Fail of the Week: Battery Pack Jack Wired Backwards

Last Saturday I had a team of teenage hackers over to build Arduino line-following robots from a kit. Everything went well with the mechanical assembly and putting all the wires on the correct pins. The first test was to check that the motors were moving in the proper direction. I’d written an Arduino program to test this. The first boy’s robot worked fine except for swapping one set of motor leads. That was anticipated because you cannot be totally sure ahead of time which way the motors are going to run.

The motor’s on the second robot didn’t turn at all. As I checked the wiring I smelled the dreaded hot electronics smell but I didn’t see any smoke. I quickly pulled the battery jack from the Arduino and – WOW! – the wires were hot. That didn’t bode well. I checked and the batteries were in the right way. A comparison with another pack showed the wires going into the pack were positioned properly. I plugged in another pack but the motors still didn’t run.

I got my multimeter, checked the voltage on the jack, and it was -5.97 V from center connector to the barrel. The other pack read 6.2 V. I had a spare board and pack so swapped those and the robot worked fine. Clearly the reverse polarity had zapped the motor control ICs. After that everyone had a good time running the robots on a course I’d laid out and went home pleased with their robots.

After they left I used the ohmmeter to check the battery pack and found the wiring was backwards, as you can see in the feature photo. A close inspection showed the wire with a white line, typically indicating positive, indeed went to the positive battery terminal. I shaved the barrel connector down to the wires and the white line wire was connected to the outside of the barrel. FAIL!

This is a particularly bad fail on the part of the battery pack supplier because how hard is it to mess up two wires? You can’t really fault the robot kit vendor because who would expect a battery pack to be bad? The vendor is sending me a new battery pack and board so I’m satisfied. Why did I have an extra board and pack, actually an entire kit? For this exact reason; something was bound to go wrong. Although what I had imagined was for one of the students to break a mechanical part or change wiring and zap something. Instead, we were faced with a self-destructing kit. Prudence paid off.

49 thoughts on “Fail of the Week: Battery Pack Jack Wired Backwards

    1. I don’t think so, on those cables that have two leads, one is black, other is black with solid or dashed white line negative is always one without marking, positive is one with white markings.

        1. That’s why I’m saying what’s the situation on the cables I’ve seen. Since I’ve also seen many adapters and connectors that use center for negative and outer barell for positive it is safe to assume that some cables also use different markings for positive and negative wire. Only way to be sure is to test it with multimeter, that’s what I always do, and that’s what example from this article shows.

    2. Don’t be so quick to assume that. Generally, the marked wire is hot. In North America, for example, electrical code requires the HOT wire to be black (or another colour in multi-phase systems), and the neutral wire to be white.

      In two-conductor zip wire, the marked (or serrated) conductor is hot, generally positive. Likewise, when the conductors themselves are coloured, the copper-coloured wire is hot, the silver one is neutral (return)

        1. Correct, the NEC (and its equivalents in other countries) don’t govern low-voltage connections, but I’ve seen an awful lot of people including bona-fide degreed engineers come through my shops who didn’t know this and simply assumed black was at ground or chassis potential. Amusing sometimes.

  1. Well, this is clearly a mistake somewhere on the production line and not enough quality control. What is even stupider is when the battery manufacturers have the battery plugs in reverse on purpose, so that you can’t easily use a pack from a competing manufacturer. I don’t know which is a correct one, but I know that Turnigy and Walkera have their plugs backwards compared to each other. And those are LiPo batteries, capable of doing much more damage.

  2. There is no typical polarity for the wire marking or plug between manufacturers and suppliers, and sometimes even within the same manufacturer. I am looking right now at four Asus supply cords with three different size plugs and both polarities (barrel vs center). Only way that I can plug for reverse polarity is, fortunately, a hard fit so I have yet to try without realizing it. Then I get to the Dell, Lenovo, IBM, and all the rest, all doing their own things as well.

    There is a reason I like USB and the upgrades to the standard for power supply use. Can still get a badly made cable, but at least there is only one correct wiring.

    1. I wanted to add a 2nd HDD to this HP laptop, so I bought the necessary motherboard-to-SATA cable off eBay, plugged it in and the laptop refused to even boot until I’d unplugged the 2nd HDD.

      It turns out HP have made laptops that use that same connector in two different configurations. What are the differences? The pin ordering is fucking REVERSED!

      How the hell is that possible they screwed up so badly?! I haven’t since bothered trying to add a 2nd HDD to this laptop because I can’t be guaranteed to get the right cable.

        1. that works good for low power loads but if it is higher power you want to put the diode across the power input so if you reverse the power supply it just blows the fuse (make sure you even have a fuse on the power input)..

          also you then need a high current diode so you dont blow that too.

          the theory behind that is you put a diode across a battery backwards and it acts as a short and blows the fuse

          1. A reverse diode to blow the fuse works great, but the leakage current through the diode can be significant, and can drain the battery quite quickly.

          2. Redhatter sez: “How’s current going to reach the diode to leak through it if the fuse is blown?”

            It doesn’t, of course. That’s the point. A reverse-polarity input will forward bias the diode, cause high current to flow and blow the fuse, protecting the downstream device.

            Under normal operating conditions, the diode will be reverse biased, and draw only its leakage current through the fuse.

            My point is that a naive engineer might select a big diode with very low forward voltage and high current capability, expecting that will be best to protect the device. But then he’s surprised when the diode might eat 5 mA while reverse biased. Depending on the device, that standby leakage current could be larger than the rest of the protected circuitry.

          3. I’m going to be using that as reverse current protection for connecting up a LiPo battery – not pulling more than a couple of hundred mA at most. I think putting a short in that would be worse than trashing the electronics! ;)

  3. If you ever re-use the USB plug & wire from some cheap Chinese device (like a USB hub that was sold as 2,0 but was actually 1.1), NEVER EVER TRUST THE COLOUR OF THE WIRES!

    I learnt that wire colour lesson when tinkering with the webcam out of an old Asus Eee. It’s a USB one that connects internally with a different connector, only I didn’t pay attention to the wire colour, so when I soldered on a new USB plug I had inadvertantly got the +V & GND the wrong way round. Because I had assumed red is always +V & black is always GND…

    1. I always, always, always check the polarity on power circuits – I’ve seen too many that were done wrong (including one or two that I got backwards ;-) ). I’ll add that some years ago there was at least one consumer electronics company that (reputedly) miswired their connectors so that if you bought an aftermarket connector/charger it wouldn’t work or would damage the product.

  4. I bought 10 18650 USB holder things, they all came with short USB micro USB leads. First one out didn’t work, no problem, at under £1 each I can cope with that. Unpacked another, that didn’t work, hmm. Unpacked a third and that didn’t work either. Of course I had only unpacked the first USB cable assuming that anyone can make a USB cable power connections correctly. Three dead chargers and one faulty USB lead later. (MAD AS HELL)

    1. That reminds me of a job that I ended up with. It was an international airport with hundreds of workstations on a LAN.

      One day I was called in when the regular guy was on leave. I worked for a different company, I was asked to do a backup to tape (3 copies).

      Several months later I was called back because the other guys had been trying to fix the LAN for 16 hours.

      The LAN had crashed and they put all three backup tapes into a faulty tape backup unit before they realized it was the tape backup unit and not the tapes that were faulty.

      Once I learnt what was going on from the remote support over the phone I laughed and grabbed a 4th backup from on top of a server rack.

      It was up and running in about 15 minutes. The support bloke asked me why I made four backups. I just told him that I always hid one backup because when a (next) backup is within hands reach it’s instinctive for a tech to try it without thinking.

      They ended being a customer for over a decade.

      1. I was a computer operator way back when. One of the other operators had a bad disk. This was a drawer disk of 8 or so platters about 15″ in diameter. They put that disk in another drawer to see if it would work. It didn’t. So they tried another good disk in both those drawers. Still nothing.

        A paper label had spun off the disk and crashed the heads on the first drive. They trashed the surface on the disk. Putting that disk in another drive crashed those heads. The two, it may have been more, other disks got trashed by those bad drive heads.

  5. With only 2 wires to connect correctly it is a 100% probability of failure if just one mistake is made and that mistake is done because only ONE bit of information is flipped. One lousy bit! Nowhere is such a coin toss played more than here. Perfection from one bit! Yikes!

  6. I get my kids to check all their components before they start building and trace their circuits before powering them up. It picks up such problems and makes them approach the task in a methodical way. It is also essential if you are using the cheapest parts you can get off ebay which we do so I can get as many of them as possible. Some sensor module kits have problems ranging from tiny SMD LEDs reversed or short circuits that let the magic smoke out the moment you power them up, but over all the cost is still cheaper, so long as you don’t waste hours looking for problems. Anyway the occasional exploding LED makes for a fun day without doing much harm.

  7. You know what they say about assumptions…
    I once finished explaining to an assembler how the “negative” terminal on an electrolytic cap was indicated with a black stripe.
    His take: colors!
    Turns out, the printed white “+” sign in +50V also coincides with the stripe.
    Don’t try this at home folks.

  8. What strikes me as the MOST annoying thing about Rud’s experience is that he got two (he only shows two, was there more?) battery holders/packs from the same supplier, and although they were wired visibly the same, they were functionally different. Now that is something that would really hack anyone off!
    Say he chose the ‘wrong’ one as being the ‘standard’, then his replacement would have hotted up, and the replacement for the replacement, and the replacement for the replacements replacement…

    Thank god there are still people out there who cut stuff up to find out what went wrong. It amazes me that there are so many gumbies out there that just keep plugging and plugging and wondering why nothing works.

    1. Actually, I had three from that order. I also had two from previous separate orders. The first completed robot worked fine so I knew its pack was okay. I had a built robot that works and its pack I used to get the 6.2V measurement and to determine that the motor IC was burned out on the controller board.

  9. Actually, from my experience the dashed wire usually means “center connector”, regardless of polarity. In this case, that’s still a fail though ;)
    I always check twice or thrice with a meter, just to be sure.
    That being said, I just had the joy of shorting li-ion batteries today!

    I was doing a quick test, so I only had a switch, no fuses. It took me a good while to find the culprit of the hot (a quickly drained) batteries, which turned out to be two wires that had melted together (midpoint) on the board during soldering.
    Fortunately only a weak short (somtimes 0 ohms, sometimes 20k).
    What made it really bad was that a motor+mosfet was connected at the source power wires, and the mosfet turned on due to the short on the control-board (control wire went to GND on N-channel), aaaand due to the short, the power was low enough to not run the motor, which made the motor act as a helping short :/
    If this hadn’t been the case, the thin wires on the board would probably have been fuses, and I would’ve quickly identified the fault.

  10. It’s like capacitors. Sometimes the marking indicates positive and sometimes the marking indicates negative.

    I never assume either way around so I always test with a meter first.

    There *was* a standard where the marked wire was signal and the other was ground but the signal was AC so there was no positive or negative.

    Now there are two standards. If there were 3 permutations then there would be 3 standards.

    Obligatory XKCD –

  11. One time I had 20 EL drive transformers. They had red/black HV output leads. One side of the HV output was connected to the DC ground, probably to save on the isolation between primary and secondary. But the Adafruit EL multiplexers also relied on this connection to drive the TRIACs.
    Guess how many of the transformers had the red output lead connected to GND?

    Exactly 10 ( 50%) of the units were wired backwards and were basically shorted out by the EL multiplexers. Luckily they were not potted but had a screw on the case, so I could open them and change the wiring.

  12. Confusing because many wires are two of the same color and the negative wire has a black line, so to suddenly have a line to indicate plus when you already marked the negative by being black.. seems stupid really. Extra information that is not just superfluous but creates confusion.

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