DC Power for Lipo battery charger?

Hi.

I've got one of those wes'tech chargers for my 3 cell lipo's 1200 batteries, now the problem I have is I cant get enough dc power to work the thing, my dc power i can adjust from 9v-14v, that would be ok for 1 to 2 cells but 3 cells i need 15v upto 26v.

I've just spotted that my Dell lappy's transformer, the output is 20v, so can i use it and what holes do i poke my wires into, I have a choice or 3 holes.

Thanks.