I bought some of these Pannier, but I noticed something! Mine were literally £12 (and bought more with a possible idea of modifying them). Do yours have the multiple modes (hi, lo, strobe?) When you switch them on/off?
The wire that came with mine seemed super thin, and current measuring 500mA.
Changing that wire to proper 0.5mm2 CSA now means the lights draw 1.5A and are noticeably brighter. This does need you to resolder the circuit board which might be a pain without a high power soldering iron - as for longevity with this mod - I can't say as mine aren't fitted yet!