It may be possible to add a battery to a Stonepoint LED Lighting A4000TH-QRXD LED Large Dual Purpose Work Light to provide backup power for 3 hours, but it would depend on the specifications of the light and the battery.

The A4000TH-QRXD LED work light is designed to be powered by an AC power source, and does not come with a built-in battery. However, it may be possible for a licensed electrician to modify the light to accept an external battery pack.

To determine what kind of battery you would need, you would need to know the power consumption of the A4000TH-QRXD LED work light in watts (W). This information is listed as 40 watts in the technical data sheet.

With this power consumption information, a qualified electrician should be able to calculate the size of the battery you would need to power the light for 3 hours. This may require data on the capacity of the battery in amp-hours (Ah) and the voltage of the battery.

It's important to note that modifying the light to accept an external battery pack may void the warranty and could potentially be dangerous if done incorrectly. If you are not comfortable with electronics and electrical work, it's best to seek the help of a professional.