As @Vostok said, you need to pay attention to the conductor sizes rather than just nebulous descriptions like "heavy duty" that don't necessarily mean anything about the electrical rating of the cable.I read the MC will detect a voltage drop along a regular extension lead - can you use a heavy duty extension lead? Can anyone recommend one that works?
I have a 1.5mm² conductor 15A 10m cable that I've used a few dozen times, and it does get noticeably warm along its length when charging at 15A for hours. You can see in this picture there's some possibly heat-related discolouration around the connection to the active pin in the plug:
Personally I would go with a thicker conductor for 15A - welding supplies stores have 15A extensions with 2.5mm² conductors, I'll be upgrading to this Unimig one:
Heavy Duty Extension Cable 15 Amps 10 Metres 2.5mm IP44 Unimig U51006
Heavy Duty Extension Cable 15 Amps 10 Metres 2.5mm IP44 Unimig U51006
www.nationalwelding.com.au