Imiev repair - precharge resistor

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mark_hetho
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Imiev repair - precharge resistor

Post by mark_hetho » Mon, 12 Nov 2018, 03:53

Does anyone know the specs for the pre-charge resistor in the imiev battery pack?

In the process of diagnosing a failure to start (go into "ready") of our imiev, we believe we have isolated the issue to the pre-charge contactor and resistor. We are able to trigger the negative, main positive and pre-charge (positive) contactors from the connector on the side of the pack (C-22). We can hear the contactors, and we get a half-pack voltage reading from the main positive and main negative (from inverter main cable to the main fuse under the car), but no voltage reading from pre charge contactor. I suspect the pre-charge resistor has burnt out.

Unfortunately there doesn't appear to be any way to access this aside from getting the battery out of the car. I'm hoping to borrow time on a hoist and drop the pack out to confirm and change over the resistor. However in order to not to tie up the hoist longer than necessary I'd like to have the part on hand, but I don't know the specs. The part is labeled in the service manual circuit diagrams as "resistor", but without specs.

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Re: Imiev repair - precharge resistor

Post by acmotor » Mon, 12 Nov 2018, 13:32

FWIW my guess would be min 300ohm max 2000ohm and rated at a few hundred W down to maybe 50W.
Duty cycle would have been ECU controlled but there is potential for it to burn out if there was a control issue.

It may be possible to operate the fast charge chademo contactor directly via 12V on D25 and connect a resistor maybe 1000ohm 100W resistor between chademo +ve at socket perhaps and G19 or G22 or G23 to simulate pre-charge. All appropriate care of course.
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Re: Imiev repair - precharge resistor

Post by mark_hetho » Mon, 12 Nov 2018, 18:21

I think the duty cycle is normally limited by the charging time of the inverter capacitor. Although the ground fault which caused the excess current was detected (based on trouble codes), it doesn't seem the ECU disengaged quickly enough - I guess there is also a delay in the contactor.

I guess simulating a pre charge via the chademo port would cross check that everything else is operational. If I had another imiev I could read the resistor value from chademo positive to inverter positive. (I've found I also had to connect one of the chademo pins to earth to engage it's contactor, along with supplying 12v on the quick charge relay.)

I guess the other way to work it out would be based on the charging time and capacity of the inverter supply smoothing capacitor. There is a trouble code for too long a (pre)charging time (P1A15), but the manual I have doesn't say how long. Given the time before the main contactor disengages without the precharge working it's maybe 7s.

Worst case scenario I have to eyeball it after dropping the pack out and hope to get a part locally before the car outstays it's welcome on the hoist.

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Re: Imiev repair - precharge resistor

Post by coulomb » Mon, 12 Nov 2018, 22:01

mark_hetho wrote:
Mon, 12 Nov 2018, 18:21
I guess the other way to work it out would be based on the charging time and capacity of the inverter supply smoothing capacitor. There is a trouble code for too long a (pre)charging time (P1A15), but the manual I have doesn't say how long. Given the time before the main contactor disengages without the precharge working it's maybe 7s.
The ohm value of the pre-charge resistor is likely not all that critical. The energy that the resistor has to absorb without exploding is fixed. So if you use a larger resistance, the current is less, but it persists for a longer time. If you use a lower resistance, the current is higher, including the initial peak current, but it doesn't last as long, so the energy is exactly the same.

I strongly recommend finding a resistor with a high pulse power rating, such as the Tyco HS series. They can withstand 25 times their nominal power rating for 1 second. A typical pre-charge time is half to one second, to get to about 90% of pack voltage. That means a pretty high initial current, but the contactors are designed to handle hundreds of amps, so this isn't an issue. Using too high a pre-charge resistor risks the situation where current drawn by the motor controller and charger output stage causes a sort of voltage divider whereby the capacitors will never charge to the threshold value that the ECU is looking for, and pre-charge will fail.

Let's pluck some figures from the air. Let's say we want pre-charge in about 1 second (7 seconds might be a timeout value, well outside ordinary operation). Let's say we want to get to 95% of pack voltage (ignoring current drawn by the motor controller and charger output). So that's about 3 time constants. So we want the time constant to be about τ = 0.33 s. The formula is τ = RC or R = τ/C. Now for a wild guess at the total capacitance of the motor controller and charger output; I'll take 1000 μF or 10⁻³ F. So R = 0.33 / 10⁻³ = 330 Ω. If the capacitance is more than 1000 μF, then the resistance will have to be lower.

Now consider the pulse power. Initially, the capacitors will be at zero volts, so the full pack voltage appears across the resistor. Let's say it's 360 V. P = E²/R = 360²/330 = 392 W. That's a lot of power, but for a very short time. With a pulse power rating of 25, the resistor could be nominally 392/25 = 15.7 W, next highest standard value is 20 W. That's physically a moderately small resistor.

But if the capacitance is more like 5000 μF, then the energy the resistor will have to absorb will be five times greater. It will have to be a fifth the resistance (say 68 Ω), and the nominal power will have to be 78.5 W, next highest standard power is 100 W.

Some values are in stock in Australia from RS Components, like the 100 Ω 100 W part (but not the 68 Ω 100 W part). The latter is 5 working days delivery, compared to next day delivery for the former. Delivery to Western Australia may be longer. You'll have to decide if this is quick enough. Perhaps a local supplier would have stock you can pick up yourself, but it seems unlikely. It might even be worth gambling on the 100 Ω 100 W resistor being close enough to buy it in advance.

Make sure you have on hand wire, heatshrink tubing, and suitable lugs. I'm pretty certain that the wires to the pre-charge resistor can be seen in this image:

Image

From http://myimiev.com/forum/viewtopic.php? ... =10#p27408 .

The orange and red insulated crimped lugs would run off to the pre-charge resistor, sadly not visible in these photos. It looks like there is plenty of room to mount the resistor to the right of the contactors; it could be simply glued into place. It won't need a heat sink, and won't get more than warm to the touch. That way, there is no need to uncover the original resistor if it's hard to get to, though it would certainly be nice to know its value (assuming it can be read from the case).

[ Edit: I note that in the next post from the one linked to above, user Kiev asks for the value of that resistor. Sadly, as far as I can tell, his question was not answered. ]

[ Edit: nearest -> next highest. You don't want to go *down* in power rating. ]
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Re: Imiev repair - precharge resistor

Post by mark_hetho » Tue, 13 Nov 2018, 04:22

Thanks for the insight.

I think 100W should be plenty, but I also can't confirm the size of the capacitor. The discharge resistor is a 15W part (from memory a white ceramic boxy part) which tells me something. I'll see what I can dig up on that side of the circuit.

I'll check what is available locally, and I might order a couple of plausible options based on that. The price isn't unreasonable. I've got a pretty good stock of wire, lugs and heatahrink in my shed, but I'll have to make sure I take them to the hoist.

I've seen that exact image! Which I thought was a good guide to what we will be looking at when we go to fish out the resistor. I imagine it might pull out by the leads but perhaps not.

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Re: Imiev repair - precharge resistor

Post by brendon_m » Tue, 13 Nov 2018, 06:06

You might want to do some research on the precharge contactor as well (get an ohm rating on the coil and find something similar). While my money would be on the resistor failing first it's possible the contacts in the contactor are cactus.

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Re: Imiev repair - precharge resistor

Post by mark_hetho » Tue, 13 Nov 2018, 06:33

All the contactor coils (negative, main positive and precharge) read 36 Ω ± 0.3Ω. But yes it's certainly possible. Luckily in the same thread referenced above someone has pulled one:
Image

Doesn't seem to be carried by digikey or rs components, so it might be tricky to track down.

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Re: Imiev repair - precharge resistor

Post by coulomb » Tue, 13 Nov 2018, 08:44

mark_hetho wrote:
Tue, 13 Nov 2018, 06:33
All the contactor coils ... read 36 Ω ± 0.3Ω. ...
Doesn't seem to be carried by digikey or rs components, so it might be tricky to track down.
Digi-Key and Mouser stock the single-contactor models in the G9EA series, like this one:
https://www.digikey.com.au/product-deta ... -ND/585480
https://au.mouser.com/ProductDetail/Omr ... 2bSdfOg%3d

The coil resistance is given as 28.8 Ω, which is some 25% less than the 36 Ω you measured, but it would probably work without modification, because it operates for only a few seconds, then (presumably) turns off. Since the pre-charge peak current is only of the order of amps, not 100 A or more, you could use something slightly cheaper, like a miniTACTOR .

At the Omron price (nearly AU$200 before GST), you won't want to buy one just in case you need it :o

[ Edit: added link to Mouser single-contactor Omron product ]
[ Edit: Added "before GST". ]
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Re: Imiev repair - precharge resistor

Post by mark_hetho » Wed, 14 Nov 2018, 05:36

Yes, the original part seems a triplet, and I know two thirds of it are working. However in terms of physically fitting a separate contactor, it might present more difficulties than parting with $200. But also true that I'll check it before ordering :)

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Re: Imiev repair - precharge resistor

Post by mark_hetho » Sun, 18 Nov 2018, 14:22

I've got some imagery of the capacitor. I believe it reads 8000F, but it's a bit unclear, it could be 800uF.
Image

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Re: Imiev repair - precharge resistor

Post by mark_hetho » Sun, 18 Nov 2018, 14:54

The discharge/bleed resistor is 15W 68K Ω.

Based on 8000uF I calculate around 41 Ω, and around 3240W of power. So with a pulse power rating of 25 we need 129W, which means a 150W part.

100 Ω 100W is still in the ballpark though.

At this stage the access to the hoist might not be so time sensitive, so I think it might be worth eyeballing the part first.

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Re: Imiev repair - precharge resistor

Post by coulomb » Sun, 18 Nov 2018, 15:05

mark_hetho wrote:
Sun, 18 Nov 2018, 14:22
I've got some imagery of the capacitor. I believe it reads 8000F, but it's a bit unclear, it could be 800uF.
It can't be 8000 F, though I admit that at first, it does look like it in the photo. It has to be 800 uF, but don't forget that there is about 220 uF [ Edit: C115 ] at the back end of the charger in parallel with that. So my original guess of 1000 uF was pretty good. I probably saw that photo some time ago and made the mental addition.

[ Edit: Added schematic from MyiMiev.com ]

iMiEV charger back end.png
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Re: Imiev repair - precharge resistor

Post by Johny » Sun, 18 Nov 2018, 15:10

It can't be 8000 F, though I admit that at first, it does look like it in the photo. It has to be 800 uF, ....
I agree it looks like self healing ceramic which makes it 800uf. Don't ask me where but I've read somewhere that the purchase time in the iMiev is around 100mS. That makes my guess for the resistor about 100 Ohms.

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Re: Imiev repair - precharge resistor

Post by coulomb » Sun, 18 Nov 2018, 17:22

Johny wrote:
Sun, 18 Nov 2018, 15:10
I agree it looks like self healing ceramic which makes it 800uf.
Self healing ceramic? Is that a thing?

I assumed it was a film capacitor. Some of them have really good peak current ratings.

Edit: and in the lower left corner, magnified below, I see "PPC", which might stand for PolyPropylene Capacitor.

Motor controller capacitor corner.jpg
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Edit 2: It might be "PP6" from another photo. But it still could stand for PolyPropylene.
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Re: Imiev repair - precharge resistor

Post by Johny » Sun, 18 Nov 2018, 17:43

coulomb wrote:
Sun, 18 Nov 2018, 17:22
Johny wrote:
Sun, 18 Nov 2018, 15:10
I agree it looks like self healing ceramic which makes it 800uf.
Self healing ceramic? Is that a thing?

I assumed it was a film capacitor. Some of them have really good peak current ratings.
sorry you're right - self healing film. I bought a whopping great 330uf for my 2nd controller to replace 2 of 5600uf 450v electros in series. Yet to be modified - sigh.

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Re: Imiev repair - precharge resistor

Post by brendon_m » Sun, 18 Nov 2018, 18:27

I read that as being PP(polypropylene) with 50% glass fibre (GF) reinforcing.
That's the code for the plastic that was injected in the mold for the outside case, nothing to do with the capacitor itself



Edit. Not nylon, that's pa6. Also clarification of what I was babbling about

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Re: Imiev repair - precharge resistor

Post by mark_hetho » Sun, 02 Dec 2018, 13:35

Ok, so we got the pack out of the car, and after a few layers of disassembly we got to the pre-charge resistor.

It's labelled 240KAG 40W "JRH", but 240kΩ on 360V is nowhere near 40W. The part was open circuit so we couldn't measure it's value.

We broke it apart to see what was inside, found burnt wire coil, and found it was made up of two coils in series. Measuring ~90% of one coil we found 9Ω, extrapolating to the full length of both coils it's around 20Ω.
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Re: Imiev repair - precharge resistor

Post by Johny » Sun, 02 Dec 2018, 14:12

From that I'd take the 240 to mean 24 and no following zeros. Next is why did it burn out. Might the bypass (main) contractor be faulty?

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Re: Imiev repair - precharge resistor

Post by mark_hetho » Sun, 02 Dec 2018, 15:27

We identified a ground fault in the DC-DC converter, which could have drawn a bit through it.

But I can't figure out is the 240K a model number? Or is there some kind of Roman numeral notation going on here where the K cancels out the zero?

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Re: Imiev repair - precharge resistor

Post by coulomb » Sun, 02 Dec 2018, 16:42

The K is the tolerance code, meaning plus or minus 10%.
[ Edit : so the 240K codes for the resistance and tolerance. ]
It's equivalent to the resistor colour code with
Red = 2
Yellow = 4
Black = 0 (number of following zeroes)
Silver = 10%
On a large resistor like that, digits and letters are more convenient than bands of colour.
So that makes the restance 24 times ten to the zeroth power, or 24 ohms.
Interestingly, that makes the instantaneous current at switch-on, assuming a 360 V pack, of 15 A, and the instantaneous power is 360 x 15 = 5400 W. So the peak power is 135 times the continuous power, assuming that the continuous power is 40 watts. That's quite a large factor. The case doesn't seem specially designed for absorbing peak power the way that aluminium cased resistors are. Maybe it's all absorbed in the grey stuff.

[ Edit: absorbing power -> absorbing peak power ]
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Re: Imiev repair - precharge resistor

Post by mark_hetho » Sun, 02 Dec 2018, 17:42

I'm familiar with resistor colour charts, but is there some kind of reference for numeric codes? I'm sure I've seen ceramic resistors with the actual values written on.

The grey stuff was like mortar - probably a good thermal buffer.

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Re: Imiev repair - precharge resistor

Post by coulomb » Sun, 02 Dec 2018, 18:01

mark_hetho wrote:
Sun, 02 Dec 2018, 17:42
... is there some kind of reference for numeric codes?
This page on SMD (Surface Mount Device) codes agrees that 240 means 24 Ω: http://www.resistorguide.com/resistor-smd-code/
This page has the tolerance code K=10%: https://www.engineeringtoolbox.com/resi ... _1656.html
I could not quickly find a page that combines them.
I'm sure I've seen ceramic resistors with the actual values written on.
Yes, it's not like they're really cramped for space there!
The grey stuff was like mortar - probably a good thermal buffer.
As in a substance that resists temperature change: yes, agreed.
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Re: Imiev repair - precharge resistor

Post by mark_hetho » Mon, 03 Dec 2018, 03:23

When I think of SMDs, I'm thinking of the tiny components that solder direct to a PCB - but I guess in a very literal way this is a surface mount device also, just onto a larger surface!

That notation makes a lot of sense on smaller components with limited space. I guess to save confusion(?) they've used it for this as well.

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