Many (most?) suppliers redistribute batteries manufactured in China, so it's the wild west. If you're considering DIY, purchase from the most reliable suppliers recommended at endless sphere from individuals who have been doing this for a long time; these are em3ev and ebikesca (Grin). If you purchase an OEM bike, FME (and reading) the batteries are safe (these manufacturers have a lot more to lose than some fly by night Chinese company that might change its name and go back into business half a block away).