I am currently filling from the battery end with the cap off and I can even manage to overfill them too. I tend to get greedy after I taryn spin a carto and filling the carto with the second set of drops. I've identified the problem anyway so now I understand why filling from the mouthpiece was so problematic for me. I could not see what I was doing and I overfilled a few cartos. I took the white cap off and still overfilled some cartos.
I am now better able to tell when the filler is saturated versus soaked. If I don't feel like popping the cap off, I may occasionally fill it from the mouthpiece end, but then I have two ends to wipe. To sit and fill more than one carto at a time, I'd rather pop off the caps, fill from battery end, spin, fill again, and wipe less.
My tissue consumption is higher when filling from the mouthpiece end!
I have to get syringes! I think keeping the center tube dry could be the key to a longer lasting cartomizer and certainly less waste.
You'll love filling with a syringe. I finally got one for myself and it's fast, clean and precise. Amazon has some - I got a couple from some random e-cig place I don't recall. They're like 99 cents each. I never liked using a condom