Locating the DC-DC converter

AusMGZSEV

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I am pulling down an MG ZS EV and having trouble locating the DC-DC converter.

I see the charger on the left, the inverter on top of the motor unit, and a HV junction box on the back firewall. There are a couple of smaller boxes but I thought the DC-DC would be larger and possibly have coolant lines.

Can any one help me locate it:
Screen Shot 2022-03-16 at 5.38.12 pm.png
 

Tim Green

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I would suggest that there may not be a straight dc/dc converter. The motor controller will take the dc and modulate it to power the motor. The battery charger will take in ac and convert to dc to charge the battery. But then I don’t know what the spec is for the rapid dc chargers is, but it could well just comply with the battery needs at about 450vdc. There will be a small dc/dc to charge the 12v battery, but this could be as low as 100 watts?
 

Coulomb

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There will be a small dc/dc to charge the 12v battery, but this could be as low as 100 watts?
Actually, the 2020 model is capable of 178 A @ 14 V, so that's 2500 W.

There IS a DC-DC converter; sometimes it's in the same box as the on-board charger. In the 2020 models, I think it's a charger/inverter/dc-dc all combined into one box, but that's from memory.

Edit: I believe that the inverter part (for V2L) is just the charger running in reverse, not a third unit.

If you open up the charger/etc box, some photos would be great. I can possibly help out with identifying blocks.
 

Tim Green

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I confess I have no idea what 12v load is required for items such as lights, aircon etc, so I guess it may need quite a large 12v supply. Most ICE cars have about 100amp (say 1500w) so I would have thought that should be enough. If fan cooled that does not have to be large.
Do we assume this car has been damaged beyond repair?
 

Coulomb

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AC to DC is different hardware to DC to AC.
It can be designed to be bidirectional. Most high power converters are indeed bidirectional.

Like the car's motor controller. When regenerating, AC (in this case, 3-phase) from the motor is converted to DC flowing into the battery. When driving, DC power from the battery flows in the other direction through the same power converter to become AC in the motor.

It's a matter of arranging the phase of the full-bridge transistors switching in relation to the AC source. There is an inductance between them, and the relative voltage across the inductance causes the power to flow one way or the other, or fall to near zero for coasting.
 

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It can be designed to be bidirectional. Most high power converters are indeed bidirectional.

Like the car's motor controller. When regenerating, AC (in this case, 3-phase) from the motor is converted to DC flowing into the battery. When driving, DC power from the battery flows in the other direction through the same power converter to become AC in the motor.

It's a matter of arranging the phase of the full-bridge transistors switching in relation to the AC source. There is an inductance between them, and the relative voltage across the inductance causes the power to flow one way or the other, or fall to near zero for coasting.
Still needs different hardware to convert DC to AC than AC to DC.
 

Coulomb

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Perhaps we'll agree to disagree then.

I'll agree that some, indeed many of the lower power AC-DC converters, start with a bridge or other rectifier, which is not reversible.

But most AC-DC converters are at least theoretically capable of connecting their AC terminals to an AC source, and having the power flow from the AC side to the DC side. Of course, this requires careful and dynamic control of the phase and amplitude of the converter's AC port. If this is not done, the converter would just blow up through massive overload.

Edit: that means that in those limited cases, the same hardware can be used to convert AC to DC as is used to convert DC to AC. In those cases, your statement that different hardware to convert in the two directions is incorrect. If you're saying that sometimes you need different hardware, then we are in agreement.
 

AusMGZSEV

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I realise this might sound a bit elitist, but I was really hoping to hear from someone who KNOWS rather than a load of theories and guesstimates.

I studied electrical engineering at university so I’m fairly up on how AC-DC and DC-AC conversion works. I’m just not entirely sure where the circuitry is in this case.
 

Coulomb

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I was really hoping to hear from someone who KNOWS rather than a load of theories and guesstimates.
Sadly, I don't think that there are many who know. I've not even seen the hint of a service manual, either.
No mention of low voltage DC output.
Yes, that top box appears to be the on-board charger only.

I got off my chair and had a look just now. My next guess is the smallish box directly below the OBC. It has some orange connectors (so pack voltage for input), and there is a weird, shiny connector at the back that connects to the loom via ≈8 mm² wire (hard to tell as it has lagging* on it). So that could be the 12 V output. Though that's not good for 178 A, that's for sure. But I think that the 2019-2021 model has a much smaller DC-DC, perhaps 80 A max. [ Edit: It also has coolant pipes. ]

* "Lagging": I can't think of the word. That black rag-like tubing that's around all the low voltage wiring. It could be heat-shrink cloth based tubing, possibly with glue or something to seal out moisture.

Edit 2: I think I was looking at the bottom of the OBC. Sigh.
 
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Coulomb

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Sigh. I deleted that post, now it has some value for you, and being a rookie moderator, I don't know how to undelete it (or I haven't been granted that power).

I now think that this is the bottom of the OBC:

Possible MG ZS EV DC-DC output connector sm.jpg
 
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