I have parallel charged 8 batteries at the same time, same bank. "ALL" cells must be close (voltage wise) before doing something like this.
I use a Duo 406 with a 1,500w power supply.
After doing this a few times, I now only charge 4 batteries at a time on one bank, the other 4 on the second bank, if I need 8 charged batteries.
I guess I was testing the limits of the 406, it worked, but when charging 8, it definetly takes a long time. In fact, I mostly only parallel 2 at a time now. They charge must faster.
After flying, I storage charge my batteries. When I know I am going to fly, I will charge them the day before, or the day of the flights.
Nothing beats have a good charger, it makes all the difference in the world.
PS: If any of my batteries are below 3.4v, I pull them from the stack and deal with it separately. Anything under 3 is usually toast. I have replaced a single cell in 6 cell batteries. I DON'T recommend this, it's dangerous, I do have an electrical background, but the only drawback is this battery even after replacing the bad cell will usually go bad within 3 or 4 months. I guess the other 5 cells can't handle a newbie in the pack.
I have taken 6 cell batteries and made them 3 cell, then use a series connector to bring them back to 22.2 volts. This seems to work OK.
DON'T TRY ANY OF THE ABOVE UNLESS YOU HAVE ELECTRICAL BACKGROUND EXPERIENCE. IT'S VERY DANGEROUS AND EASY TO CROSS WIRES which will cause a phosphorus type fire that will have to be smothered out to stop, or they will burn themselves out.
On another note, I have received brand new "bad" batteries. I always watch the first 5 minutes of charging and never leave the room when charging. One time I pressed the start button and noticed my 406 going crazy. The battery started puffing immediately and I JERK OUT the connections. There is not a doubt in my mind this one battery would have exploded in less than 15 seconds more. I didn't even think about hitting the stop button. I guess my experience kicked in as that's exactly what I would have done at work. I carefully got it out of my shop and put it on the concrete driveway. The next day I properly disposed of the battery. There was no way I was going to ship it back.