Electronic Load Based on the CL24 with Charge Control

It all started when I ordered a couple of CL24 loads a while back. They sat around for a while, and I thought, why not build a couple of complete devices around them? So I thought about it, thought about it some more, and did it.

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As I wrote in my previous article, these devices belong to the “consumer” series, but they still offer a lot of functionality. I also tried to fit it all into the most compact enclosure possible.
I should also mention right away that these two loads are part of what’s now fashionably called a limited edition, since I may not make them again.
There is also the BW150 controller, which has the same functionality but costs less. Unfortunately, its board layout differs from those of the previous models, making it much harder to use in a finished device, so I may not build anything with it either, though there are some options.

Last year, I published a review of the CL24 electronic load. It’s an interesting little model in its own right, though not without its shortcomings. What I particularly liked, and what I’d been missing on the DL24EW, was the ability to set a pause between charge-discharge and discharge-charge cycles, which the developers added after my review. You can also set the number of cycles directly, rather than being limited to CDC and CDCDC.


I also compared loads from several generations back then, and it was clear that the developers were taking my recommendations on board. Many thanks to them for that.


There won’t be many photos, since I didn’t document the process, though I did find a couple from the early stages. Sorry about the quality—I took them with a soldering iron.
Just to give you an idea of what the process sometimes called “simply installing a ready-made board in an enclosure” looked like.


Another photo of the process. This is roughly the end of the assembly, and you can see the keypad, which I ended up not needing.


Because of the enclosure’s constraints, I had to use not only a smaller (and lower-power) power module, but also a smaller protection and switching board for the charger. Unlike the original, this board has the following features:
1. Reverse-polarity protection: it simply won’t connect the charger to the battery if either one is connected the wrong way around.
2. Reverse-current protection: current from the battery won’t flow into the charger. This is critical if a DC-DC converter is used as the charger.
3. It can switch currents up to 20–25 A, with correspondingly low losses—about 0.16 V at 20 A. Of course, there’s also about 0.1 V across the shunt at 20 A, plus a little more in the wires, but I can’t do anything about that.

One drawback is that the maximum charger voltage is 50 V, while the maximum voltage in load mode is 100 V. This is important.
In fact, it can also switch 100 V, but if the polarity is reversed, the source and battery voltages add together. So 50 V (charger) + 50 V (battery) = 100 V, which puts us right at the limit. Technically, though, as long as you avoid reversing the polarity, you can use a charger and battery up to about 90 V.


By the way, about the switching module. In my CL24 review, I didn’t give it much thought, which was a big mistake. It turned out to be radically different from the module supplied with the DL24EW.
The difference is that the module on the left (DL24EW) has a protection diode connected in parallel. It’s intended to blow the fuse (though it’s debatable whether it can actually do that) and doesn’t affect normal charging.
But in the CL24 (and BW150) module, the diode is connected... in series. As a result, it further limits the maximum charging current. Worse, it prevents batteries from charging properly, especially low-voltage ones, because of the voltage drop. I only found this out recently, when one of my readers asked me for help.

That was the last straw in my indecision about whether to make my own module. The boards are already in production. I’ll assemble a batch of my modules, fully compatible with the DL24EW, CL24, and BW150, but much better than the originals. I’ll write an article about them once they arrive.


Initially, I wrote that the CL24 could handle a maximum of 150 W, and even that came with voltage limitations. For example, at 36 V or more, we’d get only up to 80 W. Pretty disappointing. So I made a few adjustments, and in the end I got 350 W without any problems.
The power module here is actually 1.5 times less powerful than the ones I usually use, so the output is 350–400 W rather than 550–600 W.


Since this load also controls charging, there are more wires inside the enclosure. The switching board also meant I had to make the module shorter, and I installed a less powerful (and quieter) fan. The result is the following specs:
Load current: 25 A; charging current: 25 A
Voltage: up to 100 V in discharge mode and up to 50 V in charging mode
Power: up to 350–400 W. It can definitely handle 350 W continuously; 400 W should be fine too, but that will depend on the ambient temperature.
Fan speed control; the temperature sensor is attached to one of the transistors.

I had to replace the current-sense shunt. I installed not only higher-power shunts, but better-quality ones, and added heatsinks, since temperature also affects accuracy.


In the end, I got two identical little boxes with exactly the same internals and specs.
And yes, it now has not only more power than the stock CL24, but also encoder control, which is definitely more convenient than buttons.



The back is just as tightly packed:
Ventilation opening
AC power cord connector
Temperature sensor connector
Charger input
Universal USB-C port, which can be used to connect not only a PC cable, but also additional WiFi and Bluetooth modules.


And on the front:
Charging mode indicator
Display
Power and measurement terminals
Encoder for adjustment and mode selection
Start/stop button
Power button.



Naturally, after replacing the load shunts, I had to calibrate it. I have to say, calibration is easier, more convenient, and more accurate here than on the DL24EW and 24MP.
You can also see that the load works perfectly well at around 380 W.


I made these loads for sale, but I probably won't list them on OLX, since this isn't a mass-produced product. The price is 7300 UAH. They'll be sold with a near-maximum set of accessories:
1. Load
2. Battery holder
3. WiFi module
4. Bluetooth module
5. Basic adapter for connecting the sources under test
6. Adapter with a QC/PD trigger
7. A pair of short cables with alligator clips
8. Power cable
9. XT-60 plug for your charger or DC-DC converter
10. External temperature sensor

How it differs from the stock CL24 and BW150 kits
1. Reverse-polarity protection in charging mode as well as load mode
2. Charging current switching at 20–25 A instead of 2–3 A, plus a low voltage drop across the switch
3. Encoder control
4. Power of 350–400 W instead of 150 W
5. A complete unit in an enclosure, not just a board
6. Built-in power supply.

How it differs from the DL24EW
1. More convenient controls
2. Ability to set a pause between charging/discharging stages
3. Updated software
4. Some bugs fixed.



Of course, it would be nice to have a terminal block on the enclosure, like the one I recently made for a customer (it turned out to be rather inconvenient). But first, there simply isn't room for one in this small enclosure, and second, it would also get in the way if you use the battery holder.
So I can throw in a couple of terminal blocks, and you can make the banana-plug leads yourself.




That's about it. I hope this will be useful to someone. I tried to make the device as neat, functional, and well-built as possible. Since I'm unlikely to make another one like it, I also made the package as complete as I could. The only thing missing, perhaps, is USB cables, but I think everyone has some at home.