hi bought a foldeble bike (with one of Thos bulky grey batteries) 2nd hand. long story short , bike goes 4 minutes ok , then get week ,then looses energy (the energy metter dont even show anything) as if , complete lose of Power. 10 minutes later ,without any Power , it gets the Power back and starts again . and the same repeat. usesless.. Engine ? Electric issue ? battery ? all ? TNX!