This what you need to know about parallel charging. When you parallel charge a batteries your are working with averages not absolutes. Here is an example, you have two 4 cell lipo batteries and have not yet connected them in parallel
(battery column 1 & 2) . Once connected in parallel to the main charging charging leads only (
average voltage column) . Main and balance lead connected and assuming all cells are able to equalize perfectly (
Bal lead connected).
Battery 1 Battery2 Average voltage Bal lead connectedCell1
3.70v 3.68v 3.69v
3.67vCell2
3.78v 3.59v 3.69v
3.67vCell3
3.69v 3.62v 3.65v
3.67vCell4
3.72v 3.60v 3.66v
3.67vTotal
14.89v 14.49v 14.69V
14.69vThe table above demonstrates actual voltage vs average voltages. The further you carry the table out (say four 6S battery packs) the larger the difference between the average voltage and actual voltage can become. When your parallel charge ends you are most likely not going to see perfectly balanced cells in each battery pack because, individual cells often have slightly different charge rates. When a individual cell charges more slowly than than another cell connected in parallel the faster charging cell will have a higher voltage and because the charger can only see the average voltage of a group of cells it cannot correct the voltage of a individual cell.
What happens when you connect to batteries together in parallel? The first thing that happens is that the batteries attempt to equalize their voltages. The battery with the higher voltage will charge the battery with the lower voltage until their voltages are equal. What is the problem with this? The real problem occurs when there is a large difference in voltage between two or more battery packs. Because there is no resistance or load between the battery packs the higher voltage pack will discharge at it's maximum capability in order to equalize the voltage difference between the batteries. As an example lets say you have two 60c battery packs, one of the packs has a 20% charge and the other has a 80% charge. When these two cells are connected in parallel, the cell with the 80% charge is going to discharge at 60c to charge the 20% pack. The problem that exists with this example is that most Lipo battery packs are not designed to be charged above 1C to 6C depending on the battery.
If careful you may charge Lipo battery packs in parallel but there are things that have to be taken into consideration. Avoid connecting batteries in parallel that have large differences in voltage (you should verify the voltages every time before before connecting them in a parallel circuit). Never connect the balancing leads to the parallel balancing board before connecting all of the main leads to the parallel charging pigtail, doing so will cause the the batteries to try to equalize their voltages through the very small gauge balancing wires (which usually pop like a fuse). Remember, no matter how small the voltage difference, equalization will occur at the maximum C that can be reached prior to the cells or battery packs reaching equal voltage. Only like battery packs should be charged together in parallel, IE. same voltage capacity / number of cells, same C rating and close in the number of cycles used or life span remaining (don't use new and old packs together in parallel when charging or flying).
I personally do not charge batteries in parallel because when batteries equalize their voltages they do so well above the recommended charging rates which shortens battery life. With a large enough difference in battery pack voltages the possibility exists for explosion or fire. When charging batteries in parallel you are balancing to the average cell voltage instead of the actual individual cell voltage. We have all experienced that in some Lipo packs batteries some individual cells charge more quickly than others when this happens in a parallel circuit, the charger is not going to see that one cell in a battery is low because the charger only sees the average of the cells connected in parallel. Remember the more cells you have in parallel the larger the average "error" will become. If you decide to charge in parallel please check closely your parallel charging pigtails, I recently saw one from a very reputable distributor / manufacture that had a battery connector installed with the polarity reversed. Had two batteries been connected to this charging lead a fire or possible explosion would have occurred.
If you choose to parallel charge your battery packs, you should preform regular battery maintenance (normal balancing charge) on a regular basis in order to truly balance the individual cells.
Of the choices you mentioned I would go with the Hyperion duo at 20amps per side or two of the PL6 and avoid parallel charging. With three battery packs you will be able to fly all day without pause for recharging. Just get a good power supply no matter which charger you decide to go with. Plenty of pilots are parallel charging their batteries, it just seems like a lot of extra work and higher associated risk to accomplish a simple task IMHO. I use two I Charger 3010Bs and a Meanwell SE 1000-24 powersupply. I can keep batteries charged and two helicopters flying all day without pause if I desire. I hope you find this information helpful. Here is a link to with photos and details to my charging station:
http://synergyrchelicopters.com/forum/index.php?topic=1252.0Rich
