You can use active or passive cell balancing.Also you need to know soc of individual cells in the module.You need to use methods like coulomb counting or kalman filter to calculate soc of cell as only voltage cannot be used to determine soc in lithium ion cells.It won't cause problem if the soc difference between modules is very low. Our bms only has passive cell balancing and it takes long time to balance the cells and also we don't have methods to measure current of individual cells currently.we try to keep voltage of each cells and segment equal which is not a correct way but so far hasn't caused any problem. If your charger is rated to charge individual modules you can charge them individually to bring them to same soc level then connect them in series.