how a battery behaves in parallel to the charger with a shared load but with a voltage difference
My Xtal ball says you are thinking of this;

51V (58.8V) 5A Li-ion / LiFePO4 Charger
Works with 14S Li-ion batteries and 16S LiFePO4 batteries.

When you put a load on the battery while charging it defeats the cut-off detected by the current in CV mode dropping below 5% ( or so) . Failing to do this can significantly reduce battery life by the amount of time spent in CV mode.

Otherwise, in CC mode, no problem since your charger will share CC (5A) between battery and load current, so it takes longer. ( just like your mobile)

But if in CV mode, you must limit the time spent charging at 58.8V if the system does not monitor Battery current separate from the load.   ( read up at Battery University site)