Warning: Attempt at own risk. A lack of knowledge or improper equipment can result in both the loss of equipment and a fire. Only attempt if you fully understand how to properly construct and use multiple battery charging setups and accept the risks of doing so. I am not responsible for accidents that may occur when using these charging methods.
What is parallel charging?
In a nutshell parallel charging allows for multiple same cell count batteries to be connected in parallel to create a single larger logical battery that can then be charged.
What does this mean to you?
Say you have (3) 3s 2200 packs that you run in your 450 sized heli. You take all 3 packs, wire them in parallel, set the charger to charge a 3s 6600mAh pack and poof you will have 3 packs done in an hour. This is assuming a few things of course but that is the basic idea and it is a good one to say the least.
How does it work you ask?
It's actually pretty simple. When you place all the packs in parallel, they all become the same voltage. I am not speaking in metaphors here, they really all do become the same voltage. Not the average of the packs either. Current actually flows between the packs and they actually all adjust voltage until they all perfectly match each other. The charger only "sees" one larger pack and charges it as if it were a single pack.
But if the charger only sees one pack, how does it balance all the packs?
All the individual packs will be balanced by the charger if a parallel balance adapter used. Using this also ties all the like cells of all the packs together (first cell in all the packs are tied together, second cell in all the packs are tied together and so on). Then not only do the packs all become the same voltage but each set of cells does also. The charger then balances all the packs as if it was charging a single pack.
Is parallel charging for me?
Anyone can do parallel charging on any charger with any batteries, so it is always an option but there are always trade-offs. Read through the "Potential Drawbacks" section for more information.
What are some specific uses of parallel charging?
450 sized heli:
Many times people start with a 450 sized heli and in the beginning they can not get enough flight time. This is what we call the "Welcome to your new addiction" stage. Parallel charging can be a great aid to the new pilot. For example with a 350W charger a 6x parallel lead set, a new pilot could charge 6 450 packs in 30min.
700 sized heli:
Larger electric helis are becoming very popular and they come with large needs in terms of power. A common battery pack for a 700 is a 12s 5000mAh made up of (2) 6s 5000mAh packs wired in series. Due to the lack of 12s chargers available, a need for charging pairs of large 6s packs has arisen. Of course you could use 2 chargers, or a dual port charger, but there is another solution, a solution that really brings out the best on the new powerful chargers offering 1000W or more of output. By using one of these new 1000W chargers and parallel charging, you can charge a pair of 6s 5000mAh packs in 25min or less.
Requirements and recommendations for parallel charging
This section is where I will address other concerns with parallel charging as they arise.
High current flow concern when connecting packs at different voltages together for parallel charging
Many people are greatly concerned that when charging multiple packs in parallel, that they be very near in voltage when you connect them, or the current that flows between them will be very great and could harm the lipos or cause greater problems. It is true that large currents can flow but the size of these currents are often greatly over-calculated. Here is some math to show a worst case scenario, connecting 2 high performance 6s 5000mAh packs, one fully charged and the other discharged.
The voltage of the charged pack is 25.2V, the voltage of the discharged pack is 22.2V, and so the voltage difference is 3.0V. The internal resistance of each pack is .018 ohms (3mohm per cell) for a total of .036 ohms.
Volts = Amps * Resistance
Amps = Volts / Resistance = 3.0V / .036ohms = 83.3A
If we only do this simple math and go no further, we find the initial current flow to be 83.3A or nearly 17C. That looks bad on paper but in reality it is nowhere near that high because of a simple concept that the straight math fails to take into account and that is the voltages of both packs will change under load. Basically the more you load (discharge) a battery, the greater voltage drop. This means that the charged pack is no longer at 25.2V when it is loaded. In fact a 6s 5000mAh lipo will likely drop to more like 22V under that high of a load. Notice that is actually lower than the discharged lipo. Likewise the discharged lipo's voltage will rise under the charging load. The actual initial current flow is not easy to pin down without testing it in real life and will change with different packs, at different voltages, and so on.
Basically it is never a good idea to connect a to packs in parallel that are at vastly different voltages but in this case the likely end result will be nothing other than 2 packs will end up half charged.
There is one concern that is still very real though and that is if the pack's balance wires are connected first, this high current could damage the balance wires and/or the balance adapter. As such the main leads should always be connected first.
In the interest of backing up the concepts above, I have done several tests. The first was to cover the voltage differences in normal use. I choose .2V per cell as the greatest possible difference between 2 packs for this test. And in the interest of making it as harsh as possible, I used a charged pack and a pack .2V per cell lower instead of a drained pack and one .2V higher. The packs used were Hyperion VX 2600mAh 35C 6s packs. These packs represent some of the higher capable packs available. Here are the numbers for the test
I used my G.T. Power watt meter to measure the current flow between the packs. I do not have the capability to log the results on a computer, so I watched the numbers closely and mentally noted them. The peak current was recorded by the watt meter. The results were
These results are approximately what I expected, maybe a little higher though. The initial (peak) current represents a rate of just over 7C and within 2sec it was below 4C. These numbers are child's play for the VX packs in terms of discharge rates and only the first second of current flow exceeded the charge rating of the packs.
All-in-all I think this shows that normal voltage differences (up to .2V per cell) between discharged packs are not problematic as long as the main leads are connected before the balance leads. Also the size and performance of the packs used are not important, as larger, higher end packs will have cause higher currents to flow between the packs but they are better suited to handle them. In other words the current is proportional to the pack capabilities and it works on both sides, charging and discharging.
The next test will be with the same packs but using a fully charged and fully discharged packs.
Parallel charging diagrams and adapters
Below are several diagrams of a typical parallel charging setups and adapters. As you can see, parallel balance charging requires 2 adapters, one for the main discharge leads and one for the balance leads. The adapters simply wire all the battery leads in parallel (i.e. all the red wires are tied together, all the black wires are tied together, etc).
Typical parallel charging setup
Alternate parallel charging setup
Parallel main leads adapter for 2 packs
Parallel balance adapter for 2x 3s packs (JST-XH)
A few notes on commercially available parallel adapters
When I first started parallel charging there were no commercially available parallel adapter cables. But how times change. Parallel charging is now common place and there are a variety of pre-made adapter cables available. Even charger companies have come on-board and are now offering "Safe" parallel adapters that include polyfuses that help protect users from battery connection mistakes.
Below are a few of the options out there.
ParaBoard from EPBuddy (see review)