Some power panels have a direct link to the battery without going through the panel circuitry.. maybe through the starter port? The starter port (usually red and black) are normally a direct connect to the battery via the internal connections on the panel. If so, you can probably connect a source of power, battery eliminator, positive to positive, and negative to negative, and charge it that way. If you do it that way, use a battery eliminator with a current meter and watch the current drawn by the battery. As it approaches 13.6 VDc, the current will drop significantly. That's time to disconnect it. Never leave it overnight, though. By the way, A battery eliminator of the 20 amp variety, is available at Radio Shack.