Someone posted the pic below on a Sondors ebike Facebook group wall. The poster said he'd charged 800+ times before this happened. The battery still works, but the charger is done.
A couple of folks seemed to think their batteries/chargers were also at risk because of the sparking they see when attaching the charger to the battery, but several others chimed in that pretty much ALL of the Sondors batteries spark when first plugging in the charger. I wouldn't be surprised if that were also the case with the RR chargers - I also own an IZIP Sumo and Prodeco Phantom, and neither of their chargers spark at all but they aren't inexpensively sourced crowdfund ebikes, either.
Several theories about what caused this emerged, including: the charger was plugged into the wall before attaching to the battery; the charger wasn't securely attached to the battery, cheap Chinese charger/battery... All I know is that I'm going to be extra careful plugging them in, never leave them unattended, and always have a fire extinguisher handy.
(Link Removed - No Longer Exists)