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How do you know the batteries are fully charged?
It is winter. The daylight hours are few and the sun is low in the sky producing MUCH less light that in summer. Then the batteries do not charge much.
My solar garden lights glow all night long (8 hours) in summer when the batteries are charged for 15 hours with very bright sunshine. Now in winter they charge weakly for 6 or 7 hours when there is sunlight and barely charge when cloudy. At night they glow for 1 or 2 hours after a day with full sunshine.
The solar garden lights used to have light dependant resistors and a plastic cover on the solar panel that got sunburned and cloudy after a few months. New ones use the solar panel to detect light and dark and they have a glass cover that never gets cloudy. Old ones used antique Ni-Cad battery cells that rusted away in a couple of months or simply failed. New ones use a modern Ni-MH battery cell in a stainless steel case. But the Chinese workers do not properly seal them so rain water destroys the electronics in a few months.
I was given some garden lights last Christmas, to replace some which had failed previously as you describe. Before putting the new ones to use I stripped them down and painted polyurethane varnish liberally on the innards (apart from the switch) and around the photocell edges. I also drilled a couple of additional drainage holes in each casing base.
So far so good. They're still working .