When we charge our batteries, you notice that as the voltage rises and gets nears the full charge level that the current backs off to a trickle as the charger then starts to discharge cells that are too high through the balance lead, so the low cell can catch up. Now think about putting one of each of the batteries I listed above on to charge at the same time, they are all lets say 4s, so no matter the mah, as the voltage rises on all of them, the charger will do the same thing since it is all really about the voltage level. In the instance I gave above the charge current would be 4.8 amps (450+650+850+1300+1550=4800 milliamps or 4.8 amps), and it is really as simple as that, each battery will get less amps as the voltage rises and the current backs off as it nears full charge. Remember, when parallel charging, all the number 1 cells in each battery are shorted together and they also charge together, and the same for cells 2-X.
Rules for parallel charging...
1) ALWAYS check each battery for NO MORE than about .2 volts difference in the entire pack between all batteries you will charge together, and consider if there is a cell that is more than about .1 different from all the other cells it will be paired/shorted with (again think of all the #1 cells among the packs, etc).
2) Always plug the mains lead in first so that the overall voltage can equalize between the batteries that are (already) connected, then plug in the balance plug second. If the voltage is .1 volt or less between, lets say all the number 1 cells, then this isn't quite as important since the current during equalization of all the shorted to each other cells will not be too great, but it is always a good rule of thumb for safety.
3) Good batteries that are charging at 1C will actually get colder as they charge instead of warmer, always grab each battery to test it for any heat a few times during the charge cycle. I like to check after the first few minutes, then again as it nears 4v per cell, and then again as it starts to lower the current as it gets to 4.2v per cell.
4) NEVER EVER leave your charging station while charging except to maybe hit the bathroom or get a drink, and check for cold batteries and any visible puffing/swelling before you walk away for that very short period.
There are 2 things to keep in mind when you parallel charge, one is that a bad battery can cause the charger to repeatedly overcharge and discharge all the other cells in the batteries as it tries to get the one weak cell up to voltage, and two, you will always get a little better balance on the battery charging them individually than you will when you charge in parallel, but it is minor and makes no difference at the field when you fly. In order to reduce the time it takes to charge, and to improve balance, I will often storage charge the batteries after a day of flying to 3.85 which also balances them at that voltage so that my charge is much faster and the balance function takes a lot less time when I go to charge to fly. Since storage level is pretty easy to achieve quickly, I will often storage charge them individually to get that better "pre" balance before they are parallel charged the next flight day.
I hope this makes some sense, I have probably missed some minor knowledge points, but these are the basics for safety.