when battery gets low I hear a beeping sound?

Try an SMC reset. Also is there a pattern to the beeps?

SMC reset for Mac laptops with non-removable batteries

These include MacBook Pro (Early 2009) and later, all models of MacBook Air, and MacBook (Late 2009).

• Shut down the computer.

• Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.

• On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.

• Release all the keys and the power button at the same time.

• Press the power button to turn on the computer. Note: The LED on the MagSafe power adapter doesn’t change states or temporarily turn-off when you reset the SMC.