Thermal cutouts

[GMM]

I was charging a car battery in situ using my bog-standard B&Q extension reel on Saturday and its thermal cutout tripped. The charger was giving about 4 amps, which makes it about 50W. I had always presumed that the thermal cutout would trip because of high current passing through it, but am I missing the point? It was quite warm and it may have got a bit warmer than you would expect in the car, but I was a bit surprised. It reset OK and worked later on with a drill. I just wondered whether some of the experts in these things would know whether it's something trivial (too warm in the car, faulty trip etc) or whether I'm just misunderstanding how these things work.

[Andy Burns]

Had you un-coiled the extension reel?

[Bob Mannix]

It shouldn't make any difference - they are rated coiled and uncoiled but I don't recall seeing a coiled rating that wouldn't drive a battery charger! Call it 100W load, that's ~400mA.

It sounds like it was in the car and got hot in the sun.

[EricP]

Mine was doing it yesterday. It just gets hot with use and shuts itself down to cool.

[GMM]

Yes, it was uncoiled, although I wouldn't imagine that would make a great deal of difference at those current. If mine's not the only one, perhaps it's not the end of the world....it doesn't seem permanent. I was just wondering why it should happen!

Thanks chaps