I checked the mAh of the included 18650 battery .
the Battery inside the solar box delivered 1250mAh down to 3,7V.
so it can be replaced by a 3500mAh if you like, but it is not really necessary.
i should last a good 4 days with no sunshine.
if replaced by a 3500, that should be about 10-12 Days.
so it depends on the situation of installation, if you expect some 3 weeks of darkness, it is also an option to put two 3500mAh inside… if there is not so much darkness in winter, the included battery may do the job.