External Discharge Resistor Load Bank

The goal of this project was to make an resistor load bank to externally discharge batteries on my dual port charger. This external discharger reduces the time it takes to discharge batteries from hours, to minutes!

Many battery chargers have the ability to discharge batteries, however they typically can only do so at a very slow rate (~10-15W, depending on the charger). After a typical day at the flying field, I inevitably have several packs that I charged, but didn't use, requiring discharging to storage voltage. With conventional internal discharging, this process will sometimes take hours, depending on how much I overestimated I would fly.

To solve this problem created by another problem, I intended to utilize the external discharge function of my Hota D6 Pro, and set out to create a low resistance, active heat dissipating, resistor load bank. And here is how I made it!

Throwing Some Numbers Out There...

But how much resistance should this load bank be, you ask? Good question. I wondered the same thing, and so I made a spreadsheet to calculate the expected current draw and power output applied to a 1Ω, 2Ω, 3Ω, etc. load. My Hota D6 is spec-ed to supply 325W or 15A per channel, and I commonly fly 3s and 4s LiPo packs. The numbers showed that applying a fully charged 3s or 4s pack to a 1Ω resistance allowed for a power output of 159W and 282W, respectively. So I decided 1Ω was the resistance for me.

Next, I did some research on resistors out there, and researched the resistor types commonly used for the purpose of an external discharger. I found that a lot of people use resistors intended for HAM radio components or DIY audio amp stuff, which are readily available on Amazon. 4Ω was a common low resistance I could find, and they were typically rated at 100W per resistor. Knowing that a parallel setup of 4 of these resistors would get me 1Ω, and that 400W allows for a good safety margin to what the charger can produce, I added a 4-pack to my shopping cart.

Simpler Ways to Discharge Batteries

Truth be told, this is NOT the simplest way to create an external discharger. Many posts on RC Groups suggest wiring up several car lights or incandescent lights to make the load bank. Before making this load bank, I tried using a PTC space heater (similar to this one). I used one channel on my charger to power the 12V fan, and the other channel to connect the PTC heater element to externally discharge the battery. It was indeed much faster than the internal discharger, but I wanted to go faster.

One final disclaimer, this discharger can only be used if there is a separate power supply for the fans, which require 12V. The Hota D6 Pro is a dual port charger with a power supply mode. I used one channel in this mode at all times to give the fans 12V regardless of what the battery is doing. Some alternative methods for powering the fans could be using any 12V wall power supply, a spare 3D printer 12V power supply, or connect it to a 3s lipo. The fans draw very low current, so I wouldn't worry about discharging a battery with them, just don't leave them plugged in.

Bill of Materials

1. 60x150x25mm Aluminum Heatsink
2. 4Ω Wire-wound resistors 4 pack
3. 2x Arctic 4028-6K 12v fans
4. Thermal paste
5. 14 AWG wire
6. 8x M3x60 screws
7. 8x M3x6 screws
8. 2x XT60 female connectors
9. PETG 3D printing filament
10. Various heat shrink

Additional Tools to Have

1. 2.5mm drill bit (Drill bit set)
2. 3x0.5mm tap (Tap set)
3. Drill press or hand drill
4. Soldering Iron
5. 3D Printer
6. Metric Allen wrenches

After getting the fans, it's a good idea to pop out the + and ground wires and hook them up to 12V to verify the fans work.

With this particular 60x150mm heat sink and resistors, I designed out a hole layout for drilling 2.5mm holes. Attached is the design drawing, and a convenient 3D printable jig. Once the 2.5mm holes are drilled, tap them with the 3x0.5mm tap.

Time for some 3D printing! I've designed two variations for the fan center spacer, one which connects between the two fans (recommended) and one that doesn't. The connected version of the center spacer also has slots cut in it for a zip tie to secure all the wires. If it is so desired to print the separated fan spacers, print two of them.

Now to install the fans.

1. Insert the 8 M3x60 screws through the fans and frame spacers, and screw into the heat sink on the grid side.
2. Guide the fan wires through the center grid on the heat sink.

I recommend cleaning off the heat sink with isopropyl alcohol or something similar before installing the resistors. It's also good practice to verify the drilled holes line up with the resistor holes before moving forward.

1. Apply a thin line of thermal paste on the resistors.
2. Line up the holes in the resistor and the heat sink and attach them with the M3x6 screws. Try not to move the resistor much until secured so as to not spread the thermal paste.
3. Rinse and repeat for all 4 resistors.

Probably the most challenging part of the build. If you don't have an active temperature-controlled soldering iron, I suggest working with some kind of wire connectors and a different wire layout, as the 14 AWG wire is difficult to solder without a lot of heat. I also recommend using a wider soldering iron tip.

1. Cut lengths of wire for the top and bottom parallel resistor connections. I left room for some extra length at 140mm (overall length).
2. Pick a side to start with. Strip and solder one end of the wire to the right-most resistor.
3. Line up the wire to go over the next resistor, and carefully strip a section of the insulation long enough for the wire to wrap around that next resistor. For me, this was about 18mm.
4. Wrap the wire around the resistor lead tightly and solder.
5. Repeat for the adjacent resistor.
6. For the 4th resistor, cut off any extra length in the wire, strip the end, and solder to the last resistor.
7. Strip away a length of the insulation in the middle of the wire. This is to solder on the main lead wire later.
8. Repeat the process for the other side of the resistors.
9. Cut lengths of wire for the main leads. The way I routed the wires, with the top passing through the bottom, the top wire had more length. The two wire lengths to line up with the fan wires were 440mm and 385mm, top and bottom wires respectively.
10. Strip the main lead wires and solder to their respective parallel wires in the center stripped location.
11. Route the wires through the bottom of the heat sink, towards the fan wires, out the front. Connect all wires together with the fan center spacer using a zip tie.

Now to solder on XT60 connectors to the resistor main leads and the fan wires. Don't forget the heat shrink before soldering!

Fan wires

1. Split out the + and ground wires from both fan wire bundles and strip. Twist the two positive wires together, and the two ground wires together (one wire coming from each fan).
2. Solder the twisted wires to the respective terminals on the XT60 connector.
3. I just used some electrical tape to tie-up the unused fan connectors. I suppose the wires could have been cut, but I didn't want to deal with exposed wire ends.

Main Resistor Leads

1. Connect either wire (+ or ground) to either connection in the XT60 connector, as the resistors are not polarized to a any direction.

Connecting the Fan

1. If using a dual channel charger - connect the fan XT60 connector to one channel on your dual channel charger of choice. Otherwise, connect the fan connector to your desired 12V power source.
2. Turn on the 12V power source to the fans.

Discharging

1. Connect the external discharge connector to the discharge port on the charger.
2. Verify the fans are on.
3. Set the target voltage at ~3.75V per cell to accommodate voltage sag (the voltage per cell will bounce back up to 3.8V after the load is removed). This will vary by battery - older battery = more voltage sag.
4. Begin the discharge function on the charger.

Post-Discharging

1. Keep the fans running until most of the heat is dissipated.

Now to take a look at how this thing performs! The total resistance is just over 1Ω, so for every volt applied, 1A can flow, and the power is the product of those two numbers (V=IR, P=IV).

One thing I found with the Hota D6 Pro is that the discharge rate, when in external discharge mode, is slightly lower than what would be possible if it was in power supply mode for all battery types. Particularly, when discharging a 3s battery in external discharge mode, the charger only allows ~2A, which I'm guessing is for battery safety reasons? So for all 3s discharges, I kept the charger in its power supply mode at full crank, and just watched it and shut it off once the battery voltage was close to where it needed to be, accounting for voltage sag under load.

Further, when in power supply mode, I tried increasing the output voltage beyond the battery voltage, but the charger could only bump up the voltage marginally above the battery voltage, reducing the discharge performance. All of that is to say, the charger's voltage management slightly affects the discharging performance, but this thing still rocks.

Results

I discharged all batteries from fully charged down to 3.8V per cell. Here's the performance I saw:

1. 2s 650 mAh LiPo: 3m
2. 3s 2200 mAh LiPo: 2m 30s*
3. 3s 4000 mAh LiPo: 10m 30s
4. 4s 2200 mAh LiPo: 8m 45s
5. 5s 1800 mAh LiPo: 4m

*This 3s battery was close to its end of life, and doesn't serve as an accurate performance metric for most 3 cell batteries.

It's interesting to note that higher voltage batteries discharge faster, as the higher voltage allows for more current to flow through the 1Ω resistance.