# Pre-Charger Indication



## Ziggythewiz (May 16, 2010)

KISS

You should have a voltmeter anyway


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## mora (Nov 11, 2009)

I use voltmeter too. I've wired green and red leds to dashboard to indicate which contactor receives +12V. But that doesn't guarantee that precharge goes on. Few times I've had contactor receive +12V but it doesn't close the circuit. Of course it was the precharge contactor that failed to operate. I can spot this kind of failure easily by reading voltmeter whenever I turn the key. No rising voltage = turn off and try again.

Don't worry, controller won't explode first. You'll likely weld your contactor to closed state after few clicks of no precharge. Ask me how I know, hehehe. My controller has been abused when it comes to capacitors. And still EV200 contactor failed first.

This precharge thing should be better in OpenRevolt. It is automated in 1000A version but it isn't finished yet.


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## Ace_bridger (Nov 22, 2011)

Thanks for responses. I have a suitable voltmeter and will mount it in the cab with the ammeter.


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## Ziggythewiz (May 16, 2010)

mora said:


> This precharge thing should be better in OpenRevolt. It is automated in 1000A version but it isn't finished yet.


Is it automated with a timer or sensors? I like the manual approach (I flick a $5 switch, wait 6-8 sec, and turn the key when I think the charge is high enough; with my controller it only gets to 70%) because if anything fails I don't turn the key. If the controller were to ever start full on, it would eat the precharge without moving the car.

If I were to do an automated circuit, I would want it sensor based with multiple switches to ensure the voltage is getting up and get it there quicker. I've seen a number of complicated precharge circuits, but they still wait 10+ seconds. Why not switch in lower value reistors as the precharge progresses to keep the current up and get there faster?


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## mora (Nov 11, 2009)

I remember it being some kind of comparison between voltage and time. First voltage rises fast and when caps are full enough rise time gets slower. When voltage rise is slow enough in some pre-set time window then controller would close the main contactor. Low value resistors must have been used as actual precharge times were pretty short when being tested. MPaulHolmes will likely give better description of this precharge circuit.


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## Ace_bridger (Nov 22, 2011)

I guess waiting for dV/dT to slow to a pre-determined rate would be sensible.

To keep the thread on track...I'll use a voltmeter to ensure that I have 144V (ish) at the controller when I turn on the ignition.


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## Ziggythewiz (May 16, 2010)

Sounds good. I don't know about the ReVolt, but on my curtis with the specified resistor I only get up to about 90V of 120 nom


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## PStechPaul (May 1, 2012)

There are some sophisticated ways to deal with precharging and I'm working on some with my DC-DC boost converter. But a very simple approach would be to use an inexpensive ($5) 24 VDC relay like this:

According to the specs, it has 650 ohms resistance and a pull-in voltage of about 80%, or about 19.2 volts (30mA). So you could add a series resistor of (120-19)/0.03 = 3.4k (5W) so the relay will pull in when the charge voltage is close enough, and wire the contacts in series with the ignition switch so it won't energize the main contactor until the voltage is at a safe level. It only wastes about 3 watts, but you could disconnect it with an auxiliary contact or other means. 

It can also be accomplished with a solid state relay which draws much less current. And a 120 VAC relay coil will also work on DC with less current, but you need to add the right resistance. 120 VDC coils exist but are rare.

Those are the simple methods. I'm planning on using a MOSFET with a zener and two resistors, for a cost of a couple dollars and only a couple milliamps draw. And depending on the amount of current involved you might be able to drive a MOSFET with PWM at a constant current until the capacitors are charged. That would be the fastest way to a full charge, and would be an interesting microcontroller project.


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## johnsiddle (Jun 22, 2011)

Hi Guy's
I am about to buy a suitable resistor for pre-charge, what value of resistor do you guy's use for use on 150v?
John


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## Ziggythewiz (May 16, 2010)

Depends on the max current you want to charge with, and how fast. Does your controller's manufacturer have a recommendation?

I think mine's a 750 ohm 20 watt


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## johnsiddle (Jun 22, 2011)

Ziggythewiz said:


> Depends on the max current you want to charge with, and how fast. Does your controller's manufacturer have a recommendation?
> 
> I think mine's a 750 ohm 20 watt


The controller will be a home designed/built I just wanted some idea of how you guys do it.

If yours is 750ohm then the charge current is about 200ma which is about 3watts at 150v.
The CR time (using 2000mfd as the capacitor value) is about 1.5 seconds, does that sound about right for your system?


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## Ziggythewiz (May 16, 2010)

That would be 30 watts. My system is 120, so it's around 20 watts. Of course, that's peak, and it ramps down quickly. Mine takes about 8 seconds.


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## johnsiddle (Jun 22, 2011)

Ziggythewiz said:


> That would be 30 watts. My system is 120, so it's around 20 watts. Of course, that's peak, and it ramps down quickly. Mine takes about 8 seconds.


You are right I lost a zero somewhere, possibly your capacitor is much larger than 2000mfd cuz 2000mfd X 750 x 1.44 = 2.16 seconds. 
Does your system just wait for 8 seconds or can you see the voltage take 8 seconds to max out?

I am surprised that the current is so low but that is a good thing.

I have just realised that I bought two kilovac relays, one to switch out the pre-charge resistor when I only needed one and a smaller relay.
I think I will remove one and keep it as a spare. 
Thanks for the info.
John


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## Ziggythewiz (May 16, 2010)

Yes, I've heard curtis uses a larger cap bank than some. I forget the actual spec though. There's nothing automatic about mine (yet). I flick a switch when I get in the car and can see the voltage rise on my main pack voltmeter. After 6-10 sec I'll turn the key to start the car and go.

Having two contactors is not a bad thing. Put one on the negative leg powered by a small switched relay, and the other on the positive with precharge resistor (hardwired if you want). Then you'd flick a switch before turning the key similar to mine, but you'd also have full controller isolation when everything is off.


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## johnsiddle (Jun 22, 2011)

Ziggythewiz said:


> Yes, I've heard curtis uses a larger cap bank than some. I forget the actual spec though. There's nothing automatic about mine (yet). I flick a switch when I get in the car and can see the voltage rise on my main pack voltmeter. After 6-10 sec I'll turn the key to start the car and go.
> 
> Having two contactors is not a bad thing. Put one on the negative leg powered by a small switched relay, and the other on the positive with precharge resistor (hardwired if you want). Then you'd flick a switch before turning the key similar to mine, but you'd also have full controller isolation when everything is off.


No I have already removed one of my contactors and replaced it with a relay, I will keep the other one as a spare.

Thinking about it, the pre-charge relay can come on with the ignition and remain on since once the main contactor engages the resistor is shorted out anyway.
I might sense the powerpack voltage with a resister and transistor and use it to automatically engage the main contactor.


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## dragonsgate (May 19, 2012)

What kind of switch should be used for a simple on / off to the precharge resistor? Would it be best to put the switch on the battery side or the controller side?


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## Ziggythewiz (May 16, 2010)

You should use either a switch rated for your pack voltage (in DC), or any automotive switch and a relay rated for the pack voltage.

That said...because the current is rediculously small, I'm using a 24V DC switch from Walmart. 

The switch (or relay) is in series with the precharge resistor, and wired across the contactor, so it's directly between the battery and contoller sides...that's the point, to safely bridge the gap (and potential difference) between an unpowered controller and the battery pack.


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## PStechPaul (May 1, 2012)

Ziggythewiz said:


> You should use either a switch rated for your pack voltage (in DC), or any automotive switch and a relay rated for the pack voltage.
> 
> That said...because the current is rediculously small, I'm using a 24V DC switch from Walmart.
> 
> The switch (or relay) is in series with the precharge resistor, and wired across the contactor, so it's directly between the battery and contoller sides...that's the point, to safely bridge the gap (and potential difference) between an unpowered controller and the battery pack.


If I understand this correctly, the switch is used to make the connection from the battery pack to the controller, after the capacitors have charged up where the voltages are nearly equal, to limit the surge current that would otherwise flow. The precharge current is not very large, but the contacts of the switch need to be able to hold the full pack voltage at the start of the precharge. And during operation, I would think the contacts would need to supply the full battery current for the charger, which could be 10 amps or so for a 1000 watt charger.

I assume this is just for the charger controller, and not the motor controller. For the motor controller, you'd need a contactor rated at several hundred amps.


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## Ziggythewiz (May 16, 2010)

You're switching the connection of the precharge resistor from the battery pack to the motor controller (for simplicity here we usually just call the motor controller 'controller' and charge controller 'charger'). You're right about the contacts needing to hold back full pack voltage, but the current will never be large because the precharge resistor is in series. After the precharge you'll switch on the main contactor (that the precharge resistor bridges) and virtually all current flow will go through there.

I'm not aware of any chargers needing an external precharge, but DC-DC converters often do.


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## dragonsgate (May 19, 2012)

I already know some of this but am asking anyway to make sure I know what I think I do. Ace asked a question and I don’t know if Ace got anything from it but I didn’t. If Ace doesn’t mind I will ask some questions with diagrams starting simple and build up to something more sophisticated with everybody’s help if you are willing to play the game. So I will ask my first question again with a little more detail and pictures. We all like pictures so get out your crayons and see what happens. We all know that a resistor of proper specification for voltage and controller can simply be bridged from one post to the other on the main contactor. This will keep the capacitors full and ready to go but it will also keep a bit of constant power to the controller even when the main contactor is open. A switch is needed to break the power to the resistor. Where dose the switch go? Before the resistor on battery side or after on controller side. It seems to me that on the controller side would allow a fairly small switch because of the "resistance". We want a relay that we can control from the drivers seat. Will one type of relay cover all setups say from 96 volts to 144 volts? If everyone is patient with me and help get through this we will then establish how to tell when the capacitors are ready with something a little more sophisticated than counting to ten.


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## Ziggythewiz (May 16, 2010)

It doesn't matter where the switch (or relay) is, it gets the same current either way. A relay rated for 144V will work fine for 96 etc.

If you monitor with a Voltmeter, which was the suggestion to Ace, it should be hooked on the controller side of the contactor.


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## johnsiddle (Jun 22, 2011)

dragonsgate said:


> I already know some of this but am asking anyway to make sure I know what I think I do. Ace asked a question and I don’t know if Ace got anything from it but I didn’t. If Ace doesn’t mind I will ask some questions with diagrams starting simple and build up to something more sophisticated with everybody’s help if you are willing to play the game. So I will ask my first question again with a little more detail and pictures. We all like pictures so get out your crayons and see what happens. We all know that a resistor of proper specification for voltage and controller can simply be bridged from one post to the other on the main contactor. This will keep the capacitors full and ready to go but it will also keep a bit of constant power to the controller even when the main contactor is open. A switch is needed to break the power to the resistor. Where dose the switch go? Before the resistor on battery side or after on controller side. It seems to me that on the controller side would allow a fairly small switch because of the "resistance". We want a relay that we can control from the drivers seat. Will one type of relay cover all setups say from 96 volts to 144 volts? If everyone is patient with me and help get through this we will then establish how to tell when the capacitors are ready with something a little more sophisticated than counting to ten.


Either picture is correct, the switch is in fact a relay the coil of which can be controlled by the ignition switch (12v).
The main contactor switch will be driven also from 12v but will be either a manual switch of some sophisticated automatic solution which fires only when the voltage at the controller is at max main battery say 144v or 96v what ever your pack voltage.
As Ziggythewiz say's if you have a voltmeter connected across the controller you can operate it manually when the voltage stops rising.

The relays used in autos are ideal and cheap from a breakers.


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## Ace_bridger (Nov 22, 2011)

I agree, either circuit is correct.

I'm using an EV200 as my main contactor, switched by a relay which is switched by the contactor driver of the motor contoller. I'm also using an EV200 as my pre-charge contactor, switched by the car ignition key.

I'll be using a suitable power resistor as discussed in the previous posts.

Then I'm going to connect a suitable volt meter across the b+ve and b-ve of the motor controller. When I switch the key I should see appropriate volts on the voltmeter. If I don't I have 10 secs (governed by the contactor driver in the controller) to switch off and try again or go and investigate.

Zigg was right, KISS!!


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## Ace_bridger (Nov 22, 2011)

I hope Mora doesn't mind me showing one of his pictures here but this photo helped me figure out what to do to ensure a safe install.

Contactors circled in black. Gold rectangular shiney thing is obviously the power resistor.

This thread has ended up being about how to pre-charge and not how we might monitor but if it helps anybody safely pre-charge without expensive mistakes then so much the better.

Hope this helps and credit to Mora.


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## onegreenev (May 18, 2012)

Ace_bridger said:


> I hope Mora doesn't mind me showing one of his pictures here but this photo helped me figure out what to do to ensure a safe install.
> 
> Contactors circled in black. Gold rectangular shiney thing is obviously the power resistor.
> 
> ...


ACE,

What about a link to the information to build the setup you have shown. For those that need to pre-charge this looks great. I had seen this before but never needed to worry about it. My Synkromotive does the recharge within the controller so I do not need an external Pre-Charge.


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## PStechPaul (May 1, 2012)

A simple and inexpensive precharge indicator would be an LED and a resistor across the charging resistor. When the voltage drops below the turn-on voltage of the LED (about 2 volts), it will turn off. You could also use an optoisolator instead, and have the transistor output pull in a relay which disables the ignition switch and main contactor until the charge is sufficiently complete. It may take a little more than that for a practical circuit, because the LED would start bright and then gradually dim and it may be hard to see. So instead you could put the resistor in series with a B-E junction which would keep the transistor saturated and turned on until the resistor voltage dropped to about 0.6 volts, at which point it would turn off. The collector could be used for an LED indicator and/or a relay.

Here is a practical circuit:


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## mora (Nov 11, 2009)

Feel free to use my pictures if you find those useful. Yes, gold thingy is power resistor. 75 ohm and 50W if I remember right. Those wires could have been much tidier, heheh. I could have added some protection between high voltage components and 12V components, like plastic/polycarbonate wall, but that is another topic.

I'm de-railing this topic a bit though. In dragonsgates pictures it reads "precharge switch removes high voltage from controller when the car is turned off". It doesn't. It precharges controller capacitors even if controller is turned off. If you want to bleed voltage down you need to connect resistor across controller terminals with inline switch for example.

I have this kind of setup in place but resistors are PTC heating elements and those are switched on/off by SSR (solild state relay). Turn on heater for few seconds when controller is switched off and voltage at controller terminals drops to zero.


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## dragonsgate (May 19, 2012)

I pulled the pictures from the NuWiki. I was busy changing the drawings and didn’t pay attention to what the caption said. I had planed to ask more questions and upgrade the precharge to a more sophisticated system closer to what Ace is after but something has come up. I just took a commission for a large sculpture so I do not think I will have much time for anything else for a while. Did I hear a sigh of relief ?


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