Battery Light Flickering/ Xmas tree dash lights

A friend and I changed the oil filter housing gasket the other day. The day after, I drove it for about 30 miles and the battery light came on, and a while later, all the dash lights. The radio also turned off as well. I went up a hill and the radio turned back on and the lights disappeared, but the battery light would flicker. The car however drove perfectly fine. (Manual, so didn't go to limp mode)

Tonight I took apart everything again to checked for loose connections, put everything back together, and the battery light seems to still flickr when the engine is revved. The dash lights came on again and the electronics cut off too. The positive cable, behind the alternator, and battery terminals in the trunk are tight as well. After a thorough inspection, I cannot find out whats causing it.

I'm assuming something is loose because they the accessories and radio would go out at certain times and reappear.

Can someone shed some light on this issue?

Necessary:
1) Check the voltage battery (engine runs)
2) Read DTC (diagnostic troble codes)
Result write here.

Your symptoms may indicate an intermittent malfunctioning of the alternator (brushes making intermittent contact). Hence the flickering battery light. The "X-mas lights" and the radio turning off and back on indicate an onboard undervoltage (around 8 ... 10V) since your battery is discharged by the several electronic circuits in your car but not charged sufficiently.

The same happened to me a few weeks ago while driving. First, the battery light came on and 2 miles later the "X-mas" lights came on, wipers started wiping slower and slower and then the engine just died.

Both my alternator (dead) and battery (weak) needed to be replaced. How old is your battery?

- Complete DIY for how to test a BMW battery & alternator (1)