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I am beyond puzzled with identifying the values of a wire wound, axial, through hole resistor that I'm replacing on a MRC Super Brain 989 relic that I burnt up after 50+ hours of charge/discharge cycles of (20) 6-cell NiMH battery sticks that make up the IMA battery in my Honda Civic Hybrid; in efforts to rebalance the pack.

The failed resistor, or actually a series of (4) identical resistors are responsible for the 10A discharge rate the 989 is capable of. I was pushing the discharge circuit beyond limits with dozens of discharges on these sticks at a 10A discharge rate at a 0.9mV/cell peak cut-off, resulting in a 6180mAh discharge capacity per stick.

The charger pauses the discharge cycle when the internal temp reaches 185ºF and resumes discharge when temp drops to 175ºF. This pause/resume would happen several times during a discharge.

All that being said....The resistors in need of replacing measure 17mm in can length and 6mm in diameter, and all 4 resistors read out on a MM with a resistance value of 0.5-0.7 Ohms.

*****THE 5-BAND COLOR SEQUENCE******

{ |BROWN| |BLACK| |SILVER| |RED| |BLACK| }

The best I can come up with is: (0.1 Ohm with 2% Tolerance) by ignoring the ending black band and treating this as a 4-Band Resistor.

The goal is to replace the resistors with a better grade, add more fans, and extend heat sink to get the charger in a workable condition to finish the project at hand.

Thanks!!

Resistor in questiong8.jpg

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    \$\begingroup\$ Post a picture of the resistor \$\endgroup\$
    – BeB00
    Jun 28, 2018 at 22:37
  • \$\begingroup\$ Why wear them out even further?! use pulse charge over Cv with an inductor from float charge CV or get a 100W Panel mount 0.6 Ohm resistor \$\endgroup\$ Jun 28, 2018 at 22:44
  • \$\begingroup\$ there is a discrepancy between your color list and the picture \$\endgroup\$
    – jsotola
    Jun 29, 2018 at 1:54
  • \$\begingroup\$ "...all 4 resistors read out on a MM with a resistance value of 0.5-0.7 Ohms" - are you saying that each resistor measures 0.5-.07 Ohms? What reading do you get with a dead short? \$\endgroup\$ Jun 29, 2018 at 2:27
  • \$\begingroup\$ If the center band is intended to be white, you would have 29 Ohms +\- 1%. It doesn’t look white to me, but maybe they thought off-white would show better? \$\endgroup\$ Jun 29, 2018 at 3:05

2 Answers 2

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Looks like a 0.1\$\Omega\$ 2% resistor, so 4 in parallel would be 0.025 ohms.

At 2.5A the power dissipation would be 0.75W, which is plausible for the appearance, but you might want to compare the exact size against similar construction resistors.

That's probably too low for your meter to measure reliably, but you can get closer by subtracting the reading you get when you short the probes from that of the resistor.

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  • \$\begingroup\$ I'm thinking 0.1 Ohms, not 0.2. OP says that first band is brown and the picture agrees with him. Brown, Black, Silver \$\endgroup\$ Jun 29, 2018 at 4:13
  • \$\begingroup\$ @DwayneReid I think you're right, they look pretty close on this screen, but the left one is clearly brown. Changed. \$\endgroup\$ Jun 29, 2018 at 4:22
  • \$\begingroup\$ I wonder why there’s an extra band. \$\endgroup\$ Jun 29, 2018 at 4:37
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    \$\begingroup\$ @Blair: Resistor values always start with a non-zero digit so, at least, you won't see black on both ends. \$\endgroup\$
    – Transistor
    Jun 29, 2018 at 6:22
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    \$\begingroup\$ @JoshuaMeehan Probably not. Your reading indicates somewhere between 0 and 0.1 or 0.2 most likely so goodness knows what the real reading is. If you can put a know current through it you can get a better reading with a multimeter on voltage range. If each resistor measures something like that and they don't appear excessively burned, I would say they're probably okay. \$\endgroup\$ Jun 29, 2018 at 18:08
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and all 4 resistors read out on a MM with a resistance value of 0.5-0.7 Ohms.

A normal (2 wire, 3½ digit 200 ohm min range) multimeter is not trustworthy at such low resistances, even if you try to zero-out the leads contact variability can mess with your readings and lack of precision is a big problem. If you want to measure such a resistor accurately then you need a 4-wire setup (if you don't have a 4-wire meter then you can use your regular meter in voltage mode in conjunction with a bench supply to provide the current).

The best I can come up with is: (0.1 Ohm with 2% Tolerance) by ignoring the ending black band and treating this as a 4-Band Resistor.

That does sound like the most likely interpretation, given that the third band is silver and given that the gap between the third and fourth bands is the widest gap.

I suspect the extra band may be an indication of temperature coefficient. While most of the references I can find online now talk about temperature coefficient being part of a 6-band code, I'm sure I have seen references to a 5-band code with a temperature coefficient band in the past.

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