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. The following is my experience about lipo battery parallel charging.
With cells in series, and no balance lead, your charger has no idea how much voltage is in an individual cell; it only knows the total voltage. If it reads 7.4V, that might be cell 1 with 4.2V and cell 2 with 3.2V, or it might be cell 1 with 3.7V and cell 2 with 3.7V, or it might be something else. Unless the charger connects to the balance lead, it just doesn’t know. So, let’s imagine it’s the first case. You start charging, with cell 1 at 4.2V and cell 2 at 3.2V, and the charger says, “ok, I need to see 8.4V before I stop, so I’ve got a long ways to go,” when in actuality it is going to overcharge cell 1 and possibly blow it up or catch it on fire, and definitely it is going to ruin or at least damage cell 1 at a bare minimum. That’s where active balancing comes in. Cells in series must be *actively* balance by a smart charger, to get them to be equal.
However, here’s what you’re missing still: cells in parallel do *not* need active balancing by a smart charger. Rather, they naturally, by the laws of physics, will perform passive balancing. Cells in parallel automatically, passively, due to voltage gradients (differences), self-balance until they are equal. If you take 2 individual LiPo cells, and place them in parallel, and wait long enough, no matter what voltages they *were*, they will equalize, automatically, to become *equal* voltages over time. One will discharge into the other, until they are equal.
So, in your case of 11.1v 2200mah lipo, when you place the whole packs in parallel, and the balance leads in parallel too, cell 1 of pack 1 is placed in parallel with cell 1 of pack 2, so both cell 1′s will automatically, by the laws of physics, passively balance until they are equal. Both cell 2′s will also passively balance until they are equal, since cell 2 of pack 1 is also in parallel with cell 2 of pack 2. However, both cell 1s (as if a large single cell) are still in series with both cell 2s (as if a large single cell), so *active* balancing is still required here, to get the cells in *series* actively balanced, and that’s where the smart charger does its job.
Therefore, when charging in parallel, you are still getting all 4 cells to be balanced, just as if you were not charging in parallel.
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) 11.1 v 3s lipo battery 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.
Remember to be careful about large voltage gradients (differences), however, as the passive balancing can be quite fierce (and potentially damaging or even dangerous), if cell voltage differences are too great before being placed in parallel–refer back to my article for warnings & recommendations on this.