I'm having the same problem. The solid 5 lights indicate your batter is fully charged. Hold the battery test button in for over 3 seconds. If no lights come on [or if all five go off] the battery is still healthy. There's a difference between battery charge and health. [See pg 85 of the owners' manual] However, if some lights come on, the battery is starting to deteriorate.
One post I read said to completely discharge and recharge the batterv 2-3 times. That's what I'm trying now. I rarely run my 1520 off the battery. It's usually running off AC power. This suggestion makes sense. I have some other applicances that the manufacturers say should be completely discharged periodically in order to extend the life the batteries. I'll see if this stops the blinking on my 1520.