Question about powering a portable compressor fridge.

[EdwardFT]

I have a new portable compressor fridge, a Dometic CDF-36, which is literally saving my bacon as the 18-year-old 3-way fridge is having trouble handling the heat. But it draws a lot of power, some of which is wasted.
My leisure batteries are 2x110Ah AGM batteries, Hymer-branded (ouch, says my wallet), 3 years old. When the portable fridge compressor runs, it draws over 4 amps, and the voltage at the socket drops by 1.2V. That's the rear 12V socket, called "TV" on the Hymer fuse board. That is not only a 10% loss, it takes the fridge a lot closer to switching off due to battery under-voltage protection. So if anybody has experience to share...
Question 1: would I gain from adding my own substantial wiring direct from the leisure battery to a dedicated fridge connection? Would 10mm2 be enough or should I use 16mm2 ? BTW I thought that Hymer used 16mm2? So is it maybe the leisure battery internal resistance that is causing the drop?
Question 2: how about if I added an extra battery close by the fridge, connected to the leisure batteries? I am thinking that the leisure batteries would maintain that local battery, but in their own time, so cable diameter would not be an issue? Or is that a dumb idea? Would I have to make it the same kind of battery as the existing leisure batteries? Could the EBL99 take a third 110Ah battery anyway?

 

[SandraL]

Cable Size Calculator - Cable sizing and selection | 12 Volt Planet

Cable Size Calculator - Read our guide to selecting cable of the correct type and size for your automotive, marine or leisure wiring application. Auto and Marine Electrical Components from 12 Volt Planet

www.12voltplanet.co.uk

If you scroll down the above link there is a cable size calculator.
High amps and distance= thicker cable.
You say volts drops by 1.2v at socket, does it also drop at battery by same? If so cable size is ok.

 

[EdwardFT]

Cable Size Calculator - Cable sizing and selection | 12 Volt Planet

Cable Size Calculator - Read our guide to selecting cable of the correct type and size for your automotive, marine or leisure wiring application. Auto and Marine Electrical Components from 12 Volt Planet

www.12voltplanet.co.uk

If you scroll down the above link there is a cable size calculator.
High amps and distance= thicker cable.
You say volts drops by 1.2v at socket, does it also drop at battery by same? If so cable size is ok.

That is a very good question. The answer so far is, I have to remove the driver's seat to get at the battery terminals, so I haven't tested that yet. But you are right, that is essential diagnostic information, that can be obtained definitively only in my MH.

 

[DBK]

We have a CDF 26 which probably has the same compressor. It originally plugged into a cigar lighter socket but this tended to overheat due to a poor connection. I replaced it with a DIN plug and socket which are like a cigar socket but smaller. This was better but over time the contacts again became soft due to overheating.

So I fitted a purpose made 12 volt plug and socket which uses flat brass pins. It is made of cream plastic and looks old fashioned but it has proved very reliable. You should be able to find them stocked by caravan accessory shops. It needs a soldering iron to wire it up.

I wouldn't worry too much about the cable thickness as 4 amps isn't much but fit thicker if you want.

The only essential with these coolboxes is solar - 200W minimum in my experience to cover spring and autumn use.

 

[EdwardFT]

We have a CDF 26 which probably has the same compressor. It originally plugged into a cigar lighter socket but this tended to overheat due to a poor connection. I replaced it with a DIN plug and socket which are like a cigar socket but smaller. This was better but over time the contacts again became soft due to overheating.

So I fitted a purpose made 12 volt plug and socket which uses flat brass pins. It is made of cream plastic and looks old fashioned but it has proved very reliable. You should be able to find them stocked by caravan accessory shops. It needs a soldering iron to wire it up.

I wouldn't worry too much about the cable thickness as 4 amps isn't much but fit thicker if you want.

The only essential with these coolboxes is solar - 200W minimum in my experience to cover spring and autumn use.

Yes, it might indeed be connections causing the voltage drop, though the socket is that smaller type, but it doesn't get hot. More solar is the plan, but so far I seem to either drive frequently, or use EHU if I am in one place for a while.

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[cmcardle75]

BTW I thought that Hymer used 16mm2? So is it maybe the leisure battery internal resistance that is causing the drop?

It is very unlikely that Hymer used 16mm2 cable to run to a 12V socket. 1.5mm2 is more likely. It is also unlikely that a large lead acid battery would drop 1.2V to supply 4A. If it did, it is fit only for recycling. A leisure battery will have a CCA rating in excess of 500A. This is defined as the current it can produce without dropping below 7.2V when fully charged. It isn't going to drop 1.2V for only 4A.

 

[Richard n Ann]

I ran our compressor cool box (3 amps running) on a 5m long 2.5mm² cable and it worked OK.
When I say 2.5mm I mean like mains flex 2.5 not automotive size

 

[EdwardFT]

It is very unlikely that Hymer used 16mm2 cable to run to a 12V socket. 1.5mm2 is more likely. It is also unlikely that a large lead acid battery would drop 1.2V to supply 4A. If it did, it is fit only for recycling. A leisure battery will have a CCA rating in excess of 500A. This is defined as the current it can produce without dropping below 7.2V when fully charged. It isn't going to drop 1.2V for only 4A.

All good points. But according to tables, 5m of 1.2mm2 copper should have a resistance of 0.086 ohms. I am calculating a resistance of 0.28 ohms (1.2V @ 4.28A). Something is up.
Next test: remove driver's seat, (1) check on-load voltage at battery, (2) if good, connect cable direct to battery.
...
Quicker test, the Hymer's own control panel by the back door. It barely twitched by 0.1V when the compressor kicked in. It must be the wiring.

 

[EdwardFT]

Oh BTW, does anyone know how to get at the built-in fridge's 12V alternator power? I could make some use of that.

 

[cmcardle75]

All good points. But according to tables, 5m of 1.2mm2 copper should have a resistance of 0.086 ohms. I am calculating a resistance o

All good points. But according to tables, 5m of 1.2mm2 copper should have a resistance of 0.086 ohms. I am calculating a resistance of 0.28 ohms (1.2V @ 4.28A). Something is up.
Next test: remove driver's seat, (1) check on-load voltage at battery, (2) if good, connect cable direct to battery.
...
Quicker test, the Hymer's own control panel by the back door. It barely twitched by 0.1V when the compressor kicked in. It must be the wiring.

f 0.28 ohms (1.2V @ 4.28A). Something is up.
Next test: remove driver's seat, (1) check on-load voltage at battery, (2) if good, connect cable direct to battery.
...
Quicker test, the Hymer's own control panel by the back door. It barely twitched by 0.1V when the compressor kicked in. It must be the wiring.

Remember (a) if the cable is 5m, the electricity has to travel 5m there and 5m back, so double your resistance. (b) cigarette lighter connections are notoriously bad, which is why they overheat so much (c) manufacturers are great for using the cheapest thinnest cable they can. Although I'd run cable in 1.5mm, Hymer might have fused at 10A which allows anything down to 0.5mm.

Doubling the length and using 0.5mm gives a 1.37V drop at 4A.

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[Reallyretired]

If the voltage drop is only at the socket, then its just the wiring, in which case if nothing else is on the same circuit and the coolbox runs Ok then you dont really need to do anything. If the battery voltage measured at the battery drops by 1.2v with a current draw of just 4A then your batteries are duff. Generally AGM batteries are not highly thought of for leisure use.

 

[EdwardFT]

Remember (a) if the cable is 5m, the electricity has to travel 5m there and 5m back, so double your resistance.
...

Ha ha! I don't think I have made such a stupid mistake for a very long time.

 

[EdwardFT]

If the voltage drop is only at the socket, then its just the wiring, in which case if nothing else is on the same circuit and the coolbox runs Ok then you dont really need to do anything. If the battery voltage measured at the battery drops by 1.2v with a current draw of just 4A then your batteries are duff. Generally AGM batteries are not highly thought of for leisure use.

The fridge runs and nothing else is on the same socket, but it is an energy loss and therefore a limitation on off-grid time. And the fridge does cut off sooner, because it has a low-voltage limit to save the battery. So I can live with it, just have to charge up more often. But IMHO its not right and I'd like to fix it.

i mean, the fridge might be essentially the only significant battery drain, because the only other drain is lights and (a) they are all LED, (b) I like low light, (c) I am an early bird so at least at this time of year I don't see much darkness. In that case a 10% energy loss means (roughly) a 5-day off-grid limit becomes a 4-½ day limit.

 

[EdwardFT]

To follow-up, with thanks for all the hints.
I just connected 5m of 2.5mm2 cable directly to the leisure battery.
No-load, 12.36 at battery, 12.35 at fridge.
Load 4.5A, 12.24 at battery, 11.97 at fridge.
I can live with that. It calculates as about 0.06 ohms, which is exactly what 10 metres (not 5!) should be.
The previous calculated 0.14 ohms does correspond to 10 metres of 1.5mm2 as cmcardle75 suggested. My opinion of Hymer sank just a little there.

PS as a solo full-timer I like the extra space I get with the front passenger seat removed, if only I had somewhere to put it.

 

[autorouter]

5m + 5m of 1.2mm2 wire will have a voltage drop of 0.57V at 4A. that's a 4% drop for 12V, and the usual acceptable criterion is 3%.

If the battery voltage is not dropping, as your panel confirms, then the rest of the voltage drop must be in the connections somewhere. Thicker cable will reduce the voltage drop, but even if the cable drop is zero, the connectors are still dropping 0.6V.

The fridge connections usually go through the Elektroblock. What model of Elektroblock is it?

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[EdwardFT]

5m + 5m of 1.2mm2 wire will have a voltage drop of 0.57V at 4A. that's a 4% drop for 12V, and the usual acceptable criterion is 3%.

If the battery voltage is not dropping, as your panel confirms, then the rest of the voltage drop must be in the connections somewhere. Thicker cable will reduce the voltage drop, but even if the cable drop is zero, the connectors are still dropping 0.6V.

The fridge connections usually go through the Elektroblock. What model of Elektroblock is it?

You are speaking of the previous case, when I connected to the Hymer wiring and was disappointed? That was connecting the portable fridge to the 12V socket just above the installed fridge. That goes through the electroblock (EBL99) at the electroblock fuse marked "TV". So although it has a 10A fuse I suppose they didn't intend the socket to draw much current. But both the fuse and the socket are labelled 10A so I would expect it to deliver 10A without much loss.

But I suppose with the EBL and the connections and the meter in the way, I might have been nickelxand-dimed to death. Whatever, I have my own wiring now and it does the job.

 

[autorouter]

You are speaking of the previous case, when I connected to the Hymer wiring and was disappointed?

Yes. I was thinking you could use the old fridge wiring through the EBL, but I see the 3-way is still fitted.

If the 3-way is generally bad on all three power sources, replacing the thermal paste in the cooling plate of the fridge section may help:

3way fridge thermal gel replacement ?

As previously discussed have received the tube of thermal gel and dismantled the fridge cooling fins which shows no sign of the old gel.Where does the gel (copper paste ) go.. between the screws or where it goes around the black tubes? Also does it go on tubes before the freezer plate slides...

www.motorhomefun.co.uk

 

[EdwardFT]

Yes. I was thinking you could use the old fridge wiring through the EBL, but I see the 3-way is still fitted.

If the 3-way is generally bad on all three power sources, replacing the thermal paste in the cooling plate of the fridge section may help:

3way fridge thermal gel replacement ?

As previously discussed have received the tube of thermal gel and dismantled the fridge cooling fins which shows no sign of the old gel.Where does the gel (copper paste ) go.. between the screws or where it goes around the black tubes? Also does it go on tubes before the freezer plate slides...

www.motorhomefun.co.uk

I might try that, little to lose.
The 3-way holds its freezer cold but the fridge section warms up easily and takes overnight to get back down to safe temperature. The fins inside the fridge are just not very cold. They used to ice up but now in 30 degree weather they don't even feel very cold. A small fan inside to circulate the air didn't make much difference. Actually a bank of PC fans outside at the top grille didn't make much difference either. Removing the grilles helped a bit, and not opening the door for hours was what really helped.
I don't care so much now I have the portable compressor fridge, anything that needs to stay cold can go in that, and then I don't mind so much if the 3-way is more of a pantry than a fridge some of the time. Actually I might even prefer it.

 

[EdwardFT]

Yes. I was thinking you could use the old fridge wiring through the EBL, but I see the 3-way is still fitted.

If the 3-way is generally bad on all three power sources, replacing the thermal paste in the cooling plate of the fridge section may help:

3way fridge thermal gel replacement ?

As previously discussed have received the tube of thermal gel and dismantled the fridge cooling fins which shows no sign of the old gel.Where does the gel (copper paste ) go.. between the screws or where it goes around the black tubes? Also does it go on tubes before the freezer plate slides...

www.motorhomefun.co.uk

Yes, if I could get at the 12V supply of the 3-way, I would use that for the portable fridge while driving.

 

[Richard n Ann]

Since I did our fridge paste 3 years ago it's been fantastic, just been in Benidorm through August and the beers are teeth shattering cold. Beers always get the prime spot next to the fins though

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[DBK]

Yes, it might indeed be connections causing the voltage drop, though the socket is that smaller type, but it doesn't get hot. More solar is the plan, but so far I seem to either drive frequently, or use EHU if I am in one place for a while.

The socket itself doesn't get hot but what was happening with mine is the contacts inside the socket which grip the plug became soft and didn't grip tightly. This was because a poor contact caused resistance and this generated heat which softened the contacts leading to an even poorer contact.

It is worth looking inside the socket and with something non-conducting see if you can bend the contacts which grip the tip of the plug more tightly. This may only work temporarily but is worth trying.

 

[EdwardFT]

The socket itself doesn't get hot but what was happening with mine is the contacts inside the socket which grip the plug became soft and didn't grip tightly. This was because a poor contact caused resistance and this generated heat which softened the contacts leading to an even poorer contact.

It is worth looking inside the socket and with something non-conducting see if you can bend the contacts which grip the tip of the plug more tightly. This may only work temporarily but is worth trying.

Thanks for the tip but sod that, trying to workaround an inadequately designed object. I have bought a "marine" cigarette-style socket which has dimples inside to grip the contacts of the plug, and markings on the surface to show where the locking dimples are. That'll do for the meantime but does anyone have recommendations for a better 12V plug and socket? Or of course I could hard-wire the supply to the cable that plugs into the fridge.

 

[cmcardle75]

Thanks for the tip but sod that, trying to workaround an inadequately designed object. I have bought a "marine" cigarette-style socket which has dimples inside to grip the contacts of the plug, and markings on the surface to show where the locking dimples are. That'll do for the meantime but does anyone have recommendations for a better 12V plug and socket? Or of course I could hard-wire the supply to the cable that plugs into the fridge.

One common alternative is the Hella/DIN type. My last caravan had a two rectangular pin type plug/socket arrangement which I've just googled to be a "Clipsal". You can also just get panel mount 5.5/2.5 or 5.5/2.1 sockets. These are very compact and easily obtained, but not good for high amperages. I'd probably err towards the Clipsal due to high amperage, firm connection and limited protrusion from the wall. They're basically a bit like small mains plugs.