There are a few reasons for this but the following seem to fix the problem for everyone as long as your battery and alternator test good.
First check your voltage drop from alternator to battery positive by putting your volt meter on dc volts and out one probe on the positive battery terminal and the other on the connection on the rear of the alternator. This must be done with car running to ensure everything is working correctly when alternator is running. The reading should be 0.1 or less. If not either get the hyundai replacement or wire in a new 4awg wire from the battery to the alternator ( you can leave original one in place). Thus alone may fix your problem but it didn't in my case.
Next you'll have to remove and check all grounds. Mine were all connected to a painted surface giving poor ground. Most are under or around the air cleaner box which you'll have to remove. I removed each ground wire and sanded to bare metal under each wire. There are 5 that I did.
Battery negative to chassis
Chassis to transmission
PCM ground (bunch of 5 small wires bolted to driver side fender inside engine bay)
Engine to chassis (passenger motor mount to fender)
2 small possibly headlight grounds( on chassis next to windshield and power steering reservoirs)
Gl hope this helps anyone with this bigggg problem