Jason wrote:

Is this battery in the keypad itself, or the brain? There's a big
battery in the brain, but it's odd shaped and I would probably have to
find it online. I didn't see one in the keypad at first glance (at
4:30am).

The battery is probably standard and available at Home Depot or computer
stores.

I'll wager it's 12V, something like 7AH.

Take the old one with you when you buy another - they do vary on connectors.

And they last 3-6 years.