Any new battery has to be cycled at least 6-8 times before it reaches 90% of its full capacity. Reaching 100% is rare and usually depends on the operationg temperature, load etc.
completely discharge and recharge the batteries 6 - 8 times...better to do so while listening to music...
You should see an increase in battery performance...
Either that or you are trying to drive a 12" MTX Subwoofer with the XP-7

hee hee
