Original post by: apple2repair ,
Hi a repair guy here. As the iphone 4S is pretty sturdy, touching the case with the battery connector should not damage the phone. And iff I understand your problem correctly it is pretty similar to not having the battery connected at all. Try to disconnect the battery and see what happens if you put the cable in. It will show the apple logo for a few seconds, than go black and restart. During an update or booting it there is a moment it needs battery power even with the usb cable connected to the phone. If if that’s the same as when you have your battery connected, you’ll need to exchange the battery. If that doesn’t work check the connector soldered to the motherboard. These are known for some wobbly soldering points. You might have damaged them when disconnecting the battery. Last at thing I can think of, is to test with an new dock connector flex, but that is not something that is usually related to error 21.