http://www.batterymart.com/battery.mv?p=BIG-ETX15L(I have had that in my motorcycle favorites for many months.)
The 2/10 amp automotive charger actually puts out 14+ volts (how much plus I'd have to get a digital voltmeter to check.) They say 12 volts so as to not confuse the lowlifes. (Sort of like only giving the English 8-y-o Scotch.) On a flooded battery such as what I got before I learned about AGM batteries you use put the acid into the battery and then put the charger on it on the 2amp setting for eight to sixteen houts.
Your battery arrived fully charged, ready to install.
Since I run my bike most every day, 365 days a year, I won't have any trouble as long as I don't leave the lights on. Again. I don't need to worry about self-discharge. Assuming you run the bike at least once every nine months you don't need to worry about self-discharge either. You should never need to put that battery on a charger/maintainer.
If it didn't arrive fully charged and ready to install it is not an AGM battery.