A new problem on my daughter's phone after she installed iOS 11.3. After taking the phone off charger the battery would drop so rapidly that you could basically see it tick down. She tried the usual steps of looking for apps that ran a lot, turning off and on, and resetting. Apple Store "genius" though it was brightness, which it obviously wasn't. Phone support first had her do a restore from backup (no help) then a restore and setup as new. Initially that seemed promising, but as soon as she logged into iCloud the problem was back. Apple decided it must be the battery, even though it checks out as being healthy. When she got to work this morning, the battery was down to 50% (in about an hour). She didn't have wifi because she had wiped all the settings and phone didn't recognize any networks. But, the battery stopped losing charge. After she waited a while to see that it stayed that way, I had her log on to the local network and the battery immediately started to drop. Turn off wifi, stopped dropping. I was sure we had it figured out. Unfortunately, when I had her log back in to the network to try something else, the battery did not resume losing charge. Something is wrong, and it appears that it may be related to wifi, but it's not consistent. Personally I don't think it's the battery. I post this to see if any of the true geniuses here have an idea of what's going on.