Question about RV battery and inverter (Scamp 5th wheel)

[John in Santa Cruz]

a group 24 is a tiny battery for an RV, even the little casitas use group 27M, and a lot of people squeeze group 31M in them. a larger RV with an inverter generally has dual (or even quad) GC-2 golf cart batteries... A typical 24M marine/rv battery is 75 amp*hours, which is about 900 watt*hours, but you should really avoid draining marine/RV batts below 50% as they are /not/ true deep cycle, so you have an effective battery capacity of 450 watt*hours.

[jwcolby123]

Fridge
Dometic RM2410
SKU 910
Input voltage (AC): 120v
Rated input current (AC): 1.3A
Rated input power (AC): 293W
Input frequency: 60Hz


Which makes no sense at all which does not surprise me. 120V * 1.3A = 156 Watts. So where does 293 Watts come from? It doesn't matter.

At the end of the day, given John's numbers for your battery, your battery won't support this thing for more than an hour or two or three. Your inverter would support this thing except it would drain your battery in a couple of hours or three.

IIRC your intention was to run the fridge on AC while traveling. Think about how long you drive. If 6 hours then 6 * 156 watts, or 6 * 293 watts whichever is correct. 900 watt hours to more than 1758 watt hours. More hours, more watts. In either case it seems that a Lead Acid battery is probably not a good fit.

Clearly either a new battery (or two) or a new idea is in order.

[Ottoscamp]

Originally Posted by John in Santa Cruz

a group 24 is a tiny battery for an RV

Yeah, I wondered about that. I bought the Scamp used and can't find anything in the paperwork to indicate what battery is specified. I'm guessing at group 24 based on size, since the battery itself does not say anything other than the part number, and the Les Schwab site doesn't label it either. This just adds to the complexity of electrical systems.

[John in Santa Cruz]

Originally Posted by jwcolby123

Fridge
Dometic RM2410
SKU 910
Input voltage (AC): 120v
Rated input current (AC): 1.3A
Rated input power (AC): 293W
Input frequency: 60Hz


Which makes no sense at all which does not surprise me. 120V * 1.3A = 156 Watts. So where does 293 Watts come from? It doesn't matter. ...

those cheap 400W inverters often can't output anything close to 400W into a inductive load like a compressor.

re that power, there's this thing called "Power Factor" or PF, and AC Watts is actually Volts * Amps * PF. But the PF ranges from 1.0 (a purely resistive load like an incandescent lamp or heater element) to usually not more than 1.3 or 1.5

that all said, I'm betting Dometic got the specs mixed up. 300 watts is on the small side of AC consumption by absorption fridges, and this is a small fridge (the larger ones can be over 700 watts).

[Ottoscamp]

Originally Posted by jwcolby123

Fridge
IIRC your intention was to run the fridge on AC while traveling. Think about how long you drive. If 6 hours then 6 * 156 watts, or 6 * 293 watts whichever is correct. 900 watt hours to more than 1758 watt hours. More hours, more watts. In either case it seems that a Lead Acid battery is probably not a good fit.

Wouldn't the battery stay charged while traveling? I thought it was designed to recharge from the electrical harness connected to the TV? Or do you mean it will discharge despite the input from the TV?

[John in Santa Cruz]

300 watts is 25 amps at 12V. if the combined wiring resistance from the truck alternator to the trailerr connector,a nd trailer connector to the trailer electrical distribution is such that 25 amps drops the voltage below about 13.5V, the battery will discharge to make up the difference.

[jwcolby123]

Originally Posted by Ottoscamp

Wouldn't the battery stay charged while traveling? I thought it was designed to recharge from the electrical harness connected to the TV? Or do you mean it will discharge despite the input from the TV?

Maybe a little. Typically the wire between the alternator and the RV is wimpy and will provide at most a few amps. You can work on that and get more but still, your fridge is asking for many amps. It seems unlikely you will ever get enough to supply more than the fridge is asking for, so the battery will constantly discharge.


It is a simple fact that an absorption fridge requires huge amounts of power to create the heat required to make the system work. As it happens, propane heat is pretty efficient. Electrical heat is simply not.

[Shocksll]

I made a video trying to test different wires lengths and guages to see if i could run the fridge on DC.

https://youtu.be/GzqmAtJ17aY

I determined that even shortening the length of the vehicle wire and increasing the size still did not provide enough amperage to run the fridge.

I think this was partly because of my vehicle only providing 14V instead of something higher like 14.5V. The other part is the 7-pin wiring only being 10awg.

[jwcolby123]

Originally Posted by Shocksll

I made a video trying to test different wires lengths and guages to see if i could run the fridge on DC.

https://youtu.be/GzqmAtJ17aY

I determined that even shortening the length of the vehicle wire and increasing the size still did not provide enough amperage to run the fridge.

I think this was partly because of my vehicle only providing 14V instead of something higher like 14.5V. The other part is the 7-pin wiring only being 10awg.

And it's not just the 7 pin but also the ground. Running my fridge on AC works just fine when plugged in, but not otherwise.


It would be cheaper (and better) to just replace the fridge with a compressor fridge.Waaaayyyy low dc power, like 10-20 watts average. In any event just give up on running the existing fridge on AC unless plugged in.

[John in Santa Cruz]

I have a DC compressor fridge, a Norcold N2175 6.2 cubic foot, and I love it, but its a fair bit more than 10-20 watts average. its about 85 watts when the compressor is running and in hot weather, it runs about 1/2 the time, so lets call it 40 watts average during the heat of day, half that at night... I'd hazard its about 50-60 amp*hours/day average, my 412AH lithium batteries will run it for a week with no sun. My 360W solar panel usually has me fully recharged by noon

[Lynn Eberhardt]

The small 3 way in my '06 Scamp draws 143 watts on start up and tapers to 130, running on AC, as measured with a Kill-A-Watt meter. Dometic website lists 135 watt draw for their newer 3.5 cu ft model. My solution is to run AC to the trailer from an adequately wired inverter in the tug while towing. I run on propane when parked without power. Uses less than 20 lbs. per month.

[jwcolby123]

Originally Posted by John in Santa Cruz

I have a DC compressor fridge, a Norcold N2175 6.2 cubic foot, and I love it, but its a fair bit more than 10-20 watts average. its about 85 watts when the compressor is running and in hot weather, it runs about 1/2 the time, so lets call it 40 watts average during the heat of day, half that at night... I'd hazard its about 50-60 amp*hours/day average, my 412AH lithium batteries will run it for a week with no sun. My 360W solar panel usually has me fully recharged by noon

I have seen the pics of your fridge and it is nice! However, my 19' scamp has the standard little absorption fridge which can't be more than about 2 cu ft or so. The spot where that goes would never fit your over-engineered...

So if I were going to replace that it should require way less than what yours draws. And IIRC this whole conversation was about running the fridge during driving, so even then, assuming your very nice fridge, it would be 8 hrs drive * 40 watts or 320 watts total draw driving down the road.

As opposed to 8 hrs driving * 130W or 1040 watts that Lynn is discussing for a fridge half your size.

Ya get my point at any rate. My rate is for my little 50qt compressor chest style fridge which really does sip power. It is rated at 40w eco or 60 w high power. But that is while running. Mine has a 270wh battery built-in, which pretty much keeps it running for just shy of a full day. In a hot truck in the middle of summer.

[Tcjmiami]

I have added what us shown in the pictures to carry my two 100ah gell cells.
I have 300watts of solar panels and a 1000 watt inverter.
This will cover my 6 cu foot dorm refrigerator most days.

[denswaer]

Thank you for this informative link. I promise not to sue anyone over it.

[computerspook]

Originally Posted by Ottoscamp

• Third, I now use a CPAP machine and rather than investing in a travel battery pack, I'd like to just plug it into the inverter. With the current setup (unintentional but appropriate pun), I'm not drawing enough current for it to work.

OK that leaves a broad area of question. Now I have always used a 12 volt supply for my CPAP (Actually BiPAP) when in my RV. Now I do this in part because if I am on shore power and the power is not so good the converter and battery can become a bit of a filter. The same thing if I am running a generator. Actually in my case I am a ham and I feed my battery and converter into a lund engineering power steam buck and boost converter for my radios and use that for the power for the breathing unit. Doubt I would do this if I did not need the power for the radios.

Now for the results. I originally had a ResMed S9. I did have an inverter which would run it off of my camper storage battery, and I had a good battery. With the inverter I would get about an hour and a half of run time. When I used the 12 volt power supply I ended up with about 2-3 hours life. Hardly workable.

I now have a Panasonic dream station. Yes the one recalled. If yours has been recalled and you have or will use it for camping definitely get it recalled first. But it has about a 2-3 hour run life on the inverter. Good news I can get over 8 off of the DC cord. The unit uses 12 volts as its actual power so it only requires a cord and not a converter. And that appears to be much more efficient. But then you will need to recharge it.

[computerspook]

Originally Posted by Shocksll

I made a video trying to test different wires lengths and guages to see if i could run the fridge on DC.

https://youtu.be/GzqmAtJ17aY

I determined that even shortening the length of the vehicle wire and increasing the size still did not provide enough amperage to run the fridge.

I think this was partly because of my vehicle only providing 14V instead of something higher like 14.5V. The other part is the 7-pin wiring only being 10awg.

The vehicle should be 13.8

[computerspook]

Originally Posted by Ottoscamp

Limited electrical knowledge, but some basic understanding -- feel free to engage in pedantry . . .

I'm pulling a 2005 19' Scamp with the factory-spec battery. The previous owner installed a small inverter in the trailer. Although I've used the inverter for some very minor 110 volt needs, it doesn't provide a lot of power for what I need. The question is: can I install a bigger inverter? Do I first need to go for a double-battery setup (which raises other questions)?

What am I trying to run? Thanks for asking!

• First, a fan would be nice. I use a clip-on in the bed area for when we're hooked up, but out of luck otherwise.
• Second, we have a two-way fridge, and I was thinking that I could plug it into the inverter will on the road rather than relying on propane, which worries me at gas stations and sometimes stops because the pilot light goes out. If we do camp without electrical, then I can switch to propane.

If I DO need a double battery setup to support a more generous inverter, where do I put the second core? (I understand the need for correct batteries and proper wiring to get 12 volts and parallel draw down.). Must it be mounted outside and near the other one? Can I place it in the storage area under the bench? Can I place both of them there? As it is, I don't store much in there, but wasn't sure if there are concerns about the batteries being inside the trailer, even if tucked away in a cabinet.

I appreciate any suggestions or insights you might have.

Best,
Paul

As someone has pointed out a really good physically bigger battery might be a better way to go. But if you really want to depend on your storage capacity of the battery(s) you are going to really need to go to a lot of them for any length.

Inverters are really pretty inefficent. So to the extent you can you want to go native power. That is in your case 12 volts. There are 12 volt fans and that is how I will go.

I also suspect that your frig will quickly drain the battery if you try to operate it on the inverter.

[Air Doug]

One thing so far not mentioned is grounding the inverter. I am no expert, but I read a lot about it and multiple grounds are supposed to risk ground loops, which can cause electrical interference in radios.
Good info is in Grounding Made Simpler, here:
https://diysolarforum.com/resources/...e-systems.159/

In my Snoozy, I had four separate grounds to the trailer chassis:
- on the tongue where the 7-pin connector entered the frame tube;
- beneath the battery compartment where all the ground wires connect to the battery;
- beneath the converter/charger;
- at a center marker/running light on the rear bumper.

I combined them all onto a single buss near the 3000 watt inverter and ran 8 ga wire to the frame and inverter.

Another thing to pay close attention to is the wire gauge specified by the inverter manufacturer. A lot of ampacity charts don't go past 200 amps at 12 V, but that is easily exceeded with a larger inverter. My inverter required 4/0 wire.

[computerspook]

Originally Posted by Air Doug

One thing so far not mentioned is grounding the inverter. I am no expert, but I read a lot about it and multiple grounds are supposed to risk ground loops, which can cause electrical interference in radios.
Good info is in Grounding Made Simpler, here:
https://diysolarforum.com/resources/...e-systems.159/

In my Snoozy, I had four separate grounds to the trailer chassis:
- on the tongue where the 7-pin connector entered the frame tube;
- beneath the battery compartment where all the ground wires connect to the battery;
- beneath the converter/charger;
- at a center marker/running light on the rear bumper.

I combined them all onto a single buss near the 3000 watt inverter and ran 8 ga wire to the frame and inverter.

Another thing to pay close attention to is the wire gauge specified by the inverter manufacturer. A lot of ampacity charts don't go past 200 amps at 12 V, but that is easily exceeded with a larger inverter. My inverter required 4/0 wire.

Yeah all grounds need to be bonded.

And certainly we all need to do what men have a well deserved bad reputation for not doing. Not reading the directions. A lot of RF noise problems from an inverter comes from not installing it according to the manual.

[John in Santa Cruz]

Originally Posted by computerspook

The vehicle should be 13.8

with newer vehicles using smart alternators, thats no longer a valid assumption. my wife's 2016 Mercedes, the system voltage can be anywhere from 12.6 to 14.4V depending on many conditions, and it can change, quite a lot, on the fly as you drive. I know newer Ford trucks do the same thing, I assume GM and Ram are much the same too. Now, the Ford trucks, if they have the full HD tow package, AND you put the truck in 'tow/haul' mode, then they maintain a higher minimum voltage