Its definitely because you have such high resistance with the nickel. Guessing nickel plated steel maybe 0.12mm or 0.15mm thick? Yeah you can't draw more than 20 or 30 amps until voltage drop is insane Also 35E's are good for 8A continuous according to the datasheet Those 15Q's are rated at 18A so huge difference
Until you do a proper load test on individual cells along with a capacity test at 0.2C you can't say for sure what you've got. The fact that those "rubbish" 35E's weigh more than the BAK cells leads me to believe they arent total junk cells
