Southwire 12/3 3-Prong Outdoor Heavy Duty Lighted Extension Cord: 100' $35, 50' Expired

I'm an electrical engineer. My advice is neither wrong, nor dangerous, but you did misread it. If he is charging a 12V battery and delivering 40A to the battery, the absolute maximum POWER that is being delivered to the battery is about 600 watts. The battery charger itself is not 100% efficient, if we assume it's 50% efficient worst case (and that strikes me as particularly bad) then the maximum instantaneous dissipation is 1200W (half in the charger, and half delivered to the battery). This assumes 100% of the power is coming from the wall, which may not be the case, since the 40A is in "starter mode", and likely had stored some energy in the charger.

So the question is how much current will be pulled through the extension cord. If you dissipate 1200W at 120V, that's 10 amps going through the extension cord (assuming a power factor of 1, I hope this is mostly reasonable), which gives you pretty good margin on the15A limit. It's also worth pointing out that "starting mode" means inherently that you aren't running it for more than a few seconds. If 12 gauge wire is rated for 15A , that almost certainly means continuously (12 gauge NM Romex would typically be rated for 20A continuous)... So, even if you exceeded 15A for a few seconds, it's a non-issue since heat takes a while to build up. It's pretty common for 120V welders to exceed 15A when the arc is first being struck, and it generally doesn't matter, the breakers don't even notice. In other words, there's a huge amount of margin.

What's more, most 120V single phase house wiring in the US is 12 or 14 gauge which is rated for 20A or 15A respectively, and tied to a 20A or 15A breaker respectively. So, it'd be really odd to manufacture a battery charger that exceeds 15A over more than a second or two since many breakers would be unhappy with it.

I was having this problem with other links. Ad-Block-Plus ended up being the problem.

Electrician here -- this is a GREAT deal ... you need to educate yourself on difference between 12 / 14 / 16 awg wire ...