Blinking battery, blue screen & shuts down.

Hello,
I have been having the same problem with my laptop. The battery light blinks when the battery charger is plugged in. This doesn't happen all the time though. Sometimes I can get it to work, but I can't bump my laptop because the screen goes haywire and I have to shut the laptop down. Any ideas what the problem could be? I know it's not the hard drive. I'm thinking it's the battery or it's not getting a good connection. I have 2 batteries and they both do the same thing, although they probably aren't full of juice anymore. Help!