You have left more out of your question than you have provided. To determine your requirements:

What do you want to know about the battery? Does it exist, or do you need to know the remaining charge?

Is the battery a 'backup' or is it the primary power source?

Is the battery replaceable, or permanent? Is it rechargable or is it a primary (non rechargable)? If it's rechargable, what is the chemistry, and how is it charged?

What type of processor are you using? What does it have for sensing? A/D, comparators, etc? Can it retain history of battery use?