Serial charging, like parallel charging, is a method to get more batteries onto your existing battery charger at the same time. Parallel charging is a popular method to put many batteries onto a single charger so that they all charge at once. (We’ve got our own parallel charging guide right here on Propwashed.) Serial charging has limitations, but it also brings a few very unique advantages. For a certain set of pilots, these features may finally fill a long-missing piece in battery management.
What is Serial Charging?
Battery cells can be connected in parallel or in series. In parallel, each cell that’s connected increases the total capacity. In series, each cell increases the voltage.
With serial charging, you connect the positive of each pack to the negative of the next, stringing them all together (in a series). It’s exactly the same as creating a battery with a higher cell count: A “4S” battery means four cells are connected in series. Because of this, when charging you no longer need to worry about matching cell counts of different packs, checking the voltage of each cell, or whether internal resistance on the cells matches. However, unlike parallel charging, you will want to keep the cell capacity roughly the same.
The immediately obvious limitation of serial charging is the maximum voltage and cell count of your charger. Every cell connected serially counts individually, regardless of the pack size. Most chargers only go up to 6S, with a few reaching up to 8S. Another issue is getting the balance lead sorted out; you basically need a purpose-built connector for each combination of batteries you want to serial charge. If you’re flying 4S or 6S batteries, it’s almost impossible to gain any advantage from serial charging. It’s also very important to use a balancing charger that keeps track of individual cell voltages, otherwise you’re likely to overcharge some of the cells.
With so many limitations, it’s no surprise that serial charging isn’t common. However, there’s one case where serial charging becomes uniquely useful: tiny whoops and other 1S craft. Stringing together 1S batteries allows you to charge as many at once as your charger’s cell count limit, with full individual monitoring. It is also far simpler to create a wiring harness for 1S. That’s where the Fractal Engineering Whoop Juice Transfer Station comes in. With a single jumper, you can select whether you will be charging from 1 to 6 1S whoop batteries.
If you already have a charger for 6S packs, chances are good that it will do balance charging. However, it’s fairly common for these chargers to not accept 1S packs or provide cables that can connect them. The serial board will allow you to make use of your existing charger, expanding your charge capacity and taking advantage of higher-quality charge equipment and capabilities. Still, 1S chargers that individually charge separate cells are inexpensive and easy to find. For charging alone, a serial board is still a tough sell.
Finally: 1S Storage
While there are lots of 1S chargers on the market, they all lack the ability to discharge and put batteries into storage charge. With the way we (ab)use them, small cells seem to degrade faster than their larger counterparts. Leaving batteries at full capacity is one of the worst things you can do to their lifespan. I often charge boatloads of batteries before an indoor event, not knowing how many I will be able to use. Any unflown packs need tended to when the day is over, and it doesn’t hurt to bring the flown packs up to a healthy storage level either.
With the serial charge board, you can easily get all of your batteries back to a stable state. Simply put batteries on the board, set the correct jumper, plug it in, and set your charger to its storage program.
While it’s not strictly necessary to pay attention to cell voltages and won’t cause damage, I’d recommend keeping the unflown packs separate from those that have been drained. Most multi-cell chargers don’t charge through the balance leads; instead they charge all cells at once, while discharging individual cells to bleed of the excess. Because of this, it’s much faster for a charger program to work with cells that are roughly similar in voltage. If time isn’t a concern, there’s no safety issue with just throwing on the nearest packs you have on hand.
There are really only two groups of people who can take advantage of serial charging: those who want to maximize the charge capability of their existing equipment for 1S packs, and those who are interested in storage charge for 1S batteries. If you’re among that group, the Fractal Engineering Whoop Balance Charging Board is a product that makes serial charging possible—and it might be the only product that exists which can.