Enabling permanent power at rear cigarette plug for fridge - D3/D4/RRS

yeah mine is 11.1v at the highest, but i doubt it will ever kick in, as i wont be running it for long enough (unless left on by accident, which could happen), also there would be quite a bit of voltage drop, so im guessing 11.1v at the plug while under 5 amps load would be about 12v on the battery once that load is disconnected and the battery rests. (i carry jumper leads, so prepared to find out the hard way) :)

with that traxide product, id guess 25 too, but on the photo i can just make out 30 amps rating, also im guessing the negative out is a pass-through and doesnt have to be used ?

I'm not sure that you have an understanding of how a 12v system works. There is no pass through of amps that don't get used. It makes heat somewhere and never gets back to the battery. It's like drinking a glass of water, if you spill half of it it doesn't get back into the glass unless you refill it from the tap.
Cheers
LuckyLes

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Originally Posted by LuckyLes

I'm not sure that you have an understanding of how a 12v system works. There is no pass through of amps that don't get used. It makes heat somewhere and never gets back to the battery. It's like drinking a glass of water, if you spill half of it it doesn't get back into the glass unless you refill it from the tap.
Cheers
LuckyLes

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sorry but i actually have plenty of experience with electronics, by pass through i mean not switched, aka common ground, so you dont have to run both the positive and the negative for the fridge through this, you can just run the positive (available via the relay socket) through this device.

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Originally Posted by haydent

yeah mine is 11.1v at the highest, but i doubt it will ever kick in, as i wont be running it for long enough (unless left on by accident, which could happen), also there would be quite a bit of voltage drop, so im guessing 11.1v at the plug while under 5 amps load would be about 12v on the battery once that load is disconnected and the battery rests. (i carry jumper leads, so prepared to find out the hard way) :)

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900mV of drop is pretty significant at only 5A. A quick back of the envelope calculation would indicate your wiring from the battery to the fridge is about 0.6mm2 (or just larger than telephone cable) to see that. I'd wager it's significantly less in practice based on the wiring size in the vehicle.

yeah, i havent measured it but will tonight, it was just a hopeful guess, maybe add a inline diode :) ....

So 0.8v voltage drop by the time it gets to the fuse on the back of the fridge, so it should cut out when the battery is at 11.9v

Yowza! That's shockingly bad for a 5A load. Can you measure and quantify each segment?

Quote:

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Originally Posted by haydent

yeah mine is 11.1v at the highest, but i doubt it will ever kick in, as i wont be running it for long enough (unless left on by accident, which could happen), also there would be quite a bit of voltage drop, so im guessing 11.1v at the plug while under 5 amps load would be about 12v on the battery once that load is disconnected and the battery rests. (i carry jumper leads, so prepared to find out the hard way) :)

with that traxide product, id guess 25 too, but on the photo i can just make out 30 amps rating, also im guessing the negative out is a pass-through and doesnt have to be used ?

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A Traxide setup uses a minimum of 4.5mm2 cable to the rear cargo area and then 2.5mm2 to each socket.

While your fridge compressor is running, it will be drawing that 5ams and causing the 0.8v voltage drop between your battery and your fridge.

The problem is not while compressor is running, it is when the compressor is NOT running.

Your fridge still draws power and because it is so low a current draw, you may only have about a 0.1v voltage drop.

But in most cases, the fridge will probably have a cut-in voltage, where once the fridge has turned off because of low voltage while the compressor is running, it does not turn back on till the voltage rises over a given set level.

With the sort of voltage drop you are seeing, you will need to experiment to see at what voltage the fridge compressor turns back on.

This would not be an issue as long as your do not forget and leave the fridge on for a long period of time.

This is the sort of situation where you could over discharge your cranking battery.

Also note, in this situation, with such a low current draw, the in-line diode will not reduce the voltage.

According to the manual, there is 4.5 meters of 1.5mm2 wire between the socket and the CJB, then there is the lighter plug itself and about 1-2m of fridge cable.

Cut in is 12.4, and the compressor run often enough that its not going to be idle long enough at low current to use any comparably significant power.

Ive seen decent diode voltage drop even at low current, not that I should need it.

Also an AGM battery would have more in reserve at 11v+ for starting than a regular crank battery.

Just because I had a couple of minutes and was curious :

According to the wiring diagram I have for the D4 there's 6.6M of 2.5mm2 cable between the CJB & the rear socket (2325mm to SJ222 then 4265mm to the socket). That accounts for 0.22V. Then there's 1M of 2.5mm2 for the earth (another 0.035v).

~16mV across fuse 55P, ~12mV across Link 18E.
1M of 6mm2 between Engine box & CJB. ~15mV (although that is assuming there's bugger all other load on the vehicle and it's asleep because there's a bit of other stuff on link 18E).

So a total of about 300mV at 5A.

I'd be interested in quantifying losses across the various connectors.