My roomba 560 may be having battery problems

My roomba 560 recently started backing off the charger and going only a foot or two, then stopping and playing the little song that means the battery is dead and shutting off. I'm sure the battery is dead, but it hapenned pretty much overnight, and I guess I figured the battery would wear out slowly over time--like it would just vacuum for less and less time. Is this the battery, or do you know? Also, are the other batteries available online as good as or better than Roomba's, and do you recommend them.