This is a very intriguing question and it is another that is based on solar lights. It is easy to assume that once the integrated battery inside of a solar light begins to become at critical levels of battery life, it would merely cause the light to dim. Well, this is a wrong assumption unless there is a specific design feature that enables you to control the light output of the light itself. In other words, if there is a low setting then you could technically create a dimming light. Yet, the light will not do that on its own unless the manufacturer allows this to happen.
It seems like 40 different answers are being thrown your way and to be honest if the light bulb is dimming it may be that it is reaching the end of its own lifespan.