Simple Precharge Circuit

[4Springs]

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!


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.

[evric]

Hi Christopher,
Great circuit. I can see one possible problem... If the ignition switch is turned to Start before the Precharge LED goes out, the current through the LED may increase to more than it should because of the connection via the Latch relay coil (depending on the coil resistance). Another diode in that horizontal connection to the LED should fix that.
Eric

[4Springs]

I can see one possible problem...

Great, thanks Eric, well spotted. And if the LED didn't pop then the contactor would come on which is also not ideal!

Circuit with suggested update:

[Astroboy]

Nice setup, i like the use of a not ready light.

I was puzzled by the precharge relay until i followed through your step-by-step description and realised it isn't what i would expect to be called a precharge relay.
When i think of a precharge relay i think of the relay that controls the precharge current to the controller. Your precharge relay is more a precharging indicator relay and precharging lockout relay. It energises in response to the precharge current. It is the not drawn, mid pack contactor controlled by the ACC voltage that is doing dual roles as the mid pack contactor and the precharge contactor. Maybe change the name of the precharge relay to precharging lockout relay or something like that?

When in precharge mode what is to stop the key being turned to start and the voltage from the start circuit going through the diode, around the latch relay coil and energising the drive contactor?

Also if the precharging LED isn't a reused dash light and is actually a LED then maybe used two LEDS in parallel, one red one yellow. That way the combined light is orange and if ever one of the LEDs fail you will still get an indicator. The change to either red or yellow light will let you know one of the LEDs has failed rather than just getting no light on LED failure.

[4Springs]

When in precharge mode what is to stop the key being turned to start and the voltage from the start circuit going through the diode, around the latch relay coil and energising the drive contactor?

Answer: Nothing at all.
It was definitely worth going to the trouble of learning a drawing program, drawing the circuit and posting it here. Nice people with book learnin point out my mistakes before I make them.
I think this amended circuit should fix that problem. Without any addition of components either!



I won't bother putting in another LED in case of failure myself; my LED is already in place and having it fail would not cause the circuit to stop working. It is there as a suggestion for anyone who might like to use this circuit though. It is also something that I might think of doing in future projects - I hadn't heard of that trick before.

[Astroboy]

I think this amended circuit should fix that problem. Without any addition of components either!

Neat.

I won't bother putting in another LED in case of failure myself; my LED is already in place and having it fail would not cause the circuit to stop working. It is there as a suggestion for anyone who might like to use this circuit though. It is also something that I might think of doing in future projects - I hadn't heard of that trick before.

I hadn't heard of it either, it just occurred to me when i was looking at the diode Eric suggested. Maybe it was the two diodes or maybe the two emitting arrows you use to indicate the light emitting diode but something there suggested the idea to me.
It does require that the operator know what the LED colour is normally and what the slight colour change signifies. So not an intuitive indicator but useful if you know what it means.
Hopefully it works now that i have suggested it.

[4Springs]

I have assembled and tested this circuit now. It is not working as expected and I can't figure out why...
The Precharing LED works as intended. It turns off about 30 seconds after turning to ACC. With a multimeter connected this corresponds to about 30V across the Drive Contactor terminals. So the Precharge Lockout relay is turning off as expected.
The Latch Relay works correctly and the car can be driven.
What is not working is the Precharge Lockout function. If I turn to START before the Precharge LED turns off then the Drive Contactor is pulled in. The Precharge LED immediately turns off, the Latch Relay latches and the car is ready to drive. This should not happen - the Precharge Lockout relay should be still breaking the circuit to earth.

Here are the results of my faultfinding efforts:
1. I pulled the Precharge Lockout relay from it's base. In this configuration the Precharge LED does not turn off and the Latch Relay cannot be latched. As expected.
2. I disconnected the Precharge LED, R1 and the diode in that section (they are connected via a plug). This made no difference to the fault.
3. I disconnected the Inertia Switch. In this configuration the Precharge LED does not turn off and the Latch Relay cannot be latched. As expected.

Somehow it seems as if the Precharge Lockout relay is turning off whenever I turn to START. But I can't figure out a way that that could happen. Can anyone give me any pointers as to what else to test?

[evric]

I know it sounds silly, but check your wiring again... If you have wired it exactly as per the last drawing above, the contactor cannot operate prior to the precharge finishing, because there is no earth connection for it until the contact closes on the lockout relay...

Normally when you turn an ignition key to start, anything connected to the ACC connection is turned off, while you turn it. Would that cause what you're seeing? - I can't see how. But this would be the reason that the precharge LED goes out.
To fix this run a diode from each of ON and ACC to the top of R1 which feeds the Precharge LED. This will prevent the LED going out when turning to START.

[4Springs]

I know it sounds silly, but check your wiring again...

I read Eric's post then went out and checked the wiring for the 27th time. Still didn't find anything, but that doesn't mean there isn't something wrong!

Normally when you turn an ignition key to start, anything connected to the ACC connection is turned off, while you turn it.

Yes, that happens in my car. My test number 2 above was designed to take ACC out of the question. You have reminded me though that the LED will always go out while in START, so I should keep that in mind while faultfinding.

Further faultfinding:
4. I unplugged the Not Ready Light section (including the earth) with no change in the fault.
5. I disconnected START and used ON instead (manually plugged in a plug at the appropriate time) - the circuit works that way as intended.
6. I unplugged ON and tried the circuit without it - the fault still occurs.
(each of the faultfinding steps are taken in isolation - everything I unplugged in number 4 was plugged back in for number 5)

So these tests narrow it down to something to do with START.
My ON has 12V during START as well. My ACC does not.

While writing this I have thought of something. My mid-pack contactor comes on with ACC and stays on at ON. What if it drops out during START? I'm pretty sure that there are some wires in the car that have ON only during ON and not during START. It's been a couple of years since I wired in that contactor - I'm sure I would have made sure it was on during START, but what if that wire/connection/diode has come off?
This is the only thing I have thought of that would cause the symptoms I'm seeing. It could also conceivably been the cause of my original controller failure!

I'm getting ahead of myself though, I need to go and test it. Unfortunately that won't be possible for a couple of days as I'll be at work. I'm now wondering whether I should take a different car...

[Astroboy]

So these tests narrow it down to something to do with START.
My ON has 12V during START as well. My ACC does not.

While writing this I have thought of something. My mid-pack contactor comes on with ACC and stays on at ON. What if it drops out during START? I'm pretty sure that there are some wires in the car that have ON only during ON and not during START. It's been a couple of years since I wired in that contactor - I'm sure I would have made sure it was on during START, but what if that wire/connection/diode has come off?
This is the only thing I have thought of that would cause the symptoms I'm seeing. It could also conceivably been the cause of my original controller failure!

I'm getting ahead of myself though, I need to go and test it. Unfortunately that won't be possible for a couple of days as I'll be at work. I'm now wondering whether I should take a different car...

This sounds very possible. If the mid pack contactor was toggling during START and allowing the pack voltage to go to the controller before the precharge was complete then it could have caused controller damage. May have also caused some contact wear in the mid pack contactor too as it would have been switching high currents.
(i just tried to add an exclamation emoticon and it shows up as a nuclear emoticon???)

BTW this is all good information for me as i was unaware that the ACC was off during START. I had always thought that the radio's momentary power off was due to voltage sag during cranking. It makes sense to depower the ACC when cranking, Gives all the current to the starter motor and protects the ACC stuff from any nasty spikes during cranking.
I must check to see what my car does.

Thank you for correcting that bit of misinformation i have been carrying around in my head for decades.

[Johny]

I may have missed something here but the way the circuit is drawn, the "precharge" relay always has controller current going though it. Therefore it always has less than 30 volts across it.
As I said - sorry if I missed something obvious.

[Astroboy]

I may have missed something here but the way the circuit is drawn, the "precharge" relay always has controller current going though it. Therefore it always has less than 30 volts across it.
As I said - sorry if I missed something obvious.

While the capacitors in the controller are precharging the current through R3 creates a voltage that causes the precharge lockout relay to be energised. Once the capacitors are charged the current stops and the precharge lockout relay is de-energised.
When the drive contactor is energised its normally open contacts close and effectively short circuit the precharge lockout relay's coil. So the precharge lockouts contacts return to the normally closed position.

At least that is how it looks to me.

[Johny]

So there is always current going through the pre charge relay and the controller caps are always charged? What turns the controller power off completely?

[Astroboy]

So there is always current going through the pre charge relay and the controller caps are always charged? What turns the controller power off completely?

The current only energises the precharge lockout relay while the controller caps get their initial charge.
Once the caps are charged the drive contactor can safely energise without fear of large arcs as the contacts close.
At that point the drive contactor's contacts connect the battery pack to the controller but at the same time are shorting out R3 and the R2/precharge relay coil. This causes the precharge relay to de-energise.
The circuit then remains like that for the entire time the car is in use.

To switch off the car i would imagine that when the ON circuit is depowered the latch relay de-energises, removing power from the drive contactor. The drive contactors's contacts open without arcing as the controller's caps are fully charged and no current is flowing.
The precharge relay doesn't energise as there is no current flowing in R3.
Then the ACC circuit is de-energised which i would imagine would remove 12v power from the controller and de-energise the mid pack contactor.
This would then be the at rest, ready for recharging state.

Some builders may also add a high resistance resistor to the controller's pack inputs to slowly discharge the caps and make them safe. Some controllers may already have this feature inbuilt.

4Springs, please correct me if i was wrong with any of this.

[4Springs]

4Springs, please correct me if i was wrong with any of this.

Top of the class Astro!
It might help to think of the Controller as a big capacitor. When 150V is first turned on the voltage at point A is 0V, so the Precharge Lockout relay "sees" 150V and energises. As the capacitor charges the voltage at point A rises, so the difference between point A and 150V+ gets less. Eventually the relay de-energises when that difference gets less than about 30V.

The controller capacitor is only discharged when 150V is removed completely. In my car this happens when the mid-pack contactor is de-energised. This is only when the key is turned to OFF and when the car is not plugged into a 240V socket. I am normally plugged in when parked so this is quite rare. I made sure I used one of the more expensive contactors with a low current usage.
The controller needs 12V to run - that is provided by the ON key position. So although it is quite often charged it is only powered up when needed to drive.

Out of interest, the controller capacitor is discharged in my car by a circuit that powers a flashing red light. Arber gave me the idea - he said just use a plug pack power supply. This is really neat - I got an old phone charger made to run from 240VAC and connected it to the 150VDC. It produces an isolated low voltage DC all the way down to about 5V. I have it powering two large red flashing LEDs under the bonnet. The idea is that if you open the bonnet and are tempted to start fiddling around with the wiring the flashing red lights might remind you to isolate it first. They are also good to tell when it has completely discharged after isolation - it takes a good minute or more to discharge enough to stop the lights flashing.

Can't wait to get out there and test that contactor. I am really hoping that it turns off when I turn to START! Fiddly to fix though, those connections are under a few dozen cells which are in turn under some pretty securely fixed wooden panels...

[Astroboy]

...
Out of interest, the controller capacitor is discharged in my car by a circuit that powers a flashing red light. Arber gave me the idea - he said just use a plug pack power supply. This is really neat - I got an old phone charger made to run from 240VAC and connected it to the 150VDC. It produces an isolated low voltage DC all the way down to about 5V. I have it powering two large red flashing LEDs under the bonnet. The idea is that if you open the bonnet and are tempted to start fiddling around with the wiring the flashing red lights might remind you to isolate it first. They are also good to tell when it has completely discharged after isolation - it takes a good minute or more to discharge enough to stop the lights flashing
...

I like that. Cheap and most likely recycling/re-purposing the phone charger.
Have you measured how low it gets the voltage when the leds go out?
Does it still keep drawing a bit of power after the leds go out and eventually discharge the capacitors down to zero?
Probably each phone charger model will have different dropout voltages and quiescent current draw.

...
Can't wait to get out there and test that contactor. I am really hoping that it turns off when I turn to START! Fiddly to fix though, those connections are under a few dozen cells which are in turn under some pretty securely fixed wooden panels
...

Maybe you could move the other end of the wire from the ACC to the ON circuit.
So the mid pack contactor came on with the ON circuit rather than the ACC.
Might save having to dismantle your pack container.
Then the next time you have to access the pack you could fix/modify the mid pack contactors setup to make it work with the ACC connection.

[evric]

[/QUOTE]
...
Then the next time you have to access the pack you could fix/modify the mid pack contactors setup to make it work with the ACC connection.[/QUOTE]

...but remember that in most cars the ACC is turned off while you START the car...

[Astroboy]

...Then the next time you have to access the pack you could fix/modify the mid pack contactors setup to make it work with the ACC connection.

...but remember that in most cars the ACC is turned off while you START the car...

4Springs has his mid pack contactor wired so that it comes on with ACC but should stay on when using START because he combined the ON and ACC supplies for that contactor. It is this diode that combines the circuits that 4Springs is hoping has failed. It would explain the strange behaviour of his pre-charge circuit as well as help explain his controller failure.
Without the combined ON and ACC supplies the mid pack contactor may have been de-energising when the START circuit was activated. This would result in the mid pack contactor re-energising whilst the drive contactor was activated and before the precharge cycle was complete.
This would mean high current switching for the mid pack contactor's contacts and large surge currents for the controller and batteries each time the car was started.

4Springs is going to check and see if the ACC is dropping on START. Also if the voltage from the battery pack going into the drive contactor/R3/precharge lockout relay also drops on START. This would confirm that the mid pack contactor was de-energising on activation of the START circuit.

[4Springs]

Huzzah! - the mid-pack contactor drops out in START!
Which would not normally be the cause for celebration, but it is if you think you may have found the original problem that has caused you $4000 in repairs!

My mid-pack contactor is supposed to be fed by ACC and ON/START, via a couple of diodes to stop the power running back into the ACC or ON/START lines. I tracked this back to some wiring under the dash (not under the battery box where I thought it might have been). One of the wires is contacting intermittently, so all I have to do is repair that fault.

I'll go over to my build thread now and ask people what types of automotive connectors are more reliable!

[4Springs]

So the simple precharge circuit now works exactly as planned! Pretty simple and cheap too - a $13 relay and a resistor I had on the shelf (R2). For my Kelly controller I used R3 = 2.2k 20W, R2 = 5k 5W. The relay is about 18k, so this adds up to about 2k for the precharge resistance which is what Kelly recommends.
As Eric pointed out there is a precharge circuit available from Zeva quite cheaply, but it charges far to quickly for a Kelly controller.

I like that. Cheap and most likely recycling/re-purposing the phone charger.
Have you measured how low it gets the voltage when the leds go out?
Does it still keep drawing a bit of power after the leds go out and eventually discharge the capacitors down to zero?
Probably each phone charger model will have different dropout voltages and quiescent current draw.

I went out and measured that the LEDs stop flashing at about 5V (the two LEDs are flashed with a circuit I made based on a 555 timer). It takes about 90 seconds to discharge to that level. From memory the charger outputs 5V, so it does really well to keep the output with such a low input. I checked the voltage about 10 minutes later and it was 2.5V.
When figuring out what to do with this precharge circuit I initially was going to re-purpose another plug pack. But I didn't have any others that would run at less than 30VDC. As you say, every model will have different characteristics.

[Astroboy]

Huzzah! - the mid-pack contactor drops out in START!
Which would not normally be the cause for celebration, but it is if you think you may have found the original problem that has caused you $4000 in repairs!
...

That should give you some peace of mind. Now you can be confident that your current controller and motor won't do a repeat performance.
And i have learnt something about the the ACC circuits behaviour that will no doubt save me from similar frustrations.
Thank you for sharing this information.

As for the plug pack and leds. That also answers another question that had been rattling around in my head for ages. How much energy is wasted by discharging the controller caps when the car is idle (switched off)? Not a big issue for daily commuters but if somebody was doing a large number of small trips it may have been significant. But if a couple of large leds drain the caps in 90 seconds then it can't be a lot.

[tonyw]

Assuming the controller has ~300 µF of caps charged to 150V, that is an energy storage of ½CV^2 , which is 3.375 J or 0.0009375 WH.

Even if you have 1 F of capacitance, that can only hold 11.25 kJ or 3.125 WH of energy at 150V.

Not very significant compared to the energy used in running the car !

[jonescg]

Please forgive the necroposting, but I have been looking for a good option for a dual relay delay timer for the Prelude's master contactor and precharge relay.

I originally used a basic 555 timer delay which closed the master precharge relay with supply voltage, and 0.5 seconds later the master contactor closed as it was supplied power via the relay on the timer board I bought off e-bay.

LM555PowerDelay.GIF (5.71 KiB) Viewed 2256 times

However at some point it stopped working, and had me stumped for a while till I worked out the timer was not activating at all, thus leaving the precharge relay on, but no master contactor.

So I am on the look out for a new timer, but preferably one which switches the precharge relay off after 1 second. The logic looks like this:

Master precharge and contactor logic.png (9.86 KiB) Viewed 2256 times

It seems I'd need two 555 timers to make this work, or a 556 timer which has two timers.

Hopefully I don't need to make one, but I guess I can if I'm keen.

[jonescg]

Seems as good a spot as any to bring this up, but the brand new GV240 contactor in the Prelude is sticking from time to time. Seems to be a bit variable though. Sometimes it sticks and other times it doesn't. The rear pack contactor is stuck and won't get off, presumably because the A/C capacitance was sufficiently high to weld it.

I'm at a loss as to why the drive contactor is sticking now, particularly since the inverter itself manages precharge. It takes 5 seconds on the dot to engage when it's working right, but sometimes it still seems to stick. Fortunately the car is still immobilised as the drivers board on the inverter has it's power cut, but still, it's not ideal.

I have a feeling it's some quirk of the inverter.

[coulomb]

Is there any way that the air conditioning controller could be powered off and then on again between pre-charge and final power off?

What about if the ignition is turned on soon after powering off? Is the pre-charge circuit guaranteed to give the full five seconds again?

Finally, is it possible that some load coming on (say wipers, headlights) could be causing a dip in the 12 V supply, so that the main contactor comes off briefly? You might not hear or notice it, but if the motor controller is running with a load on the pack, even a 10 ms dip in contactor power might cause the inverter's capacitors to drop say 50 volts and cause a splat when it comes back on. [ Edit: the solution here might be say a 1000 μF 25 V capacitor across the 12 V supply near the driver circuit for the main contactor coil. ]

That could perhaps also happen mechanically. Maybe the coil current is a bit low, and a big bump could cause the contactor to disconnect briefly. Either more current or shock absorbing for the contactor might be indicated.