12v heater while travelling.

[Richard n Ann]

12v Watts are the same as 230v Watts. The elements are approximately the same output so the performance is the same whichever power source is in use. Problem is that fridges rarely see 12v because most van builders economise on wire size. A quick check on the supply at the fridge with the engine running usually sees 11v at best.

I wondered if the exact position of the 12v heater element has any bearing on it's performance. Got no experience of where or how they're fitted, but I suppose there's only so much room at the most efficient heating area

 

[tonyidle]

I wondered if the exact position of the 12v heater element has any bearing on it's performance. Got no experience of where or how they're fitted, but I suppose there's only so much room at the most efficient heating area

The heaters slide into tubes welded to the gas 'chimney'.

 

[pyro]

The main reason not to use gas when travelling is that it is illegal, unless you have a crash cutoff device.

Thanks. I do have a gas cut off fitted, I believed it was recommended practice, but I didn't know it was actually a legal requirement, and I can't find anything on a cursory glance to suggest this. Reference to a C&MC thread dating back to 2017 infers it isn't. Gasit current advice references recommended good practices, but seems to make no reference to any legislative requirement.

12v Watts are the same as 230v Watts. The elements are approximately the same output so the performance is the same whichever power source is in use. Problem is that fridges rarely see 12v because most van builders economise on wire size. A quick check on the supply at the fridge with the engine running usually sees 11v at best.

Yes, agreed, watts is of course a measure of power (or work done) at it's simplest, I'm sure everyone's familiar with how to calculate Watts (I x V) by now. The issue here is the extent of current (I) drawn from the 12v (V) battery, which has finite capacity, not the site's 230V EHU which hasn't.

I wholeheartedly agree with you that cable size is often inadequate, thus causing a voltage drop in the 12V supply and thereby increasing current in accordance with the above formula. However, fridge manufacturers (eg Dometic) clearly state that 12V operation is not intended unless the engine is running at the time.

"To prevent the refrigerator from draining the battery, make sure that the current supplied to the caravan is cut off when the vehicle engine is not running, for example by fitting an ignition control relay"

Whilst this doesn't eliminate voltage drop of course, it is only transient and of generally lower significance whilst the alternator is charging the batteries at a high rate of charge.

It is simply for me a matter of following recommended best practices, that is turning off the gas when mobile because it is far safer and preferred, and whilst the engine is running, keeping the fridge operating on the 12V as recommended by the manufacturer. If the engine is not running, I then use gas, what could be simpler?

As for the design considerations, I leave those to the refrigeration experts who are paid to do a job. I suspect that isn't sufficient for some

 

[tonyidle]

Thanks. I do have a gas cut off fitted, I believed it was recommended practice, but I didn't know it was actually a legal requirement, and I can't find anything on a cursory glance to suggest this. Reference to a C&MC thread dating back to 2017 infers it isn't. Gasit current advice references recommended good practices, but seems to make no reference to any legislative requirement.

Yes, agreed, watts is of course a measure of power (or work done) at it's simplest, I'm sure everyone's familiar with how to calculate Watts (I x V) by now. The issue here is the extent of current (I) drawn from the 12v (V) battery, which has finite capacity, not the site's 230V EHU which hasn't.

I wholeheartedly agree with you that cable size is often inadequate, thus causing a voltage drop in the 12V supply and thereby increasing current in accordance with the above formula. However, fridge manufacturers (eg Dometic) clearly state that 12V operation is not intended unless the engine is running at the time.

"To prevent the refrigerator from draining the battery, make sure that the current supplied to the caravan is cut off when the vehicle engine is not running, for example by fitting an ignition control relay"

Whilst this doesn't eliminate voltage drop of course, it is only transient and of generally lower significance whilst the alternator is charging the batteries.

It is simply for me a matter of following recommended best practices, that is turning off the gas when mobile because it is far safer and preferred, and whilst the engine is running, keeping the fridge operating on the 12V as recommended by the manufacturer. If the engine is not running, I then use gas, what could be simpler?

As for the design considerations, I leave those to the refrigeration experts who are paid to do a job. I suspect that isn't sufficient for some

I'd never consider running a 3 way fridge on 12v unless the engine was running. A long time ago when I obsessed about such things I upgraded a car and caravan fridge and charge wiring for minimum voltage drop at the fridge. The difference was remarkable especially given that the fridge had no thermostat in use on 12v. On a decent length drive it froze the milk.

 

[pyro]

Sorry if I gave the impression that you were, we're in complete agreement .

I didn't know if it was general practice to run on 12V without a thermostat, interesting. I replaced a time worn evaporative fridge on a boat with a compressor one, and couldn't sleep all night. I got used to it in the end, but I lost a few night's sleep in the process.

I no longer obsess about such things either, I figure that's what I pay Dometic to to.

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[cmcardle75]

Sorry if I gave the impression that you were, we're in complete agreement .

I didn't know if it was general practice to run on 12V without a thermostat, interesting. I replaced a time worn evaporative fridge on a boat with a compressor one, and couldn't sleep all night. I got used to it in the end, but I lost a few night's sleep in the process.

I no longer obsess about such things either, I figure that's what I pay Dometic to to.

The 12V often doesn't have a stat (particularly on older designs), as the currents are high, making a stat expensive or needing a substantial relay. They are also traditionally installed in caravans with terrible wiring, so the nominal 120W becomes about 60 or 70W, which isn't enough to freeze anything anyway.

 

[pyro]

The 12V often doesn't have a stat (particularly on older designs), as the currents are high, making a stat expensive or needing a substantial relay. They are also traditionally installed in caravans with terrible wiring, so the nominal 120W becomes about 60 or 70W, which isn't enough to freeze anything anyway.

Wow, thanks for that, with what they charge for them, I wouldn't have thought a thermostat would have added much to the cost. Or decent cabling, for that matter.

 

[cmcardle75]

Wow, thanks for that, with what they charge for them, I wouldn't have thought a thermostat would have added much to the cost. Or decent cabling, for that matter.

The vast majority of the fridges are installed in cheap caravans with a long run of poor bell wire wiring from the car alternator through a rusty towbar socket, not expensive motorhomes with 3 metres of 6mm cabling. They probably wanted to save £10 on a relay/powerful stat as their target market didn't care.

 

[pyro]

The vast majority of the fridges are installed in cheap caravans with a long run of poor bell wire wiring from the car alternator through a rusty towbar socket, not expensive motorhomes with 3 metres of 6mm cabling. They probably wanted to save £10 on a relay/powerful stat as their target market didn't care.

Frightening when you think of the heating effect of all that substandard cabling. I haven't looked at my fridge cabling, mind you. Generally they seem to have done a nice job in the Chaussons from what I have seen.

 

[tonyidle]

Wow, thanks for that, with what they charge for them, I wouldn't have thought a thermostat would have added much to the cost. Or decent cabling, for that matter.

There are many things that Dometic could do to improve the design (they added thermostats to 12v when they started with AES I believe). Things like using stainless for everything exposed to the atmosphere, auto door lock on engine start, simple service access, non of which would have added much to the cost of production over the years. Instead (I'm told by a Dometic Service Agent) they've found a cheaper source of steel tube. It comes with inclusions in the steel that are released when the corrosive chemicals separate when the appliance isn't in use and block the only internal 'jet' or orifice.

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[Richard n Ann]

I wholeheartedly agree with you that cable size is often inadequate, thus causing a voltage drop in the 12V supply and thereby increasing current in accordance with the above formula

Wattage isn't fixed, it's only said wattage at the rated voltage. Drop the voltage and the wattage drops with it

 

[tonyidle]

Wattage isn't fixed, it's only said wattage at the rated voltage. Drop the voltage and the wattage drops with it

I wrote to MMM on that subject recently when one of their contributors stated that the Wattage of the starter motor was constant so if the voltage dropped the current increased. I only wrote to them because I felt that it was important not to spread, or validate, misinformation. They didn't publish my letter . If it worked like that the country's energy costs would disappear overnight .

 

[Richard n Ann]

Wow, thanks for that, with what they charge for them, I wouldn't have thought a thermostat would have added much to the cost. Or decent cabling, for that matter.

Well there you go

 

[cmcardle75]

Wattage isn't fixed, it's only said wattage at the rated voltage. Drop the voltage and the wattage drops with it

And in proportion the square, not directly. A 100W element rated for 12V run at 10V will only produce 69W. Despite losing only 17% of its voltage, it loses 31% of the power. Assuming a ohmic resistive load.

 

[tonyidle]

Frightening when you think of the heating effect of all that substandard cabling. I haven't looked at my fridge cabling, mind you. Generally they seem to have done a nice job in the Chaussons from what I have seen.

It isn't that the cable is not sized correctly for the current it has to carry - it is in any installation I've seen. But for high-current wiring at 12v what is then important is cable resistance that causes volt-drop and that often means at least doubling the safe cable size.

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[Russell's Dad]

My only input. Driving with fridge in 12v....no problem. On gas...no problem. Parked up in the heat on gas.....take the top vent off and get that side in the shade...no problem.

 

[pyro]

It isn't that the cable is not sized correctly for the current it has to carry - it is in any installation I've seen. But for high-current wiring at 12v what is then important is cable resistance that causes volt-drop and that often means at least doubling the safe cable size.

I assume you are referring to the phenomenon of resistivity, or rho (ρ), which is proportional to csa and the length of any specific conductor, with which I am perfectly familiar. I rather thought it to be fairly self evident from the preceding posts.

I'm unsure that I ever suggested that wattage was fixed, rather that it is universally recognised as being determined by the formula I x R.

Well there you go

Not really a great deal I can add to that seminal contribution

 

[pyro]

Typo, missed letter, P=I2R or as I stated earlier, I x V

 

[tonyidle]

I assume you are referring to the phenomenon of resistivity, or rho (ρ), which is proportional to csa and the length of any specific conductor, with which I am perfectly familiar. I rather thought it to be fairly self evident from the preceding posts.

I'm unsure that I ever suggested that wattage was fixed, rather that it is universally recognised as being determined by the formula I x R.


Not really a great deal I can add to that seminal contribution

Resistivity is interesting in itself in that it results in a particular conducter length and size exhibiting a particular resistance. From a purely practical viewpoint there are tables to make life easy.

 

[Richard n Ann]

I'm unsure that I ever suggested that wattage was fixed, rather that it is universally recognised as being determined by the formula I x R.

I don't mean to upset you, but I x R gives voltage, or to use a longer word, potential difference

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