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A look at some cheap eBay battery disconnectors that allow you to quickly disconnect a battery for working on a vehicle or preventing battery draining while a vehicle is not in use.
When I bought these I thought they might be solid brass, but it appears that all that glitters is neither gold nor brass. I wonder how well the plating will last. It's made me ponder the best metals for compatibility with the traditional lead battery terminals.
Never use one of the wire-brush terminal cleaners on these. It will quickly remove the contact plating.
Eric-O at South Main Auto seems to come across a lot of problems with ground connections onto aluminium engine blocks. The standard test for that is to measure for voltage difference across what should be solid connections when under load.
Apparently the fuse option is used as a security feature. It's wired across the isolator/disconnector and keeps the system powered to retain settings, but will blow the fuse if anyone tries to start the car.
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Let's take a look at some battery disconnectors: these are devices that you put onto one of the terminals of the battery. In this case, it's the negative terminal and you've got a thumb wheel that you can put in here and you've got a separate uh. Well, let me zoom down this, so you can see it. We've got a separate output terminal that is insulated by a plastic sort of housing.

That's gently crimped in i say gently because it really is gently crimped and it can't move, and the idea is that uh, as you screw into this with this uh thumb wheel, when it tightens down it bridges, the circuit, it pulls the two of them together and It connects this outgoing lug to the battery terminal and it seems to work quite well. They do say all you have to do is do undo a turn like that. I wouldn't recommend that, because, if there's any pressure on this at all, it can actually pull that back into contact. This is why i'm like thinking that you know it might be worth indenting that a little bit more now don't be fooled by the golden brassy color of this, because i took a file and i scuffed the end off it and it looks like aluminium underneath it Looks very silvery under there just be aware of that.

The other version is kind of more complicated. It's not as good a design in a way it has the separate terminal, plus a bush that has keying pins inside that slide down the thread like this. So you slide it down that there are no instructions uh. This thing uh goes over like this and it tells an angle and hooks over the hat, but it can still cantilever unless it's turned to the side.

This is what i think is important and in this instance, the battery terminal. You put this over the battery terminal and uh. This thread is connected to the negative of the battery and when you tighten this down, it's got a outer sleeve that comes over and it grips onto the terminal and that bridge is on. But the downside to this one is that unless you know better, this is so loose that it could, if you undo even a fair amount, any strain the battery can bring it back into contact, not so great if you're working a vehicle.

So here's what i think you're supposed to do, because it's got the uh keyed thread, and then it's got this bit at the bottom. I think you're supposed to push this down, get a pair of pliers and rotate it. So it can't then come back out on that. I'm not sure if this is official or not.

Likewise, when you put this lug over uh, i think you're supposed might not go over now, turn it uh, and if you get the angle correct with both, then theoretically, that is it locked fairly tightly in position. It's not going to go anywhere, but it's all a bit precarious this one too we're gon na file it. I found it there. This one is an aluminium core, so it relies on the plating.

To prevent oxidation. Aluminium is not a great material for direct connection, because one of the biggest strengths of aluminium is also its weakness. It's a very low resistance, which is great, it's a very robust material, which is great, but as soon as you scrape the surface of it. If you scratch aluminium, it instantly forms an oxide layer.

Again, that's how it protects itself, that's what makes it so stable, but that layer is uh electrically insulating. So if you were to wear if this was to wear down and you're to end up removing the coating, then you'd end up possibly a bad connection there. It's worth mentioning that uh to diagnose that if you had one of these and your vehicle wasn't starting, but the batteries seemed to measure okay. Well, aside from high resistance batteries, take a a probe.

Take a meter hold on. Let me just demonstrate just by what your probe uh put one on the led terminal in the middle and then on to the output terminal and measure the voltage while the vehicle has been cranked uh. This is something areacoat covers quite a lot and south main auto. If you get voltage across this voltage drop across, it means that there's high resistance, it's just worth mentioning that other than that they could be quite useful.

I would say that you know use them with care. Their primary intended market appears to be things like motor homes, boats, uh, collectible cars that you know you're not using on a daily basis and therefore it's useful to be able to disconnect the negative. Usually negative is the one that's you choose to disconnect, but it's useful to be able to just disconnect them when they're not in use just because otherwise the vehicles have a fairly steady sort of current draw on many vehicles, particularly modern ones. That can actually run your battery flat over time, but you know other than those kind of weaknesses they're all right.

It's also worth mentioning. They both have holes through them with a the facility to actually put a nut and bolt with a terminal on it. One of these - let's see where is it this one came with this little inline fuse holder too, i'm not sure if i'd trust that or not it's a little freebie thrown in um, but nothing to bolt on there with that, it's not going to bolt on there. Maybe it was maybe it was to keep so you had the option.

No, that's that's not right. I don't know, i don't know what they were. It's just a random thing that this seller included. Oh well, random, fuse holder.

Would you trust it? No uh, but there we go. That's it two brassy looking, but not actually brass um electrical disconnectors for when you want to just conveniently disconnect the battery in your vehicle when it's not going to be in use for a reasonable length of time. I could see a use for that. It would be quite a handy little device.

14 thoughts on “Ebay vehicle battery disconnectors/isolators”
  1. Avataaar/Circle Created with python_avatars Chris Hartley says:

    My big problem with either of these is where to fasten the high current cable needed to start an engine? They small hole (m5?) on a round peg isn't going to cut it. I wouldn't want to put much more than 50A through that.

    I think they are designed to look as if you can fit a high current cable inside what appears to be a barrel, it is actually solid apart from the cross drilled hole.

    My conclusion: big fail.

  2. Avataaar/Circle Created with python_avatars Aylesbury Numpty Drivers says:

    I've been getting spammed endlessly for quick release battery terminals , that look like bike wheel quick release. I just think how many people out there are changing their car battery every other day???

  3. Avataaar/Circle Created with python_avatars DrHarryT says:

    The purpose of the fuse link could/should be to put it across the break points of the electrical connection so that when you unscrew it to disable the vehicle from starting the electronic systems will still get battery power to retain memories. Then if the vehicle is attempted to start without tightening it back down the fuse will just blow if you try to start it.

  4. Avataaar/Circle Created with python_avatars Living hintz says:

    Aluminum/Aluminium is better than Copper in this application. My choice is Silver…lemme check the Nickel mine family tree and check…whoops,,,there it is! Good gravy,,,lets hope Silver holds the threads like Aluminium,,,nope. What great conductive element will hold a screw thread tight?

  5. Avataaar/Circle Created with python_avatars Trainer 2 says:

    Very cool. Similar disconnects are used for railways here in North America for terminal blocks.
    They go by AAR or AREMA terminal blocks, and they use an insulated strap between two posts with a gold colored nut. When the gold nut is tightened it then makes the connection but when loosened off it will break the connection.

    The golden color of the nut gives an easy identifyer to where the circuit can be opened since all of the other hardware is silver coloured.

    The gold nut is a pretty interesting connector since it's very reliable when maintained properly and kept free from corrosion.

  6. Avataaar/Circle Created with python_avatars Ray Gale says:

    Only yesterday afternoon I had to recover one of our Military museum vehicles that had a fail to start issue while out in public, luckily it was off the road at a weighbridge entrance. The fault was one of those clamp style battery isolation devices that had gone high resistance and nearly melted. If you look carefully the load path on both types you have is up the screw shaft then through that tiny shoulder contact area under the screw head, the slightest resistance will cause heat which softens the insulation pad causing it to extrude, which reduces clamping force making more resistance, you can see where this goes.
    They are probably OK for small loads or seasonally used equipment. DO not use them in critical applications or on large engines with high cranking currents.
    Specially don't use them on 80 year old Ford CMP Blitz gun tractors with side valve V8 engines and 6 Volt electrics.
    All our vehicles will now have proper switch style battery isolators installed now.

  7. Avataaar/Circle Created with python_avatars gary23jag says:

    I have been using the first one for a number of years, being of a much earlier production my one is made of tinned brass, it’s worked quite well save the occasional cleaning of the mating surface of the thumb wheel. I should really try to find the parasitic draw that only occurs during colder weather.

  8. Avataaar/Circle Created with python_avatars Alan Cartwright says:

    Sorry, but I'm still not clear on how the first one disconnects when the screw isn't completely removed. Does the screw not conduct between the separate parts? If not, how does the screw facilitate a connection between the two parts which are separated by a plastic barrier when it's tightened? I feel like I've misunderstood a fundamental aspect here, so any clarification would be appreciated.
    edit – nevermind, I spotted it. The screw shaft is insulated, but the cap isn't, so once the it's unscrewed the cap loses contact and breaks the circuit. Gotcha 🙂

  9. Avataaar/Circle Created with python_avatars Roberto Gatti says:

    I had the car completely turning off because of the rusted contact… frightening if you are in a tunnel .. I was luckily just parked. Always check the contact is clean if left disconnected for a long time!

  10. Avataaar/Circle Created with python_avatars Stephen Eyles says:

    Anecdote regarding battery drain down: At work a colleague had a new van delivered. A few months later he left and the van sat in the car park unused for a few months, until I needed it as mine was going for maintenance. Couldn't start it, not even dashboard lights coming on. Called the recovery people, they tried to recharge the battery but it wasn't taking charge at all. In the end it had to be carried to the main dealers for a battery replacement as the vehicle was still in warranty. The company had fitted a 24/7 dashcam and tracking device to the van which had drained the battery completely!

  11. Avataaar/Circle Created with python_avatars Al Banana says:

    I like the second one as it looks like a battery terminal that could be easily integrated into an existing system, the first one seems to require more change to what is originally installed. The fused connection should be for the always live side of the battery, to keep the radio and alarm/immobiliser enabled. I can see their utility for boats or caravans, but I wouldn't use them on my classic cars, where they are probably targeted. Better to use an intelligent charger that can keep the battery topped up.

    I had a problem with battery drain on one of my cars and installed a fancy bluetooth voltage monitor to see what was happening. Sure enough, the voltage monitor took 10x the current of the original drain.

  12. Avataaar/Circle Created with python_avatars spiffer27 says:

    Another interesting video Clive. They may be aluminium but, without handling them, I would guess that they are plated zinc die castings. Zinc would be closer to the weight of brass than aluminium which would be considerably lighter.

  13. Avataaar/Circle Created with python_avatars Michael Reis says:

    They're also nice for machines that get winterized and left in the cold, it is quicker to disconnect and remove the battery than removing the terminals, when going to store the battery in a building.

  14. Avataaar/Circle Created with python_avatars Sam Jelfs says:

    The inline fuse holder means that you can keep things like the radio powered, but if someone tries to start the car it will pop. Used more as a security device than for stopping the battery going flat while in storage.

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