Like 2old said the strap on SKS bottle mounts let you put a water bottle pretty much anywhere

https://www.amazon.com/SKS-Germany-1...37325585&psc=1

They obviously are not strong enough for a battery, especially one hanging upside down. If you want to mount a battery on the underside of your down tube you have a few options. If you have a steel frame you can drill and install rivnuts, put a bottle mount where you want it. A second option is radiator hose clamps to connect the battery mount where you want it. Since the battery will be hanging upside down in either case I would also put a velcro wrap around the battery.