Let Me Get This Straight........

[travelinbob]

Just bought a new residential fridge. Uses 6 Amp on start up and much less when running.

Also just bought a 1000W inverter.

 

Inverter will power fridge from battery and battery will be charged back up from truck while driving down the road

by the alternator on truck.

Inverter will power fridge from battery and battery will be charged back up from the convertor while hooked up to shore power.

So I'm taking 120VAC and turning it into 12 VDC and then turning it back to 120VAC.

 

Does all this make sense ?? Is all this right ???

 

[SCClockDr]

3 hours ago, travelinbob said:

Just bought a new residential fridge. Uses 6 Amp on start up and much less when running.

Also just bought a 1000W inverter.

 

Inverter will power fridge from battery and battery will be charged back up from truck while driving down the road

by the alternator on truck.

"Charged back up" would be a bit of wishful thinking while traveling. Without significant mods the truck alternator capacity is not available through the umbilical wiring. (Rule of thumb  6 amps AC = 60 amps DC)

Inverter will power fridge from battery and battery will be charged back up from the convertor while hooked up to shore power.

So I'm taking 120VAC and turning it into 12 VDC and then turning it back to 120VAC.

 AC>DC>AC is correct only on shore power or generator. It is DC>AC otherwise.

Does all this make sense ?? Is all this right ???

 

 

Your logic seems sound provided the battery capacity is sufficient.

[Devilishjim]

Well I'm thinking when hooked to shore power it is charging your battery but the fridg should be running on shore power also

[Al F]

You need to check the specs on your inverter.  Most of them have a built in transfer switch that passes through the 120V power when available to the inverter.  So you won't have the AC--DC--AC situation. 

 

[Al F]

As stated by others the 12V coming from the truck alternator will help to keep the battery charged while driving, but w/o running a separated pair of larger wires (6 or 4 gauge) from the alternator to the trailer battery there will be to much of a voltage loss in the standard trailer/truck wiring to charge the battery.  Note: you need two wires, the hot wire and the return wire to be the larger wire size.

However as long as you hook up to shore power every day at the end of your travels, you should not have a problem with the battery in the trailer discharging too far. 

[oldjohnt]

While what you say may be correct, it doesn't necessarily have to work that way. Obviously when unplugged, sure you need battery (and charging it) and Inverter power and all that 12 VDC to 120 VAC conversion going on to run the fridge. However, when plugged to shore power IFFFFFFFFFFFFF you have a proper transfer switch method, then let it choose to power the fridge direct off the incoming 120 VAC shore power by passing that 12 VDC to 120 VAC conversion.

 Such could already be built in,,,,,,,,,,,Or possibly (if available ?) be an Inverter option,,,,,,,,,,,Or the fridge could be manually unplugged from an Inverter fed receptacle and re plugged to a utility receptacle,,,,,,,,,,,,,,Or its fairly cheap n simple to wire your own transfer switch to avoid plugging and unplugging if none of the above are available options.

 Subject to the RV and wiring access and your inverter and your skills, if you choose one of the above methods we would be glad to assist you further. I use a small Haier dorm size fridge to supplement by built in RV LP Gas fridge and I currently have BOTH an Inverter powered (2 KW PSW) PLUS a utility powered receptacle right by its side for switch over. However, I'm adding a simple transfer switch to that same location in the near future to make it even easier. You are indeed correct, using battery and inverter to run the fridge when 120 VAC is available isn't a good method lol

FWIW I agree regarding using your alternator to charge house batteries while driving. Without proper cables and solid low resistance relay (if used)  contacts, voltage drop can limit just how many actual charging amps your batteries may receive. NOT to say it doesn't work and isn't used, ONLY voltage drop is a "factor" subject to wiring and cable ampacity and cable length.  Of course, battery capacity and how many charging amps its actually receiving via the alternator are necessary factors.

 As always and I subscribe, keep it simple, this is NOT rocket science and I try my level best to educate and discuss different options ! Just how I was taught and how I'm wired, I'm glad to help fellow RV'ers

 John T  Too long retired and rusty Electrical Engineer so no warranty

[Kirk W]

Quote

Just bought a new residential fridge. Uses 6 Amp on start up and much less when running.

Also just bought a 1000W inverter.

1

The key to the answer is in the inverter that you bought and the way in which things are connected. The most basic inverters do not have any choice but to do as you suggest, but if you select the best option it will be an inverter that has a transfer relay in it and also has 120V-ac power supplied to it. The most basic (cheapest) inverters do not have that ability. 

[Dutch_12078]

As Kirk said, the more basic inverters don't have the automatic transfer switch capability, but there are external transfer switches for 15 amp circuits that can be added to perform that function.

One example: Xantrex PROwatt SW Transfer Switch

[oldjohnt]

Like Dutch mentioned, while my PSW Inverter is still good quality and works great, it for sure did NOT come with a transfer switch. That's where a simple after market transfer switch or even dual receptacles (Inverter and Utility) is one cheap and easy method to eliminate the OP concern. I placed Inverter fed receptacles near my fridge plus where I power up small electronics and may add a simple TS, all was cheap simple and easy to install. Piece of cake problem solved.

John T 

[Yarome]

A few unknowns to answer your questions directly, but highly accurate information so far and some great advice regarding the different options available to you.

Just to simplify and makes some assumptions from the information you provided. It sounds like your only connections are your inverter connected directly to your batteries and then either plug your reefer directly into the inverter or to a dedicated outlet powered by your inverter. If that's the case, then yes. When on shore power you are converting 120v - 12v (battery charge) - and back to 120v via your inverter.

The truck alternator issue has been covered. With your RV's power systems on and your reefer running, about the BEST you can hope for is to keep your batteries "about" level with where you started, but more than likely you'll still notice a drop.

In that scenario, on shore power, my recommendation would be to unplug your reefer, shut down your inverter and plug your reefer into your trailers standard outlet. That would make more efficient use of your shore power and allow your batteries a better charge cycle (as opposed to having a load on during charging).

Kind of an inconvenience plugging back and forth, and you can continue as you are, but it's a little rougher on your batteries.

Ideally... as suggested, a transfer switch at a dedicated outlet for your reefer (shore/inverter) would simplify/automate the transition. You would still want to shut down your inverter while charging, but not much harm done if you don't/forget.

[Tex Bigfoot]

Some good advice above.

Two things that I can add 

You may need to add more battery power for more than a couple of hours . I had this problem when I added inverter and small Haier dorm size fridge. One battery would only last about 2 hours in hot weather. .

The other thing is if you are paying for the electric then you will want to change the fridge from inverter to shore power. It is also harder on the inverter by running it all the time.