Below is information about the difference between Desktop Mode and Longevity Mode. Once you know the difference and their functions it will easier for you to choose the option that best fits your needs.
The easiest way to get the best battery life out of your Dell laptop is to select one of the preconfigured power plans. The preonfigured power plans automatically adjust key settings that can have a significant impact on battery life, such as reducing screen brightness or dimming the display when you are not actively using your laptop.
To extend battery life as long as possible during a working session
Change the Power Plan
Change the Power Management Plan.
Keep the Vents clear.
Leaving your laptop plugged in all the time or never allowing your battery to fully charge when it's plugged in can actually reduce your battery's charge capacity and shorten its lifecycle. However, Dell has introduced two new battery charge settings, Dell Desktop Mode and Dell Longevity Mode, that help extend a laptop's battery pack lifecycle.
Desktop Mode monitors the charge status of the laptop battery, and if it spends most of the time at 100 percent charge while connected to the power adapter, Desktop Mode will automatically adjust the charge levels of the battery. The adjustments include regularly allowing the battery to discharge or drain, even while plugged in. The benefits help slow the reduction of charge capacity over time and may even decrease the need to purchase a replacement battery during the life of your laptop.
To turn on Desktop Mode in Microsoft Windows
Longevity Mode monitors the charging and discharging behavior of your battery. If you're frequently connecting and disconnecting your laptop to a power source without allowing the battery to charge fully, Longevity Mode will moderate the extent to which your battery charges to protect your laptop battery health.
To turn on Longevity Mode in Microsoft Windows