Lenovo Yoga 2 battery issues?

I've had a Lenovo Yoga 2 Pro for almost a year now (it'll be a year towards the end of August), and it has given me so many issues since.
The latest issue now revolves around the battery. Last week, my laptop just completely stopped charging, even if it was plugged in. So I went out and bought another AC adapter, and then the laptop started to work perfectly fine. I even did a BIOS update like what they recommended on their website.
But now all of the sudden, the battery isn't charging again. I even left my laptop to charge over night, and when I just opened it up it went from 7% to 4%, then shut off on me.
I do not keep my charger in my laptop for long periods of time. But I don't know why it's acting up like this. AC adapters are EXPENSIVE too - $70! I don't have that, I am a college student… Any tips?

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