# Pre-charge control, a thought.



## Jerry Liebler (Feb 1, 2011)

It is common to 'pre-charge' before activating the contactor to connect a high voltage battery to a controller. the pre-charge is through a resistor that limits the inrush current charging the controllers capacitor bank. It is also common to wait a time delay, to allow the pre-charge befor allowing the main contactor to close. Rather than using a timer wouldn't it be better to delay the contactor's activation till the voltage across the pre-charge resistor is below a set point?


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## charliehorse55 (Sep 23, 2011)

Jerry Liebler said:


> It is common to 'pre-charge' before activating the contactor to connect a high voltage battery to a controller. the pre-charge is through a resistor that limits the inrush current charging the controllers capacitor bank. It is also common to wait a time delay, to allow the pre-charge befor allowing the main contactor to close. Rather than using a timer wouldn't it be better to delay the contactor's activation till the voltage across the pre-charge resistor is below a set point?


In theory, this is the ideal solution. However, this requires creating a high voltage sensing circuit, which adds cost and complexity. Additionally, the capacitance of the controller and the impedance of the resistor are almost constant, so the difference in voltage between different precharges is minimal.


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## Jerry Liebler (Feb 1, 2011)

"However, this requires creating a high voltage sensing circuit, which adds cost and complexity." A simple CHEAP resistor divider from each side of the pre-charge resistor into a low voltage comparator. Not much cost or complexity! Probably cheaper than most timer designs.


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## Ziggythewiz (May 16, 2010)

I agree. It should be voltage based, not time based. If something goes wrong and the precharge doesn't happen you don't want to be throwing the contactor on timeout anyway.

I don't see how the voltage difference could be minimal...you go from full pack voltage to <25% of pack voltage across the resistor during precharge.

Mine's just manual, but the ideal circuit would use a staged precharge to do the 1st 50% over one resistor then the next 40 or so % across an additional one to reduce the resistance, time, and voltage gap.


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## Nabla_Operator (Aug 5, 2011)

*Re: Pre-charge control, the measurement method*

Yes, you should actuate the mains contactor when the voltage over the connections is (for example) less than 10 V.
Currently I am figuring out a system that does the job between these two isolated systems: HV battery + 12V board controller.

The simplest apparatus I found so far is with voltage to current optocoupler chip ACPL-K370. The controller sees a light (i.e. a current) as long as the voltage drop over the mains contactor is (by zenerdiode) 5,3 V for example and when the light is off, you throw in the mains contactor. 
That is similar to the manually operated systems of many DIY people who have an illuminating light bulb as long as the mains capacitor is pre charging.

I would rather measure the voltage drop in an analog way, but can't design a neat circuit with the chip above or with the AD-1B22 or the voltage to voltage chips: ACPL-C78 and -C87 either. 

Hope to read here about a marvelous solution soon.

The rest of my precharge circuit is described here.


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## major (Apr 4, 2008)

Ziggythewiz said:


> I don't see how the voltage difference could be minimal...you go from full pack voltage to <25% of pack voltage across the resistor during precharge.
> 
> Mine's just manual, but the ideal circuit would use a staged precharge to do the 1st 50% over one resistor then the next 40 or so % across an additional one to reduce the resistance, time, and voltage gap.


Hey zig,

Not sure what you're talking about. Precharge is just an RC circuit. The voltage across the resistor (and contactor) will be 5% of the battery voltage in 3 time constants (RC). After 5 time constants, it will be less than 1%. The ultimate voltage across the precharge resistor will be I * R where I is the leakage current of the caps in the controller combined with any bleed down resistance it might have.


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## PStechPaul (May 1, 2012)

I suggested an industrial SSR rather than the DIP optoisolator as a reply in the same post. Sensing the voltage across the contactor is ideal because it provides a fail-safe mechanism in case the input of the controller is shorted or drawing a heavy load. The manual method of observing a lamp is also good, as it promotes more attention to the instrumentation, but the operator could choose to ignore it and cause a damaging current surge. The time delay _assumes_ that the controller and capacitors are OK and of the expected value.

The problem with the voltage monitor across the contacts is that it would draw current continuously until the mains relay is energized, but that could be solved by an additional contact that would energize the circuit only when the start-up sequence is initiated ("ignition" keyswitch). 

It would be best to introduce some hysteresis in the sensing relay circuit so that it operates sharply and cleanly at the sense threshold, and then latches off. A simple optoisolator would show a gradual reduction of current as the voltage dropped, and might not operate the relay quickly enough. 

The proposed circuit shows a voltage divider and ADC in the controller performing the voltage sensing, which eliminates the continuous drain problem, but it also does not really compare the battery pack voltage to the controller DC bus.

I am planning to implement a similar precharge system in the circuitry for my electric tractor conversion, and I'll post the circuit when it has been designed and tested. It would probably be a good idea to build it as a separate module so that it could be used on most other EVs. Maybe I should order one of the 480V 20A DC SSRs I found for $15:
http://www.mpja.com/20A-480VDC-Solid-State-Relay/productinfo/17288%20RL/

A similar commercially available SSR for 200 VDC and 12A is about $27:
http://www.newark.com/magnecraft/6312axxmds-dc3/ssr-panel-mount-200vdc-32vdc-12a/dp/10M6328

I had also considered using an inductor and PWM on an IGBT or MOSFET to control the precharge at a constant current which would bring up the voltage in a linear manner and much more quickly than a resistor, with very little wasted energy. But the resistor is cheaper and more reliable, and the amount of energy involved in charging a 500 uF capacitor to 300 volts is just 22.5 J or 0.006 Wh.


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## PStechPaul (May 1, 2012)

I made a simulation of an idea for an automatic precharge circuit.










It keeps the main contactor S1 turned off until the voltage difference on precharge resistor R4 is less than about 20 volts, and then the contactor is pulled in by MOSFET M1. R2 simulates a 200VDC relay coil. It could also be replaced by a series resistor and an opto-relay for the more usual 12VDC relay coil. 

Actually, M1 can be used directly to replace the contactor, but that would not be practical or efficient:










I added the inductor L1 to limit the current surge when the contactor switches on full.

There is really no need to switch out the limiter resistor because it is bypassed by the contactor.


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## charliehorse55 (Sep 23, 2011)

Shouldn't it be possible to use a mosfet as a variable resistor, to achieve a constant current precharge? Operate the mosfet in triode region with it's gate voltage controlled by a feedback circuit of some sort.


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## major (Apr 4, 2008)

Here is a good report on precharge. http://liionbms.com/php/precharge.php Some motor controllers handle the precharge function. And some BMS systems handle the precharge function. 

I think you guys are making it overly complex. Heck, I've just wired the resistor permanently across the contactor on some low voltage (<50V) EVs and they worked well for decades


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## dtbaker (Jan 5, 2008)

Having great respect for Soliton engineering.... what do they do?

and is it the same or different than what the Zilla does internally? Zilla has the pre-charge stuff internal, but requires an external contactor. The Soliton design puts both the pre-charge and contactor inside.... right?


....so, with either Zilla or Soliton controllers, the pre-charge question is moot as they handle it internally.


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## major (Apr 4, 2008)

dtbaker said:


> Having great respect for Soliton engineering.... what do they do?
> 
> and is it the same or different than what the Zilla does internally? Zilla has the pre-charge stuff internal, but requires an external contactor. The Soliton design puts both the pre-charge and contactor inside.... right?
> 
> ...


Yes, both those controllers take charge of it (pun intended ). Whether the contactor is included inside the controller or external to it. This way the controller can do the proper precharge and handle certain safety concerns such as checking for a welded contactor, high resistance contactor and opening the contactor when a fault is detected. The newer Curtis and Sevcon controllers also take charge of this. And many if not all of the industrial drives I've worked on also do. However the EnerDel BMS system in my Think EV handles it. The precharge resistor is actually on the BMS mother board. The main fuses, contactors, and current sensors are all part of the EnerDel BMS and built into the battery pack. Different strokes for different folks


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## Rayco (Jul 31, 2021)

Nabla_Operator said:


> *Re: Pre-charge control, the measurement method*
> 
> Yes, you should actuate the mains contactor when the voltage over the connections is (for example) less than 10 V.
> Currently I am figuring out a system that does the job between these two isolated systems: HV battery + 12V board controller.
> ...


I watched a you tube video on a mazda rx8 conversion where his contactor welded it self closed, he installed a pre charge resistor and simple timer board ,works FANTASTIC! Simple is better.


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