You need to keep the battery filled or it will deteriorate fast and short the cells. Fill it with distilled water only (mineral free), charge it and you should be fine. Also, you should check the output of the alternator to ensure that the regulator is not overcharging (max 14.2VDC across the battery on a rev).
Not supposed to refill with electrolyte/acid after the initial fill. Before you do that, be sure that you use a hydrometer to check the specific gravity to be sure the mix is correct. They sell them at Harbor Freight Grainger or an auto store, have a little ball/scale that provides an easy red-yellow-green evaluation for dummies.