We got a solar panel installed last year (with 2, 6 volt batteries). Hooked up the batteries last week after having them stored inside for the winter. The indicators show that the battery is fully charged, but when you turn a few things on (even a few lights) the voltage goes right down to nothing. The batteries just don't seem to have much power, even though it shows as fully charged. The batteries are only a couple of years old. Any ideas?