Hold a battery to start charging it. The optimal charge is indicated by the white line going across the battery around 3/4 of the way up. When the colour goes green, then it is charged optimally. You need not hit it exactly at the line to go green. Around the green area is the orange area, which is also acceptable. Anything above or below the orange area is out of bounds. When a battery is insufficiently charged, the alarm clock will not run and collapse. If it is overcharged, it will overclock and collapse. Note that you cannot compensate between batteries; for example, you cannot undercharge the second battery if the first one is overcharged. You gain a point for successfully charging all 3 batteries and having the clock run smoothly and ring.