'Non-rechargeable batteries only' - what happens if you use rechargeables?

The Eneloops work great in my 2 canon speedlites. I’ve also had good luck with the Harbor Freight NiMh cells, but I charge them the night before I need them. They are not the low self discharge cells.

You definitely should. I’ve had very good results with those, the Powerexes, and the Energizer rechargeables. (2700 mAh on the Powerex and 2300 on the Energizer.) Last about 1.5 times the alkalines for my strobes, and 2-3x for my camera.