Help me identify a wire please!

[miniyazz]

Due to a couple of flattish front batteries I'm getting round to installing a switch to connect the front and rear batteries (via a relay). Yes I will need to replace the factory original which is dying but it will be helpful anyway if for example operating a winch for any length of time.

I have all the main wiring connected but I'm planning on using an ignition live circuit to provide power to a relay, via a switch by my driver door sill, that will bypass the DC-DC charger on demand. On ignition live so I can't accidentally leave the switch on and drain both batteries.

The cigarette lighter socket in the rear is on ignition live so I am hoping to piggy back off that cable where it runs past the driver door sill. So I took a punt at a likely cable and cut off a small strip of insulation only to find my way barred by some conductive, presumably metal, braiding (which isn't earthed). Pictures below, close up and for orientation:

So what am I looking at? I don't want to cut through the braiding to find I've hit the wrong cable. I'm pretty sure it's braiding, if I flex it a lot a tiny gap opens up to something inside which I think is another insulated cable. It conducts but isn't connected to ground and doesn't read more than a few mV ignition on or off.

Anyway in the meantime I have just insulating taped it back up but looking for a consensus.. Do I have the right wire for the rear ignitiion live wire to the cigarette lighter??

Thanks in advance!

[old Jack]

Looks like the fuel filler flap release cable from what I can see.

OJ.

[aussieintas]

I second the fuel filler cap realease cable.

[Keithyv]

+1 to that. That's not a wire.

Plenty of places to pick up a switched feed.
It may be easier to pick up the one from the socket inside the centre console. Pretty easy to get to after you pull that apart. Or pickup one from around the fuse box.

See here for diagrams, I think you are in need of some guidance

http://faq.out-club.ru/download/pajero_iv/maintenance/Service_Manual_2008_2013/2013/index_M1.htm

[Kingbrown]

Your idea has merit but just be sure that components can handle the current that will flow when joining batteries that are fully charged and flat (and other significant load currents for that matter).
A hefty solenoid would be more typical of the device that's used to join the systems together.

[miniyazz]

Thanks guys that would have been a really embarrassing cock-up...

Kingbrown, I have 0AWG cable linking the batteries (aside from a short 10cm 4AWG linking section to the relay) and the relay is 150A rated. There are also two 40A breaker switches near each battery (which haven't yet tripped despite bridging for a starter motor boost).

By my estimate the weak spots are the breaker switches which will need upgrading if I run into issues, the 4AWG cable which should be good for only a 3% voltage drop at 100A at 6ft long so easily good over 10-20cm for the 150A the relay is capable of, and of course the relay itself. Please feel free to correct me if you see anything I've missed. I'll be sure to look into solenoids if the relay proves inadequate.


Thanks for the link Keithyv, that is sure to come in handy.

[Mike DiD]

Your idea has merit but just be sure that components can handle the current that will flow when joining batteries that are fully charged and flat

Yet another Internet myth. I've actually done a worst-case test. I connected a fully charged AGM battery to a discharged wetcell battery. The maximum current drawn was 7 amps and there was a charge transfer of 5amphours when current flow stops.

A charged AGM has an open-circuit voltage 0f 13.0, but a battery needs 13.6 volts to receive charge.

[miniyazz]

Yet another Internet myth. I've actually done a worst-case test. I connected a fully charged AGM battery to a discharged wetcell battery. The maximum current drawn was 7 amps and there was a charge transfer of 5amphours when current flow stops.

A charged AGM has an open-circuit voltage 0f 13.0, but a battery needs 13.6 volts to receive charge.

There's also the drain placed on the main battery which is relatively shared between the two via path of least resistance. I.e. If my winch draws 500A (which it may do under worst case scenario) depending on a number of factors it may draw 300A from the main and (try) to draw 200A from the auxiliary.

[2bad4u]

Yet another Internet myth. I've actually done a worst-case test. I connected a fully charged AGM battery to a discharged wetcell battery. The maximum current drawn was 7 amps and there was a charge transfer of 5amphours when current flow stops.

A charged AGM has an open-circuit voltage 0f 13.0, but a battery needs 13.6 volts to receive charge.

The fact that you got 7 amps in your test, doesn't mean that it's an internet myth. It is very possible to get high current flows between the 2 batteries. Consider Ohm's law and the internal resistance of the 2 batteries. If a discharged battery has high internal resistance (which it usually would) then the current flow will be low, but what about a dead battery with low internal resistance (think shorted internal cells). It would be highly unlikely to get a "short circuit" as the dead battery effectively acts as a large resistor, but the resistance could be low enough to cause high current flows.

We are also dealing with the batteries in a vehicle with switching devices, alternators, heat, high loads, etc as opposed to a bench test. I agree with Kingbrown that consideration must be given to the high current demands that will be possible. I've seen 100A circuit breakers trip when combining 2 batteries because of the sudden in rush of current.

[miniyazz]

So today I wired in the switch, quite doable if a little finicky.

First I managed to get a shot of the back of the cigarette lighter socket in the rear:


Then figured out the right cable (second try):



Then made it a Y-connector:



Then wired in the switch (a bit tricky as it's to be illuminated when activated so needed a resistor and some extra wires and an earth) and mostly heatshrunk together:



And the finished product:



Next to wire in the relay, it's on order.

Thanks guys