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150/180 Watt Electronic Dummy Load under-voltage fix
#1
Peter of HBPowerWall fame recently posted a video on the eBay 150/180 Watt Electronic Dummy Loads, they are great units apart from the under-voltage feature that's rubbish. To fix this I’ve added a custom disconnect unit that uses a low voltage IO sense relay and a 12volt 100amp automotive relay (low contact resistance) to disconnect the battery until the unit is reset. It works like a charm and now my 100Ah, 360Wh, 40P 1S battery pack capacity tests are very consistent. If anyone is interested in this add-on under-voltage unit please add a comment and I'll post the circuit details. Cheers
Redpacket likes this post
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#2
Hello,
I got a similar dummy load too, with another pcb and smaller load-terminals but I think the other parts are mostly the same.
My unit hs a undervoltage feature, but there are two problems:
1. so far I couldn't figure out how to adjust the cut-off value (default is 2,0V)
2. the measuring is wrong. Without load activated the shown voltage is correct, but with load activated the shown voltage is round about 1V under the real voltage

Is there any option to solve these problems?
If not I would be very interested in your relais solution
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#3
Most of these cheap electronic loads seem to be very low quality based upon reviews. The only good one I am aware of is the 150W 60V/10A load described here, which also does DC IR in the latest firmware, and has a PC interface. After a few years of heavy use, my 2 units still work as new - with factory calibration still spot-on down to mV/mA. They're truly a steal at $25.
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#4
This is mine: https://de.aliexpress.com/item/400047844...0906.pic_0

I will see what the seller says about the cut-off voltage. Should be adjustable according to the description
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#5
^^^ Yes, you can set the discharge termination voltage to any value (with resolution 0.01V) and you can set a cutoff timer up to 100h (99h59m59s).  Long press the upper right button to enter the settings menu,  short press the low-right button to move between various settings, long-press low-right to exit settings. The up/low left button inc/dec values. 

Iirc this youtube review (of a low-quality clone) has the same UI so you can see how it works there (ignore the remarks there about the deficiencies of the clone - which do not exist in the genuine units; and note the genuines are only known to be reliably available from one Aliexpress seller)
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#6
I found the option to set cut-off voltage,

but now I have another problem:
When load test is running the voltage detection of the tester is wrong.

 

 




Depending on the choosen current-settings this deviaton varies.
Therefor, the cut-off function is totaly useless.

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#7
The problem is the DL24 load is using only 2-wire (vs. 4-wire / Kelvin) voltage measurement so it is reading the voltage at the end of the wires (at the load's input terminal) instead of the true voltage at the battery. This terminal voltage will be lower than the actual battery voltage (by amount I*R where R is the total resistance of the wires between the load and the battery).

The numbers in your video show that your pack resistance seems to be around 1.5 mΩ but the wires to the load seem to be around 30 times greater at 46 mΩ, so just the wires themselves account for a 0.69V drop at 15A.

As a workaround (in CC mode) you could adjust the cutoff voltage to be 0.69V lower than the actual target (but beware that the wire resistance will change as it heats up). But it's better to get a load with 4-wire support - like that I linked above. It makes little sense to design a load supporting large current but lacking 4-wire support. As I remarked before, the design of some of these cheap loads leaves much to be desired.
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#8
the used cables from the load to the pack are 2,5mm² and are fitting the terminals at the load to 100%

I thought about this workaround too, or even to recalibrate the voltage measuring at the software menue,
but I don't think it's reliable.

Yesterday the dummy load stopped discharge-test after 1,5 hours at it's default 2,0V cut-off voltage, but with the multimeter I got 3,8v.
I want to use this device for testing, not for guessing.

So can you say that the linked dummy load has a reliable cut-off function?
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#9
^^^ Even if you used higher gauge cables there's still going to be a nontrivial voltage drop in the cables at high currents like 15-20A. Further it is unlikely that this load is actually properly designed to support such high currents. I recall reports of many of the clones dying quickly at the high end of claimed current support. 

Yes, the discharge stop voltage ("sv") in the load I linked works exactly as desired in 4-wire mode, i.e. it stops discharging when the battery reaches the stop voltage as measured through the voltage sense wires (not the separate load wires) - see my answer in that thread.
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#10
I have been doing this stuff for years, and I have yet to find a better solution than a quality RC balance charger for these types of tasks.

Not only it is a 4 wire measurement, its fully aware of each cell group voltage, fully adjustable for any chemistry, ll while being cable of meting power in and out.
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