Ideas to check
Scott, The most likely cause is that the second battery that is not boiling out is not connected. Maybe the 1-2 selector switch is in series with the charger and is only connecting to one battery. Perhaps there is a bad connection (high resistance) to the full battery so that nearly all of the power goes into the boiled-out battery. You can check the voltage drop between the terminal and the wire on both ends of the battery with a 10-15A load using a digital voltmeter. The voltage drop should be under 10mv on each. If that isn't it, and if the full battery is older, it is probably because the older battery has a higher internal impedance than your new one. Again the charger power will flow to the battery with the least resistance. Old batteries have more internal impedance primarily due to some degree of sulfation. A quick check on the internal impedance and sulfation level is to first charge and then separate the batteries electrically overnight. This lets them return to their own 'at-rest' voltage. Then measure each battery without a load with a digital voltmeter. Measure to the nearest 1 mv if possible. Then apply a 15-20A load on one battery and measure the voltage across the battery terminals. Do not measure at the wires on the terminals because then you also get the drop from the connections. You want just the battery voltage itself. Do this measurement on both batteries with the same load. You will probably find that the new battery has a lower drop with the load than the older battery. This is because the older battery has a higher internal resistance and some of its voltage is being lost inside before it gets to the terminals. In any event, you want to only add DISTILLED water (available at the food stores- NOT SPRING WATER) to the battery and recharge just overnight. It would be of benefit to use the boat so that the rocking of the boat will help to mix the water with the acid in the battery before charging. I would suggest that you not use the charger when you are not on the boat, but conenct it only for a few hours to get your batteries up to full charge. Having a charger boil batteries dry is quite common if there are any faults in the system. Good luck. Rich
