Ive got a mid 2012 15" Macbook Pro with which has a magsafe 1 power port.
Is it possible to charge this model of laptop with an external battery like the 20000 Mah units you can get to charge phones / tablets ? But with a magsafe adaptor on it ?
The short answer is yes.
You just have to make sure that the battery is putting out between 12 and 18 volts. Be aware, though that the 20000 mAH is only 2AH (2 amp hours or provide 2 amperes in one hour). At that rate, you'll only get 36 watts (assuming 18V).
Will it charge? Yes. Very slowly.