Parallel Charging
TAKING CARE OF MULTIPLE PACKS EFFICIENTLY AND SAFELY
As electric power fliers, we always have to charge our batteries before we fly; that’s one of the hard-and-fast rules of this part of the RC hobby. For many years, I charged batteries one at a time, and it could take quite a while to prepare for a full day’s flying. As long as I was flying for fun, this was tolerable, taking maybe an afternoon and evening before a planned day at the field. But when I started flying a big octocopter drone for aerial-photography work, I started to realize that something had to change. That drone flew on four 6S 8Ah batteries, and I needed at least three sets for the type of work I’d do in a single day. Charging one 6S battery at a time meant something like 12 hours of charging with a single charger, so I bought more chargers. That helped, but it wasn’t the best solution.
In August 2012, I went to the IRCHA (International Radio Control Helicopter Association) Jamboree at the AMA (Academy of Model Aeronautics) flying site in Muncie, Indiana. While I was walking around the vendors’ row one day, I stumbled into the ProgressiveRC tent and had my eyes opened. What I found there can surely be found in other places, but it was my first exposure to easy and safe parallel charging. Among other things, ProgressiveRC sells combinations of chargers and power supplies built into nice cases for easy transport. It also has a wide assortment of parallel-charging boards, built with just about any connector available on the RC battery market.
I spoke with David Gray, owner of ProgressiveRC, and we discussed the types of
batteries I had and what I wanted in a charging setup. The system he recommended that day had a 24V, 1000W power supply and a single iCharger 306B charger capable of 1000W output, both built into a black plastic case. I bought that system but, within a year or so, had upgraded it by adding another iCharger 306B and changing to a 24V, 1500W power supply, a combo that ProgressiveRC sells today as the “Hardline Pro Solution.”
The iCharger 306B will charge up to 6S batteries at 30A. By parallel charging on the two chargers, I could charge those big 8Ah drone batteries eight at a time, at almost a 1C rate, resulting in a considerable time savings.
It also means that I don’t have to charge batteries in advance of a sport-flying day. My RC field has AC power, so I can load up, drive to the field, and start charging when I get there. By the time I’ve assembled the planes I want to fly and talked with friends a bit, I’ve got plenty of batteries freshly charged and ready to go.
If you’re interested in parallel charging and haven’t yet investigated the details, here are some notes that may help.
HOW MUCH POWER?
You need a charger that’s capable of putting out the power necessary for parallel charging. To figure out if a charger will work for you, start your calculations with the biggest batteries you need to charge. That way, the smaller ones you use will be covered as well. Since I’m not flying the monster drone any more, my biggest batteries these days are 6S 5Ah. To charge one pack at my normal 1C rate requires about 125W, 25V at 5A. With a charger capable of 30A output, it’s possible to charge up to six of these batteries in parallel as long as the charger can handle at least 750W output. The total power needed is the sum of the power needed to charge the individual batteries—in this case, 6 x 125W (or 750W).
To develop this much power may require a 24V power supply, so keep that in mind.
CONNECTING IN PARALLEL
Balance charging in parallel requires a special board that has the same battery plugs and balance-tap connectors as the batteries you use. They’re available from many RC vendors, with almost all the common plug types already soldered to the printed circuit board (PCB) in parallel, ready to use. I’ve bought them in the past from ProgressiveRC (progressiverc.com) and Buddy RC (buddyrc.com). The boards I use are a universal type and come from the shop with wire leads soldered to the board in place of connectors. Since I use the ancient but excellent Anderson Powerpole connector, I modify the universal boards by soldering Powerpoles in place of the wires. Newer versions of the parallel boards come with fuse protection in the power and balance-tap traces in the PCB. Because I’ve shorted out several parallel boards by inserting a balance plug at the wrong angle, I welcome this improvement.
THE RULE
Properly equipped, it’s time to start charging in bulk. To safely charge in parallel, there is one rule to abide by: The batteries to be charged should be the same cell count, in a similar state of charge, with no more than about a 0.25-volt difference. As soon as they’re plugged into the parallel board, the voltages in the batteries and their cells will equalize—that’s what “being in parallel” really means. As the voltage equalizes, current will flow from the higher-voltage battery to the lower one. High current from a large voltage difference could damage the traces on the parallel board or the batteries, so make sure the batteries are fairly close in voltage. They don’t have to be the same capacity; voltage is the key. This is another time when the practice of discharging or flying batteries down to storage voltage pays off. Doing that means all your batteries are at a similar enough voltage for parallel charging at any time.
HOOK ’EM UP!
With the balance board connected to the charger and the charger powered on, it’s time to start connecting the batteries. Always connect the battery’s power leads to the parallel board first and then connect the balance plug. Do this for each of the batteries to be charged, being careful to get all the plugs secure. Calculating the charge current is as simple as adding up the capacities of the batteries to be charged. Four 5Ah packs will require a setting of 20A, 5A for each battery. Charging batteries with different capacities still works the same way. Two 3S 5Ah and two 3S 3.2Ah batteries would require a charger setting of 16.4A for a 1C charge.
After setting the proper charge current, monitor the charging process the same as you would when charging single batteries. Never leave LiPos unattended while charging. When the charge cycle completes, detach the batteries in the reverse of the way you connected them: balance plug first, mainpower leads last.
FINAL THOUGHTS
That’s all there is to it. With proper care, parallel charging is a perfectly safe way to get several batteries charged as quickly as possible. Remember:
Charger total power output has to be enough for the job.
Only parallel-charge batteries of the same cell count.
Make sure voltages are similar—within 0.25V is a good rule to follow.
Charge current is the sum of the batteries’ capacities.
Finally, always monitor the process. Keeping a close watch on any LiPo batteries that are charging, parallel or single, is an important safety requirement.