You won't believe it but I had gone through the exact same situation as you did. Only in my case, I found an easy solution to this dead car battery problem.
I had kept my BMW car not running for months at a stretch and when summer came, I wanted to pimp my ride. To my surprise, I couldn't even open my car door with the automatic car unlock button on my car keys.
I panicked and didn't know what to do then. Somehow I figured out that I could use a "Manual Hard Metal" key attached to my car keys. Turned out that I just needed to press a small "Notch" somewhere on the automatic car keys and then pull out the "Metal" key which I used to manually unlock my car doors.
Afterwards I tried starting my BMW car engine but it wouldn't start at all. Battery was completely dead. Not even the door courtesy light was coming on.
Somewhere in my garage I had something called "Car battery Maintainer" that I bought from Canadiantire long time ago. I hooked it up to my BMW car battery for 16 hours and then pulled it off, thinking that my car battery should 100% work now.
To my disappointment, my BMW car battery was still as dead as before, as if I never charged it ever, even though I just charged it with the "Car battery Maintainer" for 16 hours non-stop.
Out of ideas, I decided to make a visit to a local Walmart nearby to see what I could buy to start up my BMW car. I didn't want to visit my local BMW car repair shop because I know they would probably suggest me getting a new car battery which would cost a lot more.
Once there at Walmart, I found a "Schumacher Electric SL1317 1000 Peak Amp Lithium Ion Jump Starter/ Power Pack, Lithium Jump Starter" for $129.97 . I was excited to try it out on my dead BMW car battery. It worked and I got it started on my 3rd try.
So I took my car for a spin for 1 hour hoping to recharge my dead car battery by running the car. When I came back home, I parked my car and turned off the engine. After a few minutes, I tried to turn it "ON" and it seemed to start without any problem whatsoever.
The next few days, I didn't get to use my BMW car since I was using my other older car I had. Then by next week, when I tried to start my BMW car again, I couldn't believe that the battery died on me again.
So I took out my "Schumacher Electric SL1317 1000 Peak Amp Lithium Ion Jump Starter/ Power Pack, Lithium Jump Starter", and tried boosting it up again. This time it wasn't so easy and no matter how many times I tried boosting it, the damn car just wouldn't start at all.
My neighbor saw me struggling to boost my car and offered to help me. He drove his cargo van upto my car with its front hood facing my BMW front hood.
Then he attached the boosting cable from his cargo van to my BMW car battery. I didn't tell him, but I thought this was even worse than my car battery booster. He was only able to provide enough juice to start up few lights inside my car but not the car engine.
So after one hour of his help to start my car, he gave up and went inside his house. I thanked him and continued my search for a solution for my dead car battery problem.
I went back to my local Walmart store and checked out other options to save my dead car battery.
I found "Schumacher 15-Amp 6-Volt/12-Volt Fully Automatic Automotive Battery Charger and Maintainer, SC1323, Auto/ Marine Battery Charger". It only cost $79.97 plus tax extra.
I went back home and looked at the weather to see if it was going to rain that day. I thought it might probably take 12 hours to charge my BMW car battery to 100%. I didn't want the rain to destroy my newly bought battery charger.
Luckily the weather had a clear sky forecast with no rain in the horizon.
So I hooked up my new "Toy" I just purchased and let it charge. Every hour I would go back to check on the status of my battery power percentage.
Instead of taking 12 hours as I thought, this wonderful cheap battery charger actually completed my charging in only 4 hours max.
I hopped into my car with joy but with a slight doubt. The engine roared much to my amusement, and I was jumping with delight. Immediately I took my car for a long drive and it didn't let me down at all.
The next few days I made an appointment with BMW repair shop to do a regular oil change and full checkup on my BMW car. The car mechanic commented that my car battery was still good.
I started chuckling inside my head because only I knew that just a few weeks ago, this very same car battery was dead as a tree log.
I never returned both purchases I made at Walmart because you never know, I might need it one day. My BMW car is still running good to this day as it never became dead ever again. Lesson learned to always run the car engine once in a while, at least once a week if not more.