That can happen with crappy / mismatched batteries, OR if you are charging internally, check your charge time.
While most of us don't like to charge internally, if you DO, then one recommended option is to let the scanner discharge until it beeps, and have your charge time set to 14 to 16 hours. That puts the least amount of stress on the batteries if you don't keep accurate details of your usage times.
In the menu settings of the BCD436HP, don't know about this other BCD scanner, there is a setting for the battery alert. It can be set to allow the batteries to get to a lower voltage before it alerts you. The 436 seems to get to that (default) alert level fairly fast so I set it to a lower level and now I'm getting MUCH more time before the alert.