Waeco Fridge Draw on Battery

Hi Derek,
Part of your reply has confused me.
"Only fit a fuse at the fridge end as a fuse up front also causes voltage drop."
Are you suggesting running an unfused wire from the battery to the back of the car?
from Marty.

As crazy as it sounds yes. As long as it is thick and heavy wire as I sugested and insulated. Just like the unfused wire from your battery to your starter motor.

No Voltage Drop.

Regards Derek.

Derek,
What happens if this insulation wears through?
Or the insulation is damaged in an accident?

Marty.

Why would it wear through ?

Depends who does the job I suspose but if it did wear through the same would happen if the starter cables wear through or are damaged in an accident.

What happens to your winch cables in an accident ? They are not fused and they are where most accidents occur.

We are tring to illiminate voltage drop not look for possible causes of fire. There are many other items of concern like the quick a/c regas jobs that use liquid LPG in the a/c, now thats cause for concern or the LPG gas take behind our kids seat in the back and we sit safe up front !

Voltage drop ! Use thick heavy wire, solder everything and fuse the fridge at the rear.

Just another thought just for the by and by. BMW fit the battery in the boot or under the rear seat in some models and it is unfused all the way the the fuse box in the bonnet.

Regards Derek.

How come the fuse at the rear has less voltage drop than if the fuse is at the front?

from Marty.

Its like sucking a golf ball through a garden hose. The closer the ball the easier it is.

Best way I can explain it in easy terms. LoL

Think of the fuse as a bottle neck in the voltage traffic.

Regards Derek

Thanks for clearing that up Derek.
And all these years I have believed V=I*R.
Good old Mr Ohm was wrong, he should have played golf instead of creating electrical laws.

from Marty.

1. Ohm's Law deals with the relationship between voltage and current in an ideal conductor. This relationship states that:

The potential difference (voltage) across an ideal conductor is proportional to the current through it.

The constant of proportionality is called the "resistance", R.

Ohm's Law is given by:

V = I R
where V is the potential difference between two points which include a resistance R. I is the current flowing through the resistance. For biological work, it is often preferable to use the conductance, g = 1/R; In this form Ohm's Law is:
I = g V
2. Material that obeys Ohm's Law is called "ohmic" or "linear" because the potential difference across it varies linearly with the current.
3. Ohm's Law can be used to solve simple circuits. A complete circuit is one which is a closed loop. It contains at least one source of voltage (thus providing an increase of potential energy), and at least one potential drop i.e., a place where potential energy decreases. The sum of the voltages around a complete circuit is zero.

4. An increase of potential energy in a circuit causes a charge to move from a lower to a higher potential (ie. voltage). Note the difference between potential energy and potential.

Because of the electrostatic force, which tries to move a positive charge from a higher to a lower potential, there must be another 'force' to move charge from a lower potential to a higher inside the battery. This so-called force is called the electromotive force, or emf. The SI unit for the emf is a volt (and thus this is not really a force, despite its name). We will use a script E, the symbol , to represent the emf.

A decrease of potential energy can occur by various means. For example, heat lost in a circuit due to some electrical resistance could be one source of energy drop.

Because energy is conserved, the potential difference across an emf must be equal to the potential difference across the rest of the circuit. That is, Ohm's Law will be satisfied: E= I R

BASICALY WE WANT AN IDEAL CONDUCTOR !

Regards Derek.

Hi Derek,
We should probably appologise to Graham for highjacking his thread the way we have. Also it is past my bedtime so I'm off to bed. Perhaps we could continue this discussion tomorrow night in our own thread, it will take me a while to digest your last post anyway.

Hi Graham,
Sorry for highjacking your thread.

nite all.

I concurr with Marty, circuit breakers[manual] @ both ends. BTW Kirchoff's Rule still exists? or have things changed recently?

eng

Laser point thermometer.

On a cold engine with a fridge that has run all night test the temperature of the wire, fuses, breakers (Auto or Manual) and see where the heat is. Normally the fuse up front or the thin wire supplied by some installers.

This is VERY dangerous advice.
NEVER run a heavy wire to the back of the car without a fuse or fusible link near the battery!
As for a fuse having less voltage drop at the end of the line....
all I can say COMPLETE AND UTTER GARBAGE.

Cheers

Thank you for your technical explanation.

I'm sure BMW are going recall all their vehicles and fit a fusable link or fuse to their rear mounted batteries.

O yes and a 7 series 740i cranks at 400amps.

I would like to see the size and price of such a fuse.

Regards Derek.

Derek,
I believe your fuse placement proposal is incorrect and also very dangerous, for the following reasons;
If you fit a fuse at the far end of the power cable, near the fridge, you then have the full length of the cable in the undesirable position that it's UNprotected, when a ‘short’ occurs, for any reason at all, and in any position with-in the length of the power cable, it will probably start a fire... :-(

If a HRC fuse is fitted to the battery (+) lug and the power cable is then inadvertently 'earthed' anywhere in its entire length the HRC fuse will blow and no damage will occur to your vehicle, power cable or the fridge, however when a fuse is fitted at the fridge end and the same 'earth' occurs, a fire is the most probable result!!

A ceramic “HRC” fuse will give a 'zero' voltage drop
size; ~35mm long
price; ~$10
They will protect the vehicle, battery & also fridge

Yes, they are really "Affordable" very effective and simple to fit.

I also don't agree with Derek's suggestion but agree that conventional fuses can be more trouble than they are worth.

IMO, for safety reasons, you must use some sort of short circuit protection. We drive these things on badly corrugated roads which easily test the wire's insulation. If you look at a BMW, Toyota or anyone else the starter morot wire is naturally unfused - its also encased with several layers of insulation, routed away from any metalwork and is held away from metalwork by specially made plastic, or insulated metal brackets. Can't tell me that anyone on this forum will go to those lengths to protect the wiring to their fridge. The problem with a short is that it will cause a fire under the bonnet, and from personal experience, I suggest that be avoided :-((

Conventional fuse holders corrode very easily and its the poor contact in the holder that causes the voltage drop.

I suggest two other alternatives. Firstly, I use fusible links, which have considerably less resistance than a fuse which is why vehicle manufacturers use them for high current circuits. Second (cheap, but workable) alternative is to solder a 40amp blade fuse into the battery end of the circuit, and cover it with insulation. By soldering the fuse, you overcome the potential resistance problem from a crappy fuse holder. But if you blow the fuse, you'll need to get the soldering iron out to fix it - but in reality it never blows unless you have a short circuit.

In fact I solder all fuses I use so they are as reliable as possible - I've had a gutful of fuse holders over the years.

Cheers
Phil

Phil, you have given some of the same reasons I use a HRC ceramic fuse, they have the added benefit that the fuse is bolted direct to the (+) battery terminal, if one 'blows' I just unbolt it, check as to why it 'blew' in the first place and fix the fault, then replace it.

I’ve never heard of any competent auto sparky connecting the fuse at the far end of the power cable, leaving the power cable exposed to risk, away from the battery when used in the usual Aux battery and fridge scenario as used in 4WD vehicles here in Australia.

The current flowing in the wire will be the same at the front as at the rear of the vehicle, therefore the voltage drop across the fuse will be the same regardless of which end you put it.

BUT, if you have a lousy fuseholder or fuse, then the higher temperatures in the engine compartment will cause it to overheat sooner.

The solution is simple - use a decent fuse and fuseholder - and put it where it provides maximum protection - at the power source.

Mike

... and good heavy wire.

Collyn,

For virtually any fridge of any size/make,condition/usage - energy consumption increases by 5% for every 1 degree increase in ambient temperature. It also increases by the same amount for every 1 degree lower the set temperature.

This explains the measurements I was getting when testing my fridge!...I was getting run-current of 6A[ramping from 4.5A after the initial peak].

I presume that the 5% change is compound?[as opposed to simple?]Looking back on the figures I am not getting a linear function, can you please point me to a link for further investigation?

Thanks....eng[late Wok]

Thanks Derek. Could you expand on your reference to 8 B&S Cable? Not sure wether this is figure 8 or single strand, and how thick it is.

8 B&S is a single core wire available in black and red.

The STD 8 B&S is 8mm2 with 93 strands of 0.32mm the insulation is 1mm thick. The outer diameter is 7mm.

Regards Derek.

Thanks Collyn,

I will look into 'M H' .

eng

. . and it's important to be aware of the huge Latent Heat of Fusion/melting. It takes the same amount of energy to freeze ONE litre of water as it does to cool EIGHTY litres of water from 2 degree to 1 degree.

And vice versa for melting - that's how ice in an Esky works.

Mike

Mike,

Good work, c.f 60l Trailblaza, measured 50% @ 45c [16l water load]
This makes it easier to compare performance...if the load were standardised! Do you know what load they used?

eng