My 2013 BMW 520d has a 105AH AGM Battery. It's time for a replacement and I intend to buy an aftermarket battery. However, 105Ah batteries are not available in my domestic market. Only 100Ah batteries are available. Can I replace my battery with a 100Ah battery? Will all the electrical functions work properly? Are there any disadvantages if any?

 

I believe your car can accept as low as an 80AH battery. You'll get more storage charge with a higher AH battery. I doubt 5AH will make that much of a difference though. I would recommend to register the battery.