Battery capacity will be affected by the cold of spring or fall.
A DC extension cable lets you power your Bear Sentry from inside your tent where you can find ways to keep your power supply a bit warmer, safely. Connect to a DC splitter cable and you could power another device at the same time.
3 meters (10 feet) should be just long enough to reach the energizer in a standard Backcountry fence layout (packable at 59g / 0.2lb).
For a larger campsite it’s perfectly fine to daisy-chain more than one.
The Base Camp fence already comes with a 3m cable but if your vehicle is set back you may need another.