I've not been using this one long enough to report on low light conditions but what I would say is that as long as the solar panel is big enough to deliver the required charge it should be fine.
You need to consider the energiser you are using and the output of the solar panel.
I read up on the experiences of people using the same one I have and indications are it should be ok .... ask me again in 6 months time ....
I don't know whether we have any electricians on here to verify this, but normally when using an energiser powered by a 12v battery you'd ideally fit a leisure battery, albeit they cost considerably more than a comparable car battery. The reason being, leisure batteries are better suited to a near total discharge and re-charge than a car battery is. However, if you are using a solar panel to keep the battery topped up, if my theory is correct you should be fine using a car battery because the discharge is not going to occur.
Pete