I am trying to DIY an external battery for my laptop. I know I could cut the laptop connector part off my laptop adaptor and then connect the connector to a battery however this will damage my AC adaptor, I am just wondering whether it is possibly I could connect the battery to the AC adaptor directly through the 3 prongs that connect to the wall outlet. in other word, the input that will go through my adaptor will be 12v Dc instead of 120 v AC. do I get 12v DC output through my adaptor if I do that?
Your proposed connection of a battery to the AC input of the Laptop PSU will not be good .The DC needed to make sure that the PSU produces enough output and does not overheat could be say 100VDC which means that you should not play around with it if you do not know what you are doing .The internal fuse in your laptop PSU will be specified for AC .This could mean that under fault conditions the PSU could burn your house down if not backed up by a proper larger more expensive DC rated fuse .Laptop PSUs designed to run on AC are optimised for cost and not efficiency .This low efficiency will make battery operation not optimum .