Does not start when touch battery charger it starts and will start for a day or 2and dead again. ConstantLy happens

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dealer said it was battery. Purchased brand new battery and it was a waste of money. Old battery was good. Car would not start the next day. Two other mechanics worked on it to no avail. Once car was jump started it would run for a day. Car is still dead in drive way. This has been going on for six months. Car only has 48,000 miles. Not reliable. Why can't this problem be fixed already? Shouldn't Hyundai mechanics know how to find a fix a problem?