Voltage and physical size is the most important. Cos if it's the wrong size it won't fit on the bike and if it's the wrong voltage , nothing will work. The ampage is the total output available from the battery. Each component only draws what it needs. The battery can't force more than is needed into the component. The amp /hour value is how long the battery will last under a constant load. 4 amps for an hour, 1 amp for 4 hours, 8 amps for 1/2 an hour. This is under test conditions. In the real world it's only a guide, because temperature, electrolite level and strength vary with age and weather conditions.