[BrianG]

The switching supply inside the charger is roughly 80% efficient from the chargers I've measured. If they were more "lossy", the extra inefficiency would be evident by heat somewhere (probably on the heatsink). So, if you are charging a 4s battery pack at 5A, the highest output power would be 84w (4.2v X 4 cells X 5A). At 80% efficiency, the input power would be 105w. At 12v input voltage, the current required would be 8.75A.

The jumpered output could very well be the issue. You could remove the jumper and run off one of the 12v outputs as a test. Generally, paralleling outputs isn't recommended because most multi-output 12v supplies have seperate FETs, but in this case where there is one ultimate 12v source, I thought it might be ok. Just to be sure, where did you put the jumper? If anything, it should be after the coils (where the original wires were).