what happens if I just plug 10 volts or 9 volts directly into it via AC-DC adapter
How important is it to you that you don't wreck your battery or the thing you're trying to charge it with?
If you have a battery and chargers that you don't care about then nothing wrong with just plugging things together to see what happens. But if you want the battery charged without damaging it and want to avoid overcharging the battery or overloading the charger, do it properly with the right charger. The battery probably costs more than the charger; just how desperate are you to avoid spending a couple of bucks?