Simple Precharge Circuit

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jonescg
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Re: Simple Precharge Circuit

Post by jonescg »

The current circuit looks like this:
HV distribution rev 1.png
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The drive contactor is activated by the controller through a 12 V supply coming from the Greatland.
The drive contactor precharge relay is activated by the key START circuit.

The master contactor precharge relay is driven by a switched supply, while the master contactor itself is actuated by a switched 12 V supply after a short delay.
It hasn't worked for a while, and it seems to be permanently welded now. Maybe later this year I will drop the battery out and replace it with a buslink, and then use a heavy duty service disconnect in the boot.
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Re: Simple Precharge Circuit

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4Springs wrote: Sun, 02 Nov 2014, 13:35 I recently had a motor controller fail, and there is a possibility that it had something to do with my precharge. So I've developed a slightly more complicated precharge circuit, but it is still very simple. I thought I'd post it here so that people can point out any fatal flaws that I might not have realised!

Image
R3 is the precharge resistor. I'm using a Kelly Controller, and they recommend 2k 20W. 150VDC is connected when I turn the keyswitch to ACC. This is not shown on the circuit, but it is because my pack is broken by a contactor which is enabled by ACC/ON/START (and also by 240VAC).
When 150VDC is first applied the Drive Contactor is open. Current runs through R3 and into the controller, which charges up over time. The voltage at point A therefore starts out at 0V, then goes up towards 150V over time. The process is logarithmic - it starts out fast and gets slower as it approaches 150V. When I asked the question "how high does it need to get?", weber replied that one rule is that the voltage over the (open) contactor should be 28V or less. On my Kelly it takes about 30 seconds to charge up to this value (122V).

The green circuitry (12V) is there to turn the Drive Contactor on when ready. I don't want to enable this contactor until the voltage across the contactor is less than 28V. Here is the process:
1. Turn key to ACC. 150V is connected. The Not Ready light shines. This is a spare light in my dash (the oil light for example). The idea is that like in a normal car, the light goes out once the engine is "running".
2. Turn key to ON. The Latch Relay is not energised, so the Drive Contactor is not energised.
3. Turn key to START. If the Precharge Relay, Charging Lockout Relay and Inertia Switch are all closed, the Latch Relay is energised. This energises the Drive Contactor, turns off the Not Ready light and you are ready to drive.
4. Turn key back to ON. The Latch Relay stays on and keeps the Drive Contactor on.

The new bit is the Precharge Relay. This is RS Components part number 8135031. The relay can handle up to 160VDC, and according to my experiments it switches on at about 50VDC, lets go at about 30VDC. I have it in the circuit so that it is switched on by the voltage across the Drive Contactor contacts. So when the 150V is turned on the contactor is open, and the Precharge Relay switches on, breaking its contacts and disabling the Latch Relay. It stays energised until the voltage across the Drive Contactor falls to about 30V, at which point it lets go and earths the Latch Relay.
R2 is there to provide a bit of protection for the Precharge Relay, which is only rated to tolerate 160VDC. The voltage in my car could be up to 175V straight after charging.

So the process from the Driver's point of view is:
1. Turn key to ACC. The Precharging LED on the dash will light, reminding you that you cannot START yet.
2. Turn key to ON. The Not Ready light will light.
3. Wait until the Precharging LED goes out. Then turn key to START. The Not Ready light will extinguish and you will be ready to drive.

If you turn the key to START too early then the Latch Relay will not energise, so you will not be able to drive. The lights on the dash will remind you what you have done.
Hello from New Zealand. I just joined. I know this is a old thread but its exactly what confuses me. This is probably more fancy that I want to do right now but something I dont understand is how do you get something that is a momentary switch (start) to disconnect permanently the precharge that ideally starts running when you turn the car to "on" just before "Start"? To be honest I dont know exactly how long it takes to precharge my basic inverter and figured it is probably ok after less than a minute? What is a safe amount of time? My inverter is a one off made in China. Can someone please kindly tell me as simply as you can how i wire my starter wiring to turn off the precharge and enable the main contactor and stay on. Ie latch as in the above circuit? Thanks for your time to reply to me.
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Re: Simple Precharge Circuit

Post by brendon_m »

A few seconds is probably enough but it comes down to the size of the capacitors and the precharge resistor.
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Dangerous thing with this circuit is it relies on the operator to know how long to precharge for and they don't slip on the key /turn it straight to start or anything similar
Could place a light across the precharge resistor (or in substitution). When the light goes out it's precharged and you can "crank it"

Or you could use a timer relay instead of the start signal to make a consistent delay

Or buy/make a proper precharge controller. ZEVA made a good one but he's closed up shop (though he open-sourced everything I believe)
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Re: Simple Precharge Circuit

Post by jonescg »

You can try a 555 timer delay, but it's hard to get them to play nice after a while. It's best if your controller manages precharge internally.
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Re: Simple Precharge Circuit

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Mons2b wrote: Fri, 31 Dec 2021, 11:11 Hello from New Zealand. I just joined. I know this is a old thread but its exactly what confuses me. This is probably more fancy that I want to do right now but something I dont understand is how do you get something that is a momentary switch (start) to disconnect permanently the precharge that ideally starts running when you turn the car to "on" just before "Start"?
Although the circuit is simple, it is not the easiest to get your head around. I'd recommend carefully reading the explanation I wrote in the first post. There is an error in the first circuit diagram though, so if you decide to build this, use the third version of the circuit diagram.
Mons2b wrote: Fri, 31 Dec 2021, 11:11 To be honest I dont know exactly how long it takes to precharge my basic inverter and figured it is probably ok after less than a minute? What is a safe amount of time?
If you have your system installed, it's best to measure the voltage. I've marked the spot on the circuit diagram with 'A'. Once you connect the system voltage, the voltage at that spot will increase from 0 to your system voltage over time. I've been told that it needs to get to within 28 V of the system voltage. So connect up a multimeter, turn on the system voltage and time how long it takes. For my system, with a 2k resistor, this takes about 30 seconds. Most other systems I've seen have been much quicker.

By the way, I started out with a timer circuit, rather than this voltage circuit. It used a 555 timer, and just lit a LED for 30 seconds. The operator needed to know not to turn to START while the light was on. I much prefer this voltage sensing circuit, as sometimes people other than me need to drive the car. This saves me giving a half-hour lesson every time I get new tyres put on!
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Re: Simple Precharge Circuit

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The safest way to get around this problem is to check the voltage across the contactor and have a timeout circuit that turns off the precharge within say 1sec if the voltage across the contactor has not dropped.
You can use a circuit similar to the one used for desat protection. What you do is you have an isolated psu making say 5v. Which drives the led of an optocoupler.
Then use a diode (voltage rating to be your bus voltage or higher) so that when the contactor closes the current running thru the LED is taken by the diode and contactor. Then when the contactor has low voltage across the optocoupler will turn off.
Sorry I do not have a paper to draw up a circuit.
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Re: Simple Precharge Circuit

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That's essentially what the ZEVA unit does
https://www.zeva.com.au/index.php?product=110
Had one on my old car and the only time I had issues was after I accidentally wired the dcdc to after the main contactor and the precharge resistor was trying to run the dcdc converter. To be fair it would drop out and not engage the main contactor due to the fault (not precharging fast enough)
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Re: Simple Precharge Circuit

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jonescg wrote: Fri, 31 Dec 2021, 13:43 You can try a 555 timer delay, but it's hard to get them to play nice after a while. It's best if your controller manages precharge internally.
The controller is simple but effective. Precharge switch was wired up to it. Manual. I thought about it hard and being that I still dont understand how you get a latch to work from start I could just wire in two timer circuits and when you turn the car on with the key one timer counts down a closed connection (precharge). The second time is open circuit and counts down to connect the other main contactor. This would be best to have a indicator bulb to show that the precharge was on encase the timer circuit failed. What do you think of that idea?
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Re: Simple Precharge Circuit

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Mons2b wrote: Mon, 03 Jan 2022, 09:58 being that I still dont understand how you get a latch to work from start
Do you mean that you'd like the operation of the 'latch relay' in the circuit explained?
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Re: Simple Precharge Circuit

Post by Mons2b »

Yes. When you turn the key in a conventional car to the start position you hold it there however long it takes for the ice to fire up. In a conversion thats not necessary but I would like to still use it but I dont know how you make a relay "stick" on after you flick a switch (the ignition key) on (start) and let it go (run) > If i cant figure it out i guess ill just wire it all to go in the run position.
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Re: Simple Precharge Circuit

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Text is too hard so I made a little video for you

https://youtu.be/cUGGZ0Xzf1c

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Re: Simple Precharge Circuit

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You need a bit more than just wiring switches and relays.
I have already implemented this on my conversion and it works like this:
When the key is turned to ON there is a relay that applies power to the inverters and the BMS and closes the negative battery contactor. The ON switch is just an input in the EV-ECU (the vehicle computer) once this happens the EV-ECU will wait for the start switch signal. Once it gets this signal it will wait until it gets communication from the inverters. The inverters will provide the bus voltage. Once it has comms the EV-ECU will ask the BMS (via CAN ) to close the precharge contactor wait about 100ms and will monitor the rate of change in voltage and the voltage level. If the voltage does not come up above a certain level and the rate of change of voltage does not drop to zero within 1s it will turn off the precharge relay. Once it determines that the conditions are righ the EV-ECU will ask the BMS to close the positive contactor and after that will enable the inverters.
So it is a bit more complicated than what can be done with relays and switches.
The method I am using sounds overly complicated but it can perform more checks and can add more safety checks than just using relays and switches.
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Re: Simple Precharge Circuit

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brendon_m wrote: Tue, 04 Jan 2022, 08:06 Text is too hard so I made a little video for you

https://youtu.be/cUGGZ0Xzf1c

This is a wonderful forum. Much more friendly than other ones I have tried to seek help from. Thanks for the kind assistance and understanding.
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Re: Simple Precharge Circuit

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Re our conversation on YouTube
If you go with the timer option then you'll want to have the BMWs ign switch turn on the timer relay and also provide power to the precharge relay via a changeover relay normally closed contacts.
This will precharge the inverter through the resistor.

Once the timer has counted out it's time it will energize the changeover relays coil which will provide power to the relay for the main contactor via the normally open contacts (which will now be closed) and the precharge relay will drop out because the normally closed contacts will now open.

Finally when you shut down the car you just turn off the key and all power will be cut to the system and if you want any interlocks for charging, rollover events, etc then just wire them inline between the ignition switch output and the timer relay/changeover relay
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The changeover relay is drawn wrong internally but I was already committed so it is what it is
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Re: Simple Precharge Circuit

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Thank you for the great diagram. I will save this in my project folder. By interlocks you mean cut offs like inertia switch, emergency stop, isolation failure etc etc?
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Re: Simple Precharge Circuit

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I have some question for all with experience.
I think it is dangerous to open the battery contactor without shutting down the inverter correctly.
If the motor is being run in field weakening disconnecting the battery would cause the bus voltage to shoot up and potentially destroy the power stage. This would put a short circuit across the motor terminals.
Does anyone know what OEM do?
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Re: Simple Precharge Circuit

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Mons2b wrote: Tue, 11 Jan 2022, 09:11 By interlocks you mean cut offs like inertia switch, emergency stop, isolation failure etc etc?
Yeah pretty much
francisco.shi wrote: Tue, 11 Jan 2022, 09:21 Does anyone know what OEM do?
OEMs will just shut down the inverter properly first, benefits of having control over the whole system. If someone turns the key off the car just sends a request to shut down rather than physically cutting power.
That's not even specific to EVs, pretty much anything with a CAN bus will be like that
If you're talking about an emergency (airbags deploying sort of thing) then they probably still send the same shut down commands but cut power to the contactors immediately, who cares if something electronic fails at that point as the cars been in a crash.
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Re: Simple Precharge Circuit

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francisco.shi wrote: Tue, 11 Jan 2022, 09:21 I think it is dangerous to open the battery contactor without shutting down the inverter correctly.
Also, opening/closing under load is very hard on the contactor - the contacts aren't rated to many high current interruptions at high voltage.
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Re: Simple Precharge Circuit

Post by francisco.shi »

That is what I was expecting.
So is it acceptable to use this method in a conversion?
This is what I am planning to do.
Does the inspector check if it is done this way or do they just turn the key off and see if the car turns off?
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Re: Simple Precharge Circuit

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francisco.shi wrote: Tue, 11 Jan 2022, 12:57 Does the inspector ... just turn the key off and see if the car turns off?
That's more than my inspector did


A lot of mine sites state an emergency stop has to shut down the machinery within 10 seconds of being pressed so you've got a reasonable window
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Re: Simple Precharge Circuit

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I have seen a few conversions with the red button which is a switch for the battery but I do n it think it is safe to have the high voltage wires coming out of the battery. My ignition switch is connected to my EV-ECU which then tells the BMS to turn the contractors on. So the EV-ECU decides when to turn the power off. Potentially the car could be turned on remotely without turning the ignition key (a bit like a Tesla) but maybe i should not mention any of this and just let him use the key to confirm the car turns off?
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Re: Simple Precharge Circuit

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Isn't the "big red button" for DC motor use where the contactor could weld closed and/or the Mosfet punches through? With the AC waveform passing through zero twice each wave, (time between high voltage and zero depends on the frequency), arcing isn't as likely to occur so would the "Big Red Button" still be required?

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Re: Simple Precharge Circuit

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T1 Terry wrote: Wed, 12 Jan 2022, 11:22 With the AC waveform passing through zero twice each wave, (time between high voltage and zero depends on the frequency), arcing isn't as likely to occur so would the "Big Red Button" still be required?
Each phase of the motor current is AC and regularly passes through zero, but as one phase passes through zero the other two are high. Overall, there is very little ripple in the current from the battery, about 6%, so the battery and hence contactor current never goes near zero when the motor is running. The little ripple is further smoothed by motor controller capacitors, though I doubt that they make a huge difference.

In summary: the problem of the main contactor welding closed is little different with AC motors versus DC. However, a shorted AC controller won't allow the motor to run at full speed and torque, so that's one less reason to have the Big Red Button in an AC conversion. Arguably, a big reason.
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