Dometic Coolmatic CD30 fridge wiring question

[Barrett1]

This is a bit preemptive but I am receiving and hoping to install my goose gear drawer system with Dometic Coolmatic CD30 fridge today. I like the idea of either hardwiring into the existing cigarette plug in the back of the truck on LHS (drivers side USA) or wiring an arb plug which screws in and wont come disconnected like a simple cigarette plug can. I say this is preemptive because I don't yet know what type of wiring the fridge has come with and haven't looked into the wiring in the factory cigarette plug behind the panel yet. I like this idea because I can turn on the overhead switch to keep fridge with power when truck is off but turn the switch off when not using the fridge and assure it is not sucking any power. Questions are as follows:

1) Is this a good idea? I have read that sometimes cigarette plugs do not have large enough gauge wire and the compressor can be iffy whether it is running or not when it should be. Maybe the grenny uses larger gauge wires?? and this is not an issue.

2) Is there a better way, I know some people are wiring straight to factory second battery, which I do have. I also have the high load switch panel if that is pertinent.

Excuse my naivety. Electrical things are really not my strong suit.

Sean

 

[Glen]

Perhaps you've already implemented a solution, but you haven't received any replies, so here goes . . .

As far as using the cigarette lighter wiring, the circuit should handle the load. Dometic says your C30 refrigerator draws 3 A at 12 VDC and I would guess the momentary draw at compressor motor start is a max of twice that (6 A). The cigarette lighter is placarded for 12 VDC / 10 A, so the circuit should safely handle the load with a 50% headroom even on startup. I didn't track down how the cigarette lighter is fused, but standard practice for a 3 A motor load would suggest a 7.5 A fuse . . . the cigarette lighter is likely fused with at least a 12.5 A fuse (25% over the circuit rating). I'm not sure why the Dometic literature says to use a 15 A fuse for a 3 A motor load . . . that's excessive.

Your idea to not use a cigarette lighter plug to connect the refrigerator is a good one : they are prone to working loose with vibration and certainly are - at best - a high-resistance connection to power. I'd also use a connector with positive retention, like a Deutsch connector. The biggest multi-pole connector they make (you need two poles) accepts 12 AWG and that is large enough for your application. I'd use 12 AWG to wire a 2-pin Deutsch connector to the cigarette lighter wiring behind the panel, then bring the wires out through a rubber grommet in the plastic interior panel. You can use insulated butt splices for 10 - 12 AWG (it should accept two 12 AWG wires in one end with a little finesse) to splice the Deutsch connector wires into the factory harness. If you want to be technically correct, you'll need what's called a "multi-wire butt connector" that is designed for two wires at one end and one wire at the other end - all of the same gauge. Also note you need a special tool to crimp Deutsch connector pins ($35 to $400, depending on brand, range of sizes and features).

Keep the wiring beyond your Deutsch connector to your refrigerator as short as reasonable to reduce voltage losses. There are lots of online calculators you can use to figure the minimum wire size you need for the total circuit length. But 3 A is a small load and your 12 AWG extension will be far below a 3% voltage drop (for the entire circuit back to the battery) as long as you keep the total circuit length (+ and - wires, including the OEM wiring) under 40 feet or so.

edit: I just looked at the wire size selection chart Dometic provides. It seems excessive for the stated load of 3 A / 12 VDC . . . it looks more like a chart for twice that load, i.e., 6 A / 12 VDC. So it might be worth measuring the actual load the refrigerator draws during operation to verify the stated 3 A draw.

 

[Barrett1]

Glen,

I was starting to lose faith i would get any reply on this but you reallycame through. I have not tackled the project yet, actually still waiting on the fridge. Im going to research some of the supplies mentioned in your writeup and hit you up with some questions as they come along.

Thanks so much,

Sean

 

[TheDocAUS]

If you have two batteries, run a fused cable to a 50 amp Anderson plug from the aux battery (use a 40 amp MIDI fuse).

Convert the Dometic cable to a 50 amp Anderson plug, and run cable to where the fridge is.

Plug the Anderson plugs together.

In Australia, these can be bought pre-made. Not sure about the US. They look like this, but that kit is using a MAXI fuse not a MIDI.

INSTALL

1. Put car to sleep first, see Rok Dr Guide;
2. Instal ring terminals over the battery terminals (they are on the top right of the image);
3. Plug in one end of the Anderson cable to the fused cable (the two Anderson plugs bottom left).
4. Feed the other cable end to the fridge location;
5. Install the lugs into the supplied Anderson plug (the lugs are in the middle of wound cable and the spare Anderson plugs are on the right side of the image);
6. Check you installed the positive wire into the positive side of the Anderson plug, and negative wiore is correct;;
7. Plug the Anderson plug into your fridge. Change connector on fridge if needed;
8. Check positive wiring is correct from battery to the fridge. Do same for the negative wire;
9. Insert MAXI fuse into the fuse holder (remove the fuse before starting if it is installed). Fuse holder is top centre of the image.
10. Wake car from sleep mode.

 

[Barrett1]

Hmm, options now. Thedocaus, do u get concerned about parasitic draw even if you turn fridge off when not using. If I use the cig lighter wires as Glen outlined in his how to, I can turn the switch off on overhead panel which in my mind prevents this issue. I guess, in a roundabout way, im getting to, what are the pros and cons of the two different ways?

Sean

 

[AaronS]

I have an Engel ... and an OEM dual battery... I just hardwired the Engel to the second battery. Two years, 37,000miles... no problems yet.

 

[Rok_Dr]

If you decide to hook it up to the starter battery then connect the negative to a chassis earth point. That way the vehicle IBS/BMS will be in circuit and correctly register the starter battery SOC. The following post has details of how I wired an Anderson outlet/inlet to my car.

https://www.theineosforum.com/threads/henrys-blog.12414322/post-1333321414

Cheers
Steve

 

[chrsbe]

I have the dometic drawer fridge and as far as i know there is no voltage monitor built in. If you hardwire it to your battery it will suck it dry. You may want to consider this.

 

[Glen]

If you have two batteries, run a fused cable to a 50 amp Anderson plug from the aux battery (use a 40 amp MIDI fuse).

The Anderson connectors are robust with low resistance, so those features make this an attractive option.

Wire size calculations use the voltage, amperage and length of wires to determine minimum wire size. A common standard for an acceptable voltage drop (due to resistance of the wire) is 3%. Distance is calculated as the length of both the positive and negative wires. The smallest gauge the SB50 50 A connector accepts is 10 AWG. Since the battery is right there under the rear seat, I'd estimate the wire required to run behind interior panels to the refrigerator (and back) would be about 14 feet (4.25 meters). Using 12 VDC, 3.0 A and 14 feet, 10 AWG yields a 0.35% voltage drop . . . about *one-tenth* of the acceptable loss. To put this another way, the 10 AWG wire could safely support 25 A or *eight* refrigerators. That is vast overkill, likely representing unnecessary expense, bulk and weight.

Using 12 VDC, 3.0 A and 14 feet again, 18 AWG results in about a 2.5% voltage drop. Would I run 18 AWG? Probably not - I'd likely up the size to 12 AWG just to make completing connections to the OEM wiring and selecting a robust connector a little easier. 12 AWG results in a 0.6% voltage drop (it would safely support 12 A or four refrigerators).

The CD30 will keep running until its auto-shutoff at 10.4V, so sizing wire for a more usual 3% voltage drop (-0.36 VDC) will have almost no effect on the operation of the refrigerator. 10.5 V is about the minimum discharge level for lead acid, so the refrigerator cutting out at 10.40 + 0.36 = 10.76 V battery voltage is actually a good thing.

do u get concerned about parasitic draw even if you turn fridge off when not using.

In my experience, if you turn an RV electric refrigerator off using the refrigerator controls, it's really off and there is no load or parasitic draw. So wiring to the battery shouldn't be a problem. The suggestion by @Rok_Dr to connect the negative cable to a chassis ground (and not directly to the battery post) so the Battery Management System (BMS) can see and report on the load is spot on.

Despite all my wire size calculations above, there is one other consideration that might recommend the connector and implied wire size @TheDocAUS suggests. I have run into situations (in an RV, not a Grenny) where a BMS incorrectly detects a refrigerator compressor motor startup as a short circuit. This is likely due to no filtering on the motor (such as no capacitor across the motor leads) or some odd interaction of back EMF and the BMS short detection circuit, and in my case it was dependent on state of charge (SOC) being less than 65%. For my Isotherm 12 VDC refrigerator (6 A load), I had to shorten the wire to about 8 feet and increase the gauge to 10 AWG to keep the LiFePO4 battery BMS from "detecting" a short circuit condition. Like @TheDocAUS 's solution, this would run multiples of my refrigerator, but it did solve the false short-circuit detection problem.

Side note: While trying to solve my own false short circuit detection problem, I contacted technicians representing the RV manufacturer (they integrated all the systems), the refrigerator manufacturer (Isotherm), the battery and BMS manufacturer and the power distribution panel manufacturer. They all pointed the finger at the other guys and had no helpful insight or solutions.

 

[Glen]

I have the dometic drawer fridge and as far as i know there is no voltage monitor built in. If you hardwire it to your battery it will suck it dry. You may want to consider this.

Dometic's documentation for the CD30 says it will automatically shut off at 10.4 VDC and then it won't turn back on until it detects 11.7 VDC. This is a common feature on RV refrigerators.

 

[HereinDaRockies]

I have the dometic drawer fridge and as far as i know there is no voltage monitor built in. If you hardwire it to your battery it will suck it dry. You may want to consider this.

There is a voltage monitor:

Switch-off voltage: 10.4 V at 12 Vg