To be fair, RAC19 was running at 20V without mishap and has since had his NUC running at 12V for some hours without problems, but Intel also say that potential damage can occur if less than 11.6v.
I just checked no load voltages my multimeter.
NUC power adapter 19.36 Volts
MaxOak K2 “20 Volt” output 19.27 Volts
MaxOak K2 “12 Volt” output 12.07 Volts
These would all be a little lower with a load connected but I would need a break-out-box to get measurements under load, which I might try to do.
As always, anyone not prepared to a risk, should stay strictly with the manufacturers advice. Personally, I am happy to use the MaxOak battery on either “12 Volts” or “20 Volts” but I would not want induce anyone else to take a risk that they are not prepared to live with.
RAC19, I trust you far more than either KayoMaxstar or Intel. But I don't want to fry my £525 investment and not have the benefit of the Intel warranty. It is proving difficult to obtain any manufacturer guidance.
The fact that the "20v' (19.27v) output of the MaxOak is less than the NUC 'brick' (19.36v) suggests there is no risk from using the MaxOak K2, and it does seems to be working fine for you. But unhelpfully, nowhere in the NUC Support Forum is there any guidance as regards using external power except advice from Intel contributors that it is wise to use a DC/DC 19v voltage regulator to ensure 19v. No thread ends with any actual battery recommendation for the NUC.
Yet most battery manufacturers guarantee compatibility with specific laptops (Dell, HP, Toshiba etc) but the Intel NUC isn't listed I have since found just one battery "guaranteed" to be compatible with an NUC at 19v (from www.psaparts.co.uk), but none that also simultaneously output 12v for other devices. This is the PSA model VP-V6N56D has 1x 19v (can be varied); 1x10.8v, 2 x 5v. But this doesnt work as 10.8v is too low for my camera. I am going to try with my Tracer 8aH. But am now struggling to find a cigarette lighter connector that fits a NUC. I find this all surprising, notably the lack of help from Intel.