SAMSUNG GALAXY S3 it says its 100% battery then in 5 secs its 20%

[DJ REGGIE]

please help. this happened more than 3 times

 

[Golfdriver97]

please help. this happened more than 3 times

Welcome to the forums.
The battery may possibly be going bad. Ordering a new one wouldn't be a bad idea. You can order one here: ShopAndroid - #1 Store for Android Accessories

For the one you are currently using, I would remove it and see if you can see any bulges in the battery, look at the connection pins, etc. If that all seems good, I would turn off the phone and let it completely charge while off, and see what happens.

 

[Rukbat]

And in 5 seconds more, does it come back to 95%? Then the battery has a dendrite. It's a manufacturing defect (but try to get them to replace it under the "manufacturing defects" warranty).

Does it come back to 5% and stay there, or drop lower? It's either a bad battery, or something in the phone is shorting, killing the battery very quickly. (Only monitoring the current drain at the battery terminals over a few of these cycles will determine which.) But you can get a couple of new batteries and a charger from some Amazon partners for bout $15 for the package.