wiring diagrams were people use 100 ohm resistors, and shut off the controller when not in use. I suspect it would be hard to test ohms on a light bulb because it would change when they get hot, and if there isn't enough resistance, one could damage the contacts in the controller.


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## EVfun (Mar 14, 2010)

I'm not going to respond to anything until you get that font size back to normal. I feel like I'm being shouted at, and that doesn't make me feel very helpful. My head hurts.


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## gojo (Feb 1, 2011)

EVfun said:


> I'm not going to respond to anything until you get that font size back to normal. I feel like I'm being shouted at, and that doesn't make me feel very helpful. My head hurts.



Sorry. I have old eyes, and so do some of the people I send e-mails to, so I thought I was just making it easier for others to read. You will know about these things some day (hopefully).
Is this ok? I am at 14, but I can still drop it to 12 and see ok with readers?
Actually I had already dropped to 16 before you posted. For some reason it shows up larger when I post then when I print it on Word Perfect. 

Thank you for pointing this out, and my apologies to all.


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## DJBecker (Nov 3, 2010)

Precharge power and thermal

Design point: 
A 200V pack (that's 48 cells at 4.16V per cell) and a 10,000uF capacitor bank:
0.5*0.010*40,000 = 200 Joules or 200W power for a full second!
Charge = 0.010uF * 200V = 2 amp-seconds (AKA 2 columbs)

That's a fair amount of power to put into a resistor. And with the time
scale of a precharge in a few seconds, the resistance and power rating
and is less important than the thermal mass.

It surprises many people that changing the resistor doesn't change the heat problem. It doesn't. Roughly half of the power still ends up as heat in the resistor. A higher resistor just makes the job of the precharge relay easier, at the expense of taking longer to finish the charge.

But if you have the contactor at the motor controller, there is a much better approach. Treat the precharge as a power supply problem. Just deliver one or two amps for a second, at a reasonable efficiency. That's a much easier task. A buck converter can do this with very little heat, while having a low initial current and avoiding the slow tail that takes most of the time.

Note: Even with logic-level control of the precharge circuit, you should still put in a precharge relay as a safety against the drive transistor shorting. But the relay can be tiny since it handles zero switching current and only a brief, modest current during operation.


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## EVfun (Mar 14, 2010)

gojo said:


> love simplicity.
> The bulb tells you when your caps are charged, and you can use it as a switch by just unscrewing it.
> Why did you leave it on all the time?
> I have been leaving my 750 ohm resistor on all the time, because I have no way of know how long it takes to charge the caps in the controller, but that gives me problems with my Iota DC DC because the resistor will also charge those caps all the time.
> ...




It is better, at least the new size gets a whole post in the window without scrolling.

1. I left it on all the time because it's a pain to keep unscrewing the bulb. 

2. The Iota should not be on the controller side of the main contactor. It prevents precharging because there it starts up under load. Nothing should be on the controller side of the main contactor except the controller cable and the precharge wire. I have always left my DC to DC running all the time. I've used Todd, Iota, and Kelly converters.

3. The precharge resistance value isn't very important. The lower the value the greater the heat it will make. This is generally not an issue, but if the controller fails you could be dissipating full pack voltage across it. 120 volts across a 750 ohm resistor is about 19 watts. 120 volts across a 100 ohm resistor would be 144 watts -- that would be a large resistor. The light bulb changes resistance depending on the voltage across it, so this isn't an issue. 

4. The bulb size was chosen by measuring the voltage drop across it when the contactor was open. I wanted less than 5 volts to minimize inrush. It is important to use a ksi relay if you leave the controller precharged. If you have a precharged controller and ksi powered the car will move about 6 inches if you push the throttle. You would also need a larger light bulb to fully precharge (about a 25 watt bulb.)


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## gojo (Feb 1, 2011)

EVfun said:


> It is better, at least the new size gets a whole post in the window without scrolling.
> 
> 1. I left it on all the time because it's a pain to keep unscrewing the bulb.
> 
> ...


Good stuff. 
This is just what I need to know. I already have the 750 ohm, 25 watt precharge resistor and a KYI switch. I kind of learned by trial and error, to only have the control cable, and precharge on the motor side of my switch.
I am thinking the only thing I need to do different at this point is a higher wattage bulb for my Iota precharge. The 7 watt brings my pack voltage down from say 127 volts (present state of charge) to between 79 and 88 volts ( the voltage pulsates for some reason). So simple math shows there is still about 50 volts arcing across my DC to DC switch with the 7 watt precharge bulb.
I have checked on C7 base bulbs ( the size we both are using ) and I can get 15, 25, and 40 watt sizes to replace the 7 watt. I think a maybe the 40 watt would bring my pack voltage across the switch up closer to the 132+ pack voltage and not arc across my DC DC switch so badly. The only concern here is if the 40 watt would stress the caps. You mentioned a 25 watt for charging the controller, it might be smart to try 25 watts for my DC to DC first and see what kind of an arc I get. 
There seems to be no arc when I screw in the 7 watt precharge light bulb, I would have to experiment with the 40 watt bulb, and listen for an arc in the socket. I have no reason to leave the Iota on constantly. I have a wimpy 200 watt Victor DC DC to the car’s key switch to activate the main relay switch, and to use as a back-up for the Iota if needed to get home on. .


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## Ziggythewiz (May 16, 2010)

I don't believe a higher wattage charging circuit will raise your precharge voltage any, only a lower resistance will do that.

You shouldn't get much of an arc when connecting any load with significant resistance. Big arcs come from low resistance contacts, as in your contactor.


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## gojo (Feb 1, 2011)

Ziggythewiz said:


> I don't believe a higher wattage charging circuit will raise your precharge voltage any, only a lower resistance will do that.
> 
> You shouldn't get much of an arc when connecting any load with significant resistance. Big arcs come from low resistance contacts, as in your contactor.


Perhaps my post is a bit confusing. I am trying to precharge both my controller, and my Iota DC to DC. The controller is no problem as long as I don’t run anything but the cables to the controller, and the precharge (750 ohm 25 watt resistor) on the motor side of my main relay switch.
In order to do this, I need to precharge my DC DC on the battery side of my main switch. When I turn on the manual switch for the Iota, I get a major arc, so I added a 7 watt light bulb to precharge the caps in the Iota. The arc isn’t as strong, but it will still ruin the switch I am using. So, when I am talking about increasing the wattage, I am talking about using a lower resistance light bulb. My thinking is the lower resistance precharge will lower the voltage difference across my DC DC switch. Right now with the 7 watt bulb I have about a 50 volt difference. If I use a lower resistance bulb (25 or 40 watt) wouldn’t that divert more of the energy away from my switch? If I can get the difference down to around 15 volts there shouldn’t be much of an arc. 
The arc only occurs when I close the switch. I think I am in effect making a snuber circuit using the capacitors in the Iota, and the light bulb for my resistor. The light also pulsates ( about 10 volts ). If I can catch the pulsation at a higher voltage there seems to be less of an arc.


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