An engineer friend of mine wants to construct a mains fed stairwell 8-15W LED ceiling light fed by the existing mains lighting circuit through its wall rocker light switches. He is, however, set on incorporating dim-up and dim-down behaviour and on powering the light through the dim-down phase following switch-off from an 18650 lithium battery . I anticipate criticism on using an 18650 but, if the switched mains constraint is accepted, the general idea seems to present a challenging engineering problem for which I can see no practical solution. It would be so interesting to hear your comments and any suggestions for other ways of meeting the requirement.