orig. battery was weak, didn't hold much charge
The first battery from iFixit resulted in a phone that appeared bricked. Phone would start but not boot, not able to restore or leave recovery mode. DFU mode could be achieved but nothing else.
Mike picked up my comment on the "meta" site and suggested I send a note to support@ifixit.
The second battery arrived, I removed the first replacement and reinstalled the phone's original battery --- it booted up perfectly. I removed that and installed the new, second battery from iFixit and all's well.
If you fear the worst, reinstall your old part and verify you can get back to the original condition that required a fix (if applicable, of course.)
And go slow with the back cover. I bent it where it meets the antenna cover and now doesn't close completely.