Dell E7240 stuck at 0.59ghz

Hi I have dell E7240 laptop and I bought new battery..But my processer processor speed stuck at 0.59 GHz on battery but when I plug in the power adaptor the core speed goes to 2 GHz and normal speed but when I remove the adaptor back the processor goes back to 0.59 gz and it slows on the laptop I tried updating the drivers and updating the BIOS but all ine vain please if you have any soluation please tell me.