My car battery went dead after I charged my power bank in the car

[jasonroy]

This is a new one and I'm hoping someone can explain what happened because I'm genuinely confused. I have a 2017 Mitsubishi Lancer and yesterday I was sitting in my car while my wife was shopping. I had a large power bank – one of those 20,000 mAh ones that can charge a laptop plugged into my car's USB port to charge it. The engine was off. I was sitting there for maybe 45 minutes, listening to music on my phone through headphones, not even using the car's speakers. When my wife came back, I turned the key and got nothing but clicking. Dead battery. I had to call roadside assistance for a jump start. I've charged my phone in the car many times with no problem, but this was the first time I charged a large power bank. I've been researching impact of charging devices in car and I understand that charging a phone uses a small amount of power. But a power bank is much bigger. Does it draw more power from the car's USB port? How much more? The power bank takes maybe 5 or 6 hours to fully charge from a wall outlet at home, so 45 minutes in the car probably only gave it a small amount of charge. But that small amount was enough to kill my battery? That seems crazy to me. My car battery is supposed to be able to run the radio for hours, right? So why would charging a power bank for 45 minutes drain it completely? Is it possible that my battery was already weak and the power bank charging was just the final straw? The battery is about two years old, which in Sharjah might mean it's near the end. Or could there be something wrong with my car's USB port that makes it draw more power than it should? How can I test if my battery is healthy or if there's a drain problem? I don't want to be afraid to use my car's USB port for anything other than a phone, but I also don't want to get stranded again. Should I only charge things when the engine is running from now on? Is that the safe rule? Please share your knowledge about how much power different devices draw from a car battery and how long you can safely use them with the engine off. I'm not a technical person so simple explanations would really help. I just want to understand what went wrong so I don't repeat the same mistake. Any advice would be greatly appreciated.