OK, so I've got some news. I had the seller send me a new charger in replacement for the one that fried its thermistor. Just had it opened and the interesting thing is that the location of the inrush limiter thermistor is different on this one. I thought these super cheap China electronics were only manufactured once in a super batch with no possibility to correct flaws that customers discovered along the way. But perhaps I was wrong.
The great thing with getting this new unit is that I could easily look up the ratings for the inrush current limiter thermistor. It's marked "1.5D-15" which means that it produces a resistance of 1,5 Ohms @ 25 °C and has a diameter of 15 mm. Checked the remaining properties, which show that it should be able to handle 8 Amps continuous @ 25 °C.
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The million dollar question is of course: did the manufacturer put in a more potent thermistor in this this second version of the charger that I got, or will it soon be fried like the old one?
Anyway, this gives us some good aim while choosing a replacement thermistor. It should produce a nominal resistance around 1,5 Ohms and be able to handle at least 8 Amps. You’ll find them in any well-stocked electronics part-shop. I would step up and put in a stronger one that handles up to 12 Amps while I’m at it.
It’s really easy to locate the thermistor. Just disassemble the casing and follow the to wires from the mains connector. One of them meets the thermistor directly at the circuit board. The thermistor has a piece of shrink tube around itself. Good luck!