Different voltage from each cig lighter socket?

[phiiiiil]

I bought a little battery monitor gadget the other day that plugs into a cigarette lighter socket. I plugged it in the front socket and it told me the battery was dead! Plugged into the rear however it reports it is fully charged.

A bit of investigation reveals that with nothing plugged in to either the rear socket is ~12.7V but the front was 12.1V, enough of a drop to make the gadget think the battery was on its last legs.

I can't for the life of me think how the voltage could be different between the two sockets when presumably they are both connected to the same fuse. What's going on?

Phil

[scanner]

Easy - Crap wiring and/or bad connections.

Rear one is usually lowest voltage as it is at the end of a long thin wire.

[apole]

Hi Phil,

I rewired both of mine as they are fed by very thin wires. I bought some 25amp cable from a shop on ebay, example item number 370084078729, then wired via a fuse direct from leisure battery. Get a crimping set and ensure all the joins are good.

[phiiiiil]

Easy - Crap wiring and/or bad connections.

Rear one is usually lowest voltage as it is at the end of a long thin wire.

That's all I could think of, and I'll certainly be having a look at replacing the wires and joints, but I'm still a bit uneasy about the "why" - as I understand it just a voltmeter on its own shouldn't be anywhere near enough of a load to cause a drop of over 5% unless it's broken, but the reading from the rear one is okay, so then the wire to the front one or its connections must be resistive enough that surely the socket wouldn't be able to actually power anything? Unless the things I've plugged into it (satnav, chargers etc) have been much more tolerant of low voltage supplies than I give them credit for.

I'm no electrical expert by any stretch of the imagination but I can't quite get my head around it...

Phil

[apole]

Hi,

The bigger wires will help with getting the power to the outlets. Things like phone chargers and sat navs are not very voltage sensitive and draw very little current, wheras an inverter or say a coolbox draw much more power and are more sensitive to a voltage drop.

The voltmeter could be being affected by a poor connection, maybe the front lighter has poor/dirty connections at the rear, or has been used alot as a cigarette lighter and suffered from that, the rear is only a power outlet wheras the front is used as a cigarette lighter, that could be the reason.

Anyway changing the wires are easy when you have the time to spare and won't cost much and unless it's causing a specific problem can be left.

See how clean the contacts are in the front lighter, may just need cleaning to remove some surface tarnishing.

Andy

[stilldesperate]

Unless the things I've plugged into it (satnav, chargers etc) have been much more tolerant of low voltage supplies than I give them credit for.

I'm no electrical expert by any stretch of the imagination but I can't quite get my head around it...

Phil

The voltage drop at the socket would be relative to the current drawn. Thin wires, poor connections all add to the resistance. (voltage drop is current x resistance) Satnavs, etc work at pretty low current drain, so the voltage won't drop too much. Run a fridge from the socket, at 4amps, and the voltage will fall.

Do what Apole suggests if you're bothered, rewire with higher amperage cable.

The table below is typical for multistrand cable.

Wire dia Nr strands/dia Dc resistance (ohms/Km) Max current (Amps)
0.50 16/0.2 0.94 4
0.75 24/0.2 1.20 7
1.00 32/0.2 1.34 11
1.50 48/0.2 1.64 14
2.50 80/0.2 2.08 19
4.00 56/0.3 2.61 26

Hope this helps,

SD (Just seen Andys post - all good stuff!)

[stilldesperate]

Ooops, the tabs on the table didn't work out!

Point was, use 16 (strands) /0.2(mm dia) and the max current is 4A

use 32 (strands) /0.2(mm dia) and the max current is 11A

use 48 (strands) /0.2(mm dia) and the max current is 14A

Hope this helps,

SD