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Bobster

Testing BMS cables

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Hi all,

This isn't a PROBLEM, but it is a PUZZLE (to me at any rate).

It was necessary to install a modified cable between the BMS and inverter. This was initially achieved by taking a regular ethernet cable and cutting all wires except those connecting pins 4 and 5. Easily done. Worked well. Was untidy.

Then time came to tidy up. A neighbour who had an RJ45 crimping tool helped me. We took the same cable, removed one head, threaded it through the correct strain reliever/grommet on the inverter and replaced the head with just pins 4 and 5 connected. (I had a temporary cable in place whilst this was going on). Neighbour says he has a test kit ( https://tinyurl.com/qvauw49). It has a terminator which goes on one end and the tester sits at the other end. The tester showed that only pin 5 was connected. This went on a while. No matter what we did, the test kit always showed pin 5 connected and an open circuit on 4 (assuming no light means open circuit). Neighbour tells me there must be a break in the cable somewhere.

Eventually I proceeded as follows
1) Cut the head off one end of the cable, exposed all the conductors, stripped some insulation from the blue pair (pins 4 and 5 on  a standard ethernet cable) and tested with a multimeter. This showed short circuit end-to-end on both circuits.
2) Fitted a new head for pins 4 & 5 only. Repeated test with multimeter. This shows all good
3) Tested with the test kit again. This shows pin 5 good, pin 4 open circuit.
4) Put this cable into the system. It worked.
5) Tested the temporary (butchered) cable that had just come out. Multimeter shows it's good, cable tester shows pin 5 good, pin 4 not.
6) Took an absolutely unmodified cable and used the tester. All LEDs bar "G" lit up in turn - so the pin 4 LED does work.

So now what? Why does the cable work, the multimeter show that it's good, but the tester show that it's not? One though that occurs to me is that the tester is testing for 1 gig ethernet, and something about the modified cable doesn't meet the necessary standard. After all, the multimeter is just showing that there's a DC short circuit.

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2 hours ago, Bobster said:

but the tester show that it's not?

The tester probably employs some smart-ass trick that requires more than half the connectors to be connected to test the other half, and pin 4 just happens to fall into a set with too few elements. Or something. Ignore it and move on with your life 🙂

 

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19 minutes ago, plonkster said:

 Ignore it and move on with your life 🙂

Wait! Help is at hand...

I had one of these and it's IC blew when I connected it to a circuit that I shouldn't have. So I thought let me replace the IC with a rotary switch and make a robust tester instead.

So here's the schematic: The supply is simple: the positive is connected to its LED on the master unit then through the cable that you are testing and through the LED on the slave.

The return comes back through ALL the other channels and each channel is connected to the negative through its own  1k resistor..

PS: Do I get paid for this info??

004.JPG

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2 hours ago, plonkster said:

The tester probably employs some smart-ass trick that requires more than half the connectors to be connected to test the other half, and pin 4 just happens to fall into a set with too few elements. Or something. Ignore it and move on with your life 🙂

 

Yes. Pins 4 & 5 only is not a common scenario.

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Just now, Bobster said:

The tester probably employs some smart-ass trick that requires more than half the connectors to be connected to test the other half, and pin 4 just happens to fall into a set with too few elements. Or something. Ignore it and move on with your life

My poor neighbour spent several hours over a week testing and retesting and refitting heads. Though he never once tried the multimeter (which I use because it's the only form of testing available to me). So we both move on.

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