DIY Charging Harness for 1S Batteries

If you fly micro/nano quads, you probably use a bunch of single cell LiPos. The balance charger I use for my 3S and 4S packs (Accucel S60) doesn’t support 1S batteries. In order to take advantage of some of the features of the charger (charge/discharge to storage, measuring IR, etc) I decided to make a simple wiring harness to let me treat multiple 1S batteries as a single multi-cell pack.

WARNING: This tutorial involves doing something with rechargeable batteries. Please note that batteries (all batteries, but LiPo chemistry in particular) are volatile if treated incorrectly. Overcharging and other forms of misuse can and will cause a fire.

Supplies Needed

Here’s what you need to build one of these harnesses:

  • Balance plug with a size that matches the number of batteries you want to charge simultaneously. For 3 batteries you need a 3S balance plug, 4 batteries needs a 4S plug, etc. It’s probably easiest to buy the connector with leads already attached unless you already have a crimper tool. Here’s an example on Amazon for a 3S plug (I didn’t spend time shopping around – this is likely not the best price out there). Keep in mind that we’re going to be connecting batteries in series: All connectors on the harness must have a battery attached. Feel free to go all the way up to 6S (or whatever your charger supports), but know that you’ll always need to plug 6 batteries in.
  • Connector for the charger – this is the connector that goes to the main terminals on the charger. Banana plugs are common on most chargers. I went with XT60 because I already have an XT60 to banana plug for my normal 3S/4S packs.
  • Connectors for the batteries you use. My batteries use molex connectors, but go with whatever your batteries have.
  • Wire of sufficient gauge for the expected current transfer. In practice, you’re probably going to buy connectors with leads already attached. You can most likely just use those wires with no problem.
  • Good quality balancing charger. On ordinary multi-cell packs, the cells are typically going to be relatively close to one another in voltage. In this setup, that’s not necessarily true. A well designed charger will detect large voltage discrepancies and supply/draw current from the balance port to prevent overcharging. A poorly designed (or cheap) charger will ignore this discrepancy and the higher voltage batteries will be overcharged, risking damage and a fire.

Building

Essentially we’re connecting a set of batteries in series from the negative to the positive terminal of the charger. Additionally, there’s a wire at each junction that goes back to the balance port (so the charger can measure the voltage of each individual battery). By wiring up the batteries in this way, the charger can treat the group as a single, multi-cell pack.

Here are the build steps:

  1. Daisy chain the battery connectors in series, connecting black on one battery to red on the next connector in the series.
  2. Attach the main charger leads to the ends of the daisy chain (black to black and red to red).
  3. Attach the ground lead on the balance plug to the ground wire on the end of the daisy chain.
  4. Attach the lead furthest from ground on the balance plug to the opposite end of the daisy chain.
  5. Attach the remaining balance leads to the junctions in the battery connector daisy chain, in order.

Here’s a few photos/diagrams:

Wiring block diagram of harness for 1S batteries

Block diagram of the harness connections. 3 batteries are pictured here, but follow the pattern to extend out to more.

Photo of connection points on harness for charging 1S batteries

Shouldn’t all soldering be color coded? Solder together the wires of the matching color.

Photo of fully assembled harness for 1S batteries

All connections made

Completed harness for 1S batteries

End result after heat shrinking the connections

Charging three 1S batteries simultaneously

Example usage

Testing and Using

It’s always a good idea to double check your work before plugging stuff in, especially when working with LiPos. I recommend sanity checking all the connections to ensure none of the battery connector pins are shorted out. Also, verify the balance port leads are in the correct order. You can do this by connecting batteries to the harness and checking the voltage differential at the balance port.

Usage of this harness is pretty similar to charging an ordinary multi-cell pack. There are a few differences worth highlighting:

Charge/Discharge Rate

Conventional wisdom says you should charge at 1C, meaning one times the capacity. In this case, the capacity is the sum of the capacities of all the batteries. So, for three 300mAh batteries, I set my charge current to 0.9A. If you plan on charging batteries with different capacities, use the smallest capacity of the group as the capacity for all of them when you do this math.

Charging 1S Batteries with Different States of Charge

I talked about this a little bit in the supplies section. Proper chargers should be able to handle balancing batteries with a wide discrepancy in state of charge (voltage) across cells. Some chargers will do this automatically, but others have a specific “balance” charge mode. Either way, keep a close eye on the voltages the first few times you use this harness.

Also, chargers typically supply current at a much slower rate through the balance port than through the main supply leads. This means charging batteries with large differences in voltage is slower (more time spent supplying current through the balance port). In other words, charging batteries with similar states of charge will be faster and more efficient.

Charging 1S Batteries with Different Capacities

I haven’t actually tried this, but I believe it will work just fine. A similar caveat applies regarding the balance port. The batteries with the higher capacity will achieve voltage slower. The charger will have to use the balance port more, and incresae the overall charge time. Again, charging batteries of the same capacity will be faster and more efficient.

Parallel Charging Groups of 1S Batteries

In theory you could build several of these harnesses and connect them up through a standard LiPo balance board. I haven’t tried this either. Parallel charging comes with its own set of rules and guidelines, though. Don’t try this if you aren’t already familiar with parallel charging ordinary packs.

Balance Charger Advanced Features

The main reason I did this was because my charger can set batteries to storage charge. Storage charge is just as important for 1S batteries as it is for multi-cell packs, and I didn’t have a way to do it otherwise. However, some chargers have additional features like measuring ESR or IR on a per-cell (or total pack) basis. Take these values with a grain of salt since there are other factors at play (like the resistance added by the connectors on each battery).

Conclusion

There you have it! Hopefully this straightforward wiring harness helps improve your 1S battery charging experience. Again, please re-read the warning at the start of the post and make sure you’re staying safe when dealing with LiPos.

Questions? Suggestions? Leave them in the comments below!

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