[folivier]

Hmmm, my older Freedom 2800 floats at about 13.4Vdc and will do a variable float charge of a few amps to keep my BBs charged. But I did have to look at their chart and select flooded lead acid as the battery type and warm as temperature environment. That gave me the float value of 13.4Vdc. You might have to do something similar to trick it to give you the values you need. Oh, did you disconnect or turn off the temperature compensation on your Magnum? By setting the temp. that allows the charging values to stay the same.
Ours are working great!