How can I tell how much charge my 512 has?

On the top of the 512, there are three lights that tell you how much charge the 512’s battery has. When the 512 needs to be charged (only 1 of the lights will be lit), there is a rapid succession of 5 electronic beeps or bongs that will interrupt the music before resuming it to let you know that the battery is low. It will repeat this sound periodically until it is plugged in to charge.