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Discussion Starter · #1 ·
Long story short, I wanted to replace the moulded UK three pin plug on the end of the plug-in charger cable that came with the car. The reason's not really important but it's the sort of thing you'd consider doing if you wanted to pass the cable through a hole in a wall that was too small to fit the plug through. After checking out the usual sources and not finding any good reason why I shouldn't just get on and do it, I cut the plug end off . . .

. . . only to find that instead of the conventional 3 wire (live/neutral/earth) cable, there are 6 - the usual three plus three thinner wires. I've completely mangled the plug - it's the only way I could get it apart - and from what I can see there are a few other smaller wires in the remains. I can't see any electronics in what remains of the plug; some of the ends of the smaller wires terminate together and there's what looks like a resistor between a pair of them.

So what's going on? As far as I can tell, the 3 additional wires are there to monitor the temperature of the power wires in the cable. Depending on power source and draw, the wires can heat up as the battery charges or if something goes wrong, and if they get too hot bad things happen. Genuine Smart/Mercedes chargers have a temperature light on them, and I can't see there's any way they'll be monitoring the battery or anything in the car. So my best guess is that it's a protection circuit of some kind for the cable. It's a great concept, but it does make fixing the cable very challenging if something goes wrong (like the cable frays or the plug gets damaged).

Fortunately I've got another EV charge cable, so I'm not completely stuffed - I just feel a bit stupid about it (and a couple of hundred quids' worth of charger worse off . . .). Looking at other Mercedes chargers it looks like it's a common feature, but much less so with other manufacturers - it's not a feature I can see on another charger I've got (Kia). That may explain why I didn't see any advice/warnings online about not hacking it up. Most sites suggest it's just a case of changing the plug, but that's clearly not the case when it comes to Merc/Smart.

Obviously my experience is limited to the UK, but I'm pretty sure Merc/Smart supply the same chargers regardless of location.
 

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I've done this on a couple of chargers (chevy, tesla). Both included extra wires for monitoring temperature at the connector. Try just leaving the extra wires disconnected, there's a 50-50 chance the charger will think the temperature is OK. If not, you'll have to do some research. Not sure who manufactures the smart charger, but could be some company that would actually talk to you. For example, Clipper Creek made Chevy's.
 

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I guess this is why the manual warns you to never use an extension cord with the Smart charging box - it would defeat being able to monitor for excessive heating at the wall receptacle end of the cord. Of course, with the cord being only a short "pigtail" you have to use an extension cord for most practical uses of the charging box anyway.

But as far as temperature sensors, they probably use small thermistors that are molded into the plastic of the wall plug - they are little things only a couple mm in diameter. If you can't find what particular thermistor they used, you could "spoof" it by trying various value resistors until it works.
 

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Discussion Starter · #5 ·
I think I've been too freaked out by my stupidity to rationalise what comes next, so thanks for the comments.

Hallcp - that's a great (and now I think about it, pretty obvious) suggestion to try the three main wires and see what happens. I've not done that yet 'cos I took your other bit of advice and found the manufacturer/distributor details. And I've sent them a service enquiry message that I'm hoping will make them laugh if nothing else.

Yinzer - I find the pigtail part of the cable incredibly frustrating. No idea why they made that design choice; makes that part of the cable very hard to handle/store compared to the straight section.

For anyone interested, the manufacturer/distributor is an outfit called Delphi Technologies. The webpage that promotes their chargers contains some interesting information that's not explicitly detailed in the charger manual - specifically that the chargers have a couple of protection circuits in them.

Shame I didn't see any of that before I got busy with the cutters . . .
 

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Discussion Starter · #7 ·
Thanks Len. Unfortunately, without a schematic, I think I'm stuffed.

And that's pretty much the response I got from Delphi - or more accurately, Borg Warner who I assume are the parent company. A very polite response to my request for help confirmed that Delphi weren't the original manufacturer - another outfit called Aptiv take that credit, with the product being white labelled (although I suspect there may be further subcontractors in the chain). BW said that they don't have a schematic and so couldn't help from that perspective. They've also got a charger in bits on a bench somewhere, but it's not the same generation as mine. Whilst clearly sympathetic, they couldn't help.

I've still yet to try wiring without the temperature circuit. We'll see how that goes.
 

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I guess this is why the manual warns you to never use an extension cord with the Smart charging box - it would defeat being able to monitor for excessive heating at the wall receptacle end of the cord.
When I had my charger plugged into an extension cord, said cord did become quite warm. It actually made it easier to coil when I was done.
 

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Did modify my BMW I3 factory supplied 120 V charger/cord noting unusual... 3 wires only. Replaced short pig tail with molded wire - plug & 4 ft wire section. This way it does not hang from receptacle, but rests on the ground. Made sure to use proper AWG wire thickness. Modification works just fine.
 

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Discussion Starter · #11 ·
I get the impression it's a Merc/Smart thing rather than a voltage/territory thing. There's plenty of other chargers from different makers that don't have any sort of temperature monitoring.

I've got my eye on a used one from another manufacturer right now - it's a Kia like the one I've already got. Looks like a much simpler design.
 
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