Support Crazy Battery

Hi All,
I have a weird problem in my Galaxy.
it started long time ago but lately it becomes impossible.

my battery get empty and then full again within seconds.
here a common scenario (which happen yesterday for example):
phone was charged all night. when i woke up, the battery was 100% charged, unplugging the charger and battery is 70% charged. when i got to work it was about 50%. after a few minutes it was about 12%. then, i turned off the phone and charged it by USB. after a few hours, turned it on and got 4%. answered a phone call, and surprisingly got 50%. and then about 60%, after a few minutes it got back to 50% and stayed like that until the end of the day. i went to sleep with the phone charged (it was about 50% before i went to sleep). woke up in the morning and - no phone. cant charge it, cant turned it on. nothing.

this happens a lot!!!
at least once in two weeks my phone is charged all night, but in the morning the battery is completely empty. i need to play with it for a few hours until its ready to charge itself while turned off...

if anyone had this problem, I'll be glad for some help.
i don't think that the problem is in the battery itself, but some kind of a bug.

Thank you

 

yes i am.
Im using open home which shows the battery level and power manager.

I got a new battery today and the problem continued.
so, i got back to the support and after waiting for an hour, the technician said that it happens because too many applications are open.
I think it might really be the cause of the battery running out very quick, but it doesn't explain the charging problems.

anyway, he didn't know to answer my question how should i close the applications (without task manager which i showed him)

so now my question is how should i get out of applications in order for them to close. task manager did show a huge amount of open applications.

thank you