For example a 11" series motor is pulling around 20-30A @ 12V freewheeling, no load.
The question is, will the inductive type load that is motor presenting be a problem for charger that is used to be connected to LiPO battery pack.
40-80A is a more typical unloaded draw for a 9" to 11" motor, and it still takes a brief burst of several hundred amps to accelerate the motor from a stop. Also, yeah, the charger *might* not like seeing an inductive load. Depends on a lot of things. You could sidestep all of this by putting a battery in parallel with the motor and charger.
An inverter welder - at least in DC output mode - *should* have accurate current regulation. Once again, though, you need the battery to deliver that initial burst of current.
Finally, the ideal method for breaking motors in - to develop a nice patina on the commutator bars and to fully cure the winding varnish (e.g. - Kostov) - is to run them at 1500-2000 rpm (11" to 9") at, say, 50% of the continuous or 1hr current rating. If that rating is 200A then you will want to load the motor just enough to make it pull 100A. This is only ~1.2kW input to the motor, but it is way outside the province of the usual charger. Not a problem at all for a motor controller - they usually have accurate current regulation (notable exception is the Curtis 1221/1231) - and you can usually get just the right amount of loading by having the motor drive the transmission in neutral.
Major may have something to say about this, but since I derived this process from him in the first place...