The Default Battery Preset is a very helpful setting.  When you submit your feeds to Amazon to create Offers on existing ASIN listings on the FBA Inbound Shipment screen, the values from the Battery Preset are included in the feeds to Amazon.

Each of your Stock SKUs in your catalog within can have it's own Battery Preset selection.  When new Stock SKUs are created, this default setting is used.  

During the technical onboarding of your account, we include several Battery Presets so that you don't have to.  Mostly, these are alkaline (non-rechargeable) batteries.  In the screenshot above, you can see a few custom entries that were added in a test account -- like the lithium batteries in an Oculus VR set.

You can edit the Battery Presets or add more in the Battery Presets tab of the Table Setup screen.

The Global default value is Batteries not required.