You can't turn your shield off automatically once it's up. But you can turn it off prior to it going up if you see incoming that you can stop easily (such as a spam attack). The only way to "turn off" a shield while it's already up is to relocate (which isn't really turning it off).
And you CAN recharge your shield while it's up. BUT it won't add to the shield. It'll only prep the next shield (which you'll have to turn back on after the 1st one wears off). Basically, it's just a prep that you can spend your energy on.