In size these batteries are quite large. They are called Lithium Titanate Oxid.
Voltage is 2.3v and they store 40amp/hr.
Physical size is 160mm Long by 66mm across. With poles added the battery is 202mm long.
Their big plus is their ability to being charged/discharged almost daily and last for decades.
Their Website :
The 40amp/hr battery :
LTO Batteries (Various sizes) :
Developing story, more to come.