One never really "buys" a battery, we basically just rent them. I have seen new ones fail. They fail at any age. Use a hydrometer on them. Make sure your connections are taken off, cleaned, and tight when you put them back on. Use a voltmeter at the battery and then at the inverter. A long run of light wire with a poor connection will drop a volt easily. Can you unplug the dish after it locks on? Twelve volts part that is of course. Of course you could measure the actual DC current being used and then do the math. End of story doing it that way.