Lithium-polymer cells have revolutionized electric flight, but as any e-flier knows, LiPo charging and maintenance can be time consuming. For optimum service life, LiPos should be stored around 3.8 volts per cell, which creates a bottleneck for spur-of-the-moment flying, and recharging batteries at the field can be a chore when you’d rather be in the air. Many electric fliers compensate for this by using a whole phalanx of chargers, but all those chargers are an extra expense, extra gear to transport, and they each have to be operated individually. A neater, more compact solution is to parallel charge groups of batteries simultaneously with a single charger equipped for parallel charging.
How it works
As the name implies, parallel charging works by connecting a group of batteries in parallel, to a single charger, using a wiring harness or a parallel charging board. To the charger, the batteries then look like a single pack with the sum of their capacities. For example, six 1000-3S packs appear to the charger as a single 6000-3S pack and can be charged accordingly. The batteries do need to be the same cell count, but the cool part is that they don’t even have to be the same capacity; pack voltage will equalize during the charge cycle so that even packs of different sizes reach full charge at the same time. The only catch is that the batteries need to be at roughly similar voltage levels before connecting them.
It’s this last caveat that made me a little slow to embrace parallel charging. Like many of you, I don’t always “fly out” my batteries, so I find myself with a pile of packs to charge, all at various levels of discharge. But, a proper parallel charge setup will tolerate a reasonable voltage spread, so it’s really not much of a problem.
What happens when a group of batteries are connected in parallel is that current flows from the fuller packs to the emptier ones. As long as the voltage difference isn’t too great, the current won’t be high enough to be an issue. For example, in bench tests I found that two 5000-6S packs with a 0.25V differential showed an initial current flow of 12 amps, and this tapered off rapidly to 8 amps after 30 seconds and less than 7 amps after a minute (in other words, well within normal charging limits). By all means, you should keep a pocket battery monitor handy so that you can check the voltage of each pack before you connect it; you don’t want to be plugging in a fully charged pack with a group of discharged packs.
Let’s get started
For this demonstration, we’ll use the iCharger 306B from Progressive RC (progressiverc.com). While there are other chargers suitable for parallel operation, the 306B has a couple of features that make it particularly adept. First, it can provide up to 30 amps output so that even batteries in the 5000-6S class can be charged at their optimum rate. Second, it can handle higher input voltage than most other charges, further boosting its effective power. Making the most of these features depends of course on your power supply or generator; high-rate charging requires a power source with enough muscle for the job (see “Selecting a Suitable Power Supply”).
The methodology described here applies to all suitable chargers, but in combination with Progressive RC’s parallel boards, the iCharger 306B makes the process particularly simple. There are only three items required: a suitable charger like the 306B, a parallel board with connectors compatible with your batteries, and a balancing cable to connect the two. Progressive RC supplies parallel boards with the most popular connectors, as well as a “universal” board with plain wire leads so that you can install the connectors of your choice.
With the charger connected to your power source, you can then connect each of the packs to the parallel board. It’s wise to check each pack with a voltage monitor to make sure none are fully charged and all are in the same general voltage range. Always connect the power leads before the balance leads, as the heavier cable can handle the initial surge as the packs begin to equalize.
With several packs connected to the parallel board, all the cell number ones are connected together, all the cell number twos are connected, and so on. Thus, current continually flows from the higher voltage cells to the lower voltage cells, therefore keeping them equalized throughout the charge cycle.
Selecting a Suitable Power Supply
The iCharger 306B from Progressive RC is a remarkably capable unit for the price. It can produce up to 30 amps output, but it has an additional performance edge in that it’s rated for up to 38 volts input. Why is this important? Because while charge rate is set based on amperage, when the battery being charged is of higher voltage than the power source, the charger has to step up the voltage by trading amps for volts. In the case of modern chargers like the 306B, this is done by a switching circuit called a buck-boost DC to DC converter.
The figure below shows how this works. In the upper section we see a typical hobby-grade 120V switching power supply with a rated output of 30 amps at 13.6 volts. On the effective output side of the table, you can see that this power supply will indeed allow 30 amps output for charging 3S packs, but the effective output gradually tapers off for larger battery packs, with the charger output limited to just 15 amps when charging 6S packs. Note that this table takes into account the fact that charger circuitry is about 90% efficient.
Now look at the lower section, which shows a high-voltage power supply. This can be accomplished either with the high-voltage units available from suppliers like Progressive RC, or by connecting two conventional 12V supplies in series (check with the manufacturer to make sure your unit is suitable for series operation). With input voltage doubled, watts out are also doubled, allowing a high-voltage charger like the 306B to maintain its rated output all the way up to 6S packs. Just as with power systems, where charging is concerned, voltage is your friend.
|Input Power Source||Effective Charger Output|
|DC/DC Boost Converter Efficiency: 90%|
The iCharger 306B from Progressive RC is a great choice for parallel charging. With 1,000 watts of power, it has the capacity to charge up to six large 6S packs simultaneously. The 306B also includes features like storage charging (adjusting voltage for long-term storage), and it handles all popular battery chemistries. This really is a “do-it-all” charger.
Setting charge current
Progressive RC’s tidy parallel boards keep clutter to a bare minimum. This shot shows how even packs of different capacities can be parallel charged together. The packs must all have the same cell count and should be at similar charge levels, but the charger can take care of the rest. Here, four 2200-3S packs and two 3300-3S packs are being charged together. The voltage level in the packs equalizes during the charge cycle so that all packs will be completely charged at the same time.
With the batteries connected, it’s time to set the charge current. The charger sees the cluster of packs as a single battery with the sum of their capacities. So, if you wanted to charge six 2200-3S packs at 1C, you’d set the charger for 13.2 amps (6 × 2.2 × 1 = 13.2). If you wanted to charge these same six packs at 2C, you’d select 26.4 amps. Your power source has to be capable of providing the required power, particularly if you’re charging 5S or 6S packs.
With the batteries connected and the charger set to the correct amperage, you can begin the charge cycle, and before you know it, you’ll have up to six batteries—all perfectly balanced and ready to fly. That’s all there is to it! With a high-output charger like the iCharger 306B and parallel charging capability, you can literally charge packs faster than you can empty them. No more sitting around waiting for the charge cycle to end, just hour after hour of hassle-free flying.