Found these, hope they help. Basically it slowly recharges to 100% when you don't upvote. The higher your % the more the upvote is worth. Basically you will have less of an effect on the value of the post if you spam the upvote, you also get paid less if you upvote every post even if a post gets popular.
https://steemit.com/steem/@pfunk/i-feel-like-the-voting-power-recharge-mechanism-is-flawed
https://steemit.com/steemit/@aaseb/please-help-me-understand-the-voting-power-recharge-rate
RE: what happens if my voting power goes to zero ??