Battery indicator

I’m confused how a battery indicator works?  One time, I got 4 bars, and it got full charged in less than one hour.  Most of the times, I get five full bars, and I recharge Instinct.  It keeps recharging for several hours, and the battery indicator never stop animating.  Whenever I want to recharge it, it doesn’t recharge at all, and battery indicator is not animating.  So, what is the deal?