I bought a simple solar set from Amazon about three years ago : two 15W panels, a solar fan, a light and a box. the box has two 12V batteries (Exide). As the batteries did not hold long anymore I replaced them about 2 months ago. For about a month that worked great : the fan and a light both worked during the day and the whole evening. But lately the fan stops already around 5pm. What can be the reason?