My FPS drop whenever i disconnect the battery cable.

asad814 said:

and BTW some times when batteries become old they can not give much power when you do heavy tasks......so thats not unnecessary thing

Not completely true, if the battery becomes too old or it is has a defect, which causes it to not provide the power the system needs, that will cause the system to crash/shut down unexpectedly. A higher load requires higher power delivery. A fully charged battery might be able to provide the power needed to run the GPU and CPU at 100% load, but as the battery's charge drops the maximum voltage it can provide decreases(you can download HWiNFO and observe how the voltage drops with every percentage, then plug in the charger and see how the voltage ramps up) and with voltage the maximum power it can deliver also reduces. It used to be able to deliver enough power to supply the components, but now when the charge has dropped to 30%, it supplies less power not enough to feed the components, which will cause the system to shut down/crash. To avoid this, the power limit of the CPU and GPU should be reduced, which in our predators is around 50%. With every charge the battery's wear level increases, which over time reduces the battery's power output. Also as iamtherealappa mentioned, the battery would be gone within 30-60minutes, which in many cases won't be enough to satisfy you and then you will be left without a laptop for the rest of the day, if you are away from the charger.