As mentioned in my serial charging article, there is no dedicated hardware (at least that I am aware of) to serial charge your 2S batteries, but the technique is the same as with 1S batteries - instead of building a 6S battery from six, single 1S batteries we are building a 6S battery from three 2S batteries.
Building this kind of adapter can be accomplished with a hand full of passive components and some minimal soldering skills. A 3D printer capable of printing TPU for the enclosure comes in handy, but you can also use liquid tape to secure the contacts on the bottom of the adapter.
You will need the following materials for the adapter:
- A small piece of prototype board
- 3x 3 pin, JST XH 2.54 connectors (the receptacles for your 2S balance leads)
- 1x 7 pin, JST XH 2.54 plug preferably with wire (balance plug for a 6S battery)
- 1x XT30 or XT60 plug, depending on what your parallel board can handle
- Some silicon wire for the charge lead
- Some heat shrink tubing
- 3D printed TPU cover or some liquid tape for isolation
I suggest building multiple of those adapters at once, you can save on material price and if you build six, you can easily get a parallel charging board full and charge 18 2S batteries at once.
You will also need a charger capable of charging 6S batteries, if you do not already have one, I highly recommend the ISDT Q6 Pro charger.
Score and break the prototyping board to proper size 15x6 holes worked perfectly and will also fit the 3D printed TPU cover. Score the holes at 16x7, break it and grind the board down.
Place the JST XH connectors next to each other, solder them to the board and connect the last pin with the first pin of the connector next to it. This wires the 2S packs in series.
Increase the diameter of the holes so that you are able to pull the wires of the balance lead through them. You can do this by using a 1mm drill or simply by scraping the holes with a hobby knife.
Solder the wires of the 7 pin balance lead, double check that you get the polarization right.
Solder the charging lead to the outer contacts of the three connectors, add your battery plug of choice.
After you are done with soldering double check that you did not short out anything, best check with a multimeter that none of the pins on the 7 pin balance lead is shorted to the pin next to it. Double check it, mistakes here may short out your batteries!
At this point you are basically done. I would highly recommend to insulate the bottom of the board either with liquid tape or - preferably - with the 3D printed TPU case. Should you go with the TPU case you need to cut it open at the hole designated for the charging cable, pry it apart and stick the cables and boards in it the slot. You might need to file your board down a little bit, should you have a hard time fitting it inside the case. Once everything is inside, glue the piece you just cut back together.
When it comes to charging there are a couple of things you have to consider. When you only charge three 2S batteries as one 6S battery, make sure that the batteries have same capacity. The voltage difference does not matter in this case.
Basically you want to make sure, that you only charge batteries of the same capacity on one adapter. The voltage difference on the different adapters should be as low as possible, preferably not more than 0.1V. I like to bring my batteries to storage voltage after flying. I do this with six similar batteries at a time on a parallel charging board - this only takes a couple of minutes. When I want to fly the next time, all my batteries are already on the same level (or at least within 0.1V) and I simply hook them up all together.
I try to charge with 1C, except if I have to be out quickly, then I charge with 2C and can be done with 18 batteries in about half an hour. Keep in mind that charging your batteries with too high of a current can significantly shorten their life or even destroy them.
When on the road I take my charger, a big battery and one of the serial adapters and charge 3 batteries at once with 2C. In total I have 12 batteries - this way I am done charging the first set when I reach the end of my first cycle of the 12 batteries. So I can basically fly the batteries back to back with almost no downtime.