Converter - Battery - Inverter - Converter Loop

Great Horned Owl · Jul 18, 2012

I am in the process of setting up my 5er. I put in a good sized battery bank and an inverter. When boondocking, I will plug the 30 amp plug into an outlet that is wired to the inverter. Before turning on the inverter, I will turn off the breaker to the converter.

I was thinking of wiring a relay to turn off the converter whenever the inverter is on, but I can't decide if it is really needed. My concern is that sooner or later, I will forget to turn off the converter. The inverter will be drawing battery power to supply the converter which will be charging the batteries. I am not sure exactly what will happen, but I suspect that it will not be good. At best, it will probably deplete the batteries fairly quickly.

Does anybody know what will really happen?

Lou Schneider · Jul 19, 2012

Nothing catastrophic will happen - as long as the wiring between the converter and batteries is properly sized to handle the converter's maximum output.

What will happen is the inverter will pull power from the batteries and the converter output current will increase as it tries to keep the battery voltage up. At the same time the inverter will draw more current, pulling more current from the converter, and so on.

The limiting factor in the loop is the converter's maximum output current, which will be flowing through the wiring between converter and batteries.

Each stage in the conversion is less than 100% efficient, so the converter and inverter will hum merrily along, dissipating your battery charge into waste heat. So don't leave the loop connected too long.

The easiest solution is to put a 120 volt on-off switch for the converter in a convenient spot next to the inverter. You just have to break the black wire that connects the converter to the circuit breaker and run it to the switch and back. You can use a piece of romex for this. Then turn off the converter switch when you plug the shore power cord into the inverter and turn it on when you remove the cord to plug into shore power.

John From Detroit · Jul 19, 2012

It might have been simpler to get an inverter/converter then run your shore cord to the inverter's AC-IN and let the internal auto transfer switch do the work.

Since the converter/charger is built into the inverter it knows when to shut it off.

These units also protect you (to a point) against brown outs and surges as they monitor incoming voltage, frequency and waveform in many cases and if it's not "Within spec" they switch to inverter mode.

Of course that only works so long as the batteries hold out.

Alfa38User · Jul 19, 2012

What John said, depending on the particular inverter. Some may not have the charge function built in.

It seems to me that a 2 way 120V ac switch (3 terminals) would be the answer. The 'up' side connected to the inverter supply, the "down" side to the converter supply would solve the dilemma neatly. Select as required and it could be mounted wherever convenient and easy to wire.

Lou Schneider · Jul 19, 2012

It seems to me that a 2 way 120V ac switch (3 terminals) would be the answer. The 'up' side connected to the inverter supply, the "down" side to the converter supply would solve the dilemma neatly.

You'd need a double pole 2 way switch (6 terminals) to switch the neutral along with the hot line. Otherwise the neutral will remain connected to both the inverter and shore power.

Most stand alone inverters will short out if their neutral terminal has a path to the battery negative - and that's exactly what will happen when you plug the shore power cord into the park outlet with the neutral common to both. The path is from the park neutral to park ground through the bond in the park's main power buss, then to the battery negative through the grounded RV chassis.

Plugging the shore power cord into either the inverter or shore power is a more positive way of keeping things separated.

Alfa38User · Jul 19, 2012

Good Catch Lou!!! Thanks

Great Horned Owl · Jul 19, 2012

Thanks guys, but I must not have explained very well. The converter is in the power center, inside, under the fridge. The inverter is in an outside, basement, storage compartment. running any kind of connection between them will be tough. I did manage to run a remote on/off for the inverter into the bedroom.

The output of the inverter goes to a 30 amp RV style receptacle that is mounted under the 5er's front overhang. Nothing else is connected to the inverter. The RV power cord will either be plugged into that receptacle or into shore power. It would not be possible to be connected to both at the same time.

My concern is that some day, I will forget to turn off the converter before connecting up to the inverter. A foolproof system would be a n.c. relay activated by power from the inverter with the contacts disconnecting the power to the converter.

Although doable, there is no convenient way to route the wiring. If the only consequence of forgetting to turn off the converter will be a run down battery, a blown fuse, or a tripped breaker, I won't bother. If more serious damage could result, I'll find a way to run the wires.

With the batteries acting like a huge capacitor, this becomes a classic under damped, positive feedback system. Unfortunately, there are too many unknown variables for me to be able to calculate the time constant.

Joel

Gary RV_Wizard · Jul 19, 2012

I thought your initial explanation was pretty clear. 8)

I think I would just hang a sign next to the inverter receptacle, reminding me to shut of the converter when I plugged in the shore cord.

jje1960 · Feb 23, 2016

Great Horned Owl said:
I am in the process of setting up my 5er. I put in a good sized battery bank and an inverter. When boondocking, I will plug the 30 amp plug into an outlet that is wired to the inverter. Before turning on the inverter, I will turn off the breaker to the converter.

I was thinking of wiring a relay to turn off the converter whenever the inverter is on, but I can't decide if it is really needed. My concern is that sooner or later, I will forget to turn off the converter. The inverter will be drawing battery power to supply the converter which will be charging the batteries. I am not sure exactly what will happen, but I suspect that it will not be good. At best, it will probably deplete the batteries fairly quickly.

Does anybody know what will really happen?

Realize this is a very old thread... However, this is exactly what I want to do, wondering how this worked out? For years, I've used a long extension cord (10 gauge) mounted from the back of the SRX, plugged into the main 50amp trailer plug with an adapter, run under the trailer on clips, stored in the drivers side propane compartment. When I wish to power the trailer via generator in the truck bed, simply drop the cord through the bottom of the compartment and into the bed, plug into the generator, works great. Now, I've added another matching battery and mounted a 2000 watt pure sine wave inverter above the battery bank with a protection fuze block. Instead of plugging the shore power run into the generator in the bed, I'm intending to just plug it into the inverter. Simply turning off the breaker in the cabin panel for the converter should allow use of the internal receptacles for the TV, charging laptop, fan or what-ever. This is basically the exact situation you have outlined, question, how has this worked out? Were the cabin receptacles available and the converter protected by turning the breaker off? Many thanks, this seems doable as long as (as pointed out) the breaker for the converter is off. Thanks!

Gary RV_Wizard · Feb 23, 2016

If you have a separate breaker for the converter/charger, yes it works. There may be other things on that breaker, though, so check it out first.

The issue here is that sooner or later you WILL forget to turn off the converter. Or somebody will notice the breaker and helpfully turn it on for you. The charger-battery-inverter loop will cause an immediate heavy drain on the batteries. It's a runaway condition, because the more the charger strives to replace the lost amps, the greater the load on the inverter and thus increased amp draw.

Let's say you have a charger capable of 60A. It pulls 7-8 amps from the inverter to do that (around a 10% overhead). To supply that,the inverter pulls about 75-80A from the batteries. See where this is going?

jje1960 · Feb 24, 2016

Gary RV Roamer said:
If you have a separate breaker for the converter/charger, yes it works. There may be other things on that breaker, though, so check it out first.

The issue here is that sooner or later you WILL forget to turn off the converter. Or somebody will notice the breaker and helpfully turn it on for you. The charger-battery-inverter loop will cause an immediate heavy drain on the batteries. It's a runaway condition, because the more the charger strives to replace the lost amps, the greater the load on the inverter and thus increased amp draw.

Let's say you have a charger capable of 60A. It pulls 7-8 amps from the inverter to do that (around a 10% overhead). To supply that,the inverter pulls about 75-80A from the batteries. See where this is going?

Many thanks Gary. Agree, extreme attention to cutting off the converter breaker is going to be paramount. Now that summer is coming I can take a look at the most practical way to make a permanent mod that ensures fail-safe process. I've been wanting a nice inverter to use for a number of years now and looking forward to the obvious benefits! We love our week on the dry and it's a nice option to not need the generator running for the time we don't require the airconditioner. For now, I'll definitely hang that sign next to the inverter switch that warns to TURN OFF CONVERTER! Thanks again!

Clay L · Feb 24, 2016

Although I used an automatic transfer switch to disconnect from shore power and switch the inverter output to the ac in the service panel when the inverter was turned on I still had to switch the converter off. I did not have room to put in a sub-panel which would have been the best approach.
I did forget once to turn the converter off when I turned the inverter on. All that happened was that the inverter alarmed and switched off.
My converter and refrigerator were on the same breaker which was handy because I did not want to run the refrigerator on ac. When the breaker was turned off the refrigerator just switched over to propane which is what I wanted to have happen anyway.

jje1960 · Feb 28, 2016

Almost finished the project. Added the extra battery, installed the inverter and fuze, all connected up. All that is left to do, run the remote switch to the cabin (easy day, already had extension cord run with old small inverter last year) and install a AC receptacle box in the drivers side propane compartment to plug in the cord that is the existing run to the trailers main plug in the rear. Now, just use the coiled up cord (that we normally plug into the generator in the truck bed) into the AC receptacle and turn on the inverter from the cabin with remote switch. Everything tested out great, used the breaker to turn off the converter, turned on the inverter and plugged it in, everything inside the cabin works fine, the receptacles are at 120V, microwave works great. Gotta love it when a project comes together. We will see how it goes with the two batteries, however I suspect I'm not going to be able to resist going to four batteries for doubling to 220ah... We will see. Another two blue tops at $176.00 each would be another project investment, need to make sure it's really necessary.

Frank B · Feb 28, 2016

jje1960:

I do exactly the same thing with a 2500 watt pure sine inverter, which is mounted in a storage area of my trailer close to the batteries (yes, it has adequate cooling). I installed a power bar screwed to the top of the storage area close to the outside access door. Now I just plug an extension cord into the power bar, and plug the trailer's power cord (with adapter) into that. I have done that for a long time with two different trailers. I have, on occasion, forgotten to turn off the converter. Nothing dire has happened. It just drains the battery quickly, and makes the converter work hard.

However, also remember to turn your fridge to propane only, and turn off the electric water heater, if you have one. Those two will kill batteries fast, but both will also run just fine on propane. Fridges in particular may have an automatic setting that will favor AC power over propane if available. The fridge does not know if the AC is coming from your inverter, so will happily run on electricity and drain your batteries unnecessarily.

One additional thing that I do make sure to do now is to turn off the inverter when I am not using it. Mine has a 'key fob' remote that allows me to do that easily. Leaving the inverter running when I do not need the AC also kills the batteries. The inverter itself has an 'idling current' which can be fairly high on a cheap unit like mine.

And, btw, I have 6 GC-2's in mine.

It is NOT too much.

Frank.

jje1960 · Feb 29, 2016

Frank B said:
jje1960:

I do exactly the same thing with a 2500 watt pure sine inverter, which is mounted in a storage area of my trailer close to the batteries (yes, it has adequate cooling). I installed a power bar screwed to the top of the storage area close to the outside access door. Now I just plug an extension cord into the power bar, and plug the trailer's power cord (with adapter) into that. I have done that for a long time with two different trailers. I have, on occasion, forgotten to turn off the converter. Nothing dire has happened. It just drains the battery quickly, and makes the converter work hard.

However, also remember to turn your fridge to propane only, and turn off the electric water heater, if you have one. Those two will kill batteries fast, but both will also run just fine on propane. Fridges in particular may have an automatic setting that will favor AC power over propane if available. The fridge does not know if the AC is coming from your inverter, so will happily run on electricity and drain your batteries unnecessarily.

One additional thing that I do make sure to do now is to turn off the inverter when I am not using it. Mine has a 'key fob' remote that allows me to do that easily. Leaving the inverter running when I do not need the AC also kills the batteries. The inverter itself has an 'idling current' which can be fairly high on a cheap unit like mine.

And, btw, I have 6 GC-2's in mine. It is NOT too much.

Frank.

Thanks for the tips Frank. The fridge and hot water heater no problem, I always have them on dedicated propane while traveling, however great tip. As for the inverter, yes, mine has a remote with led that I'm mounting inside the cabin right next to the control panel in the hallway. Also going to put a sticker there with reminder to TURN OFF CONVERTER (thanks Tom) so I don't forget. Thanks for you insight!

jje1960 · Mar 12, 2016

Finished the mod. End result is 220 Ah of maintenance free AGM battery bank, battery cut-off switch, 2000 watt inverter with 200 amp fuse, with remote on/off switch inside the cabin. Lastly, a wired plug in the left side compartment to plug all this into the main power inlet. Don't need to go outside anymore (rain or shine) to start or plug in the generator (although gen has remote) while not on shore power while traveling Our week of off-grid at Bristol will use much less fuel now as well, just use the generator to charge the batteries a couple of times a day, as long as the aircon does not need to be used (although always need it). This all looks real complicated, however I did all the research during the winter and was actually less than two hours of work Total cost is like $850.00 considering I already had one battery. Sounds like a lot, however I definitely think it's in the investment/comfort zone I was playing around with it yesterday and it was nicel to see the plugs in the rv show 120V without shore power being plugged in Had the main aircon running in fan mode, TV and satellite box running and microwaved a cup of water. Pretty cool for us.

calinb · Mar 12, 2016

Great Horned Owl said:
Does anybody know what will really happen?

Yeah--it will just deplete your battery faster.

I solved the problem very simply in our family truck camper. I wired the 30A shore power cable to a two wall socket 110 VAC electrical box in the cabinet that houses the converter, inverter, and power distribution / fuse / breaker panel. The converter and power distribution box are both equipped with three-prong AC plugs on the end of their power cables. When I'm connected to shore power, both power cables get plugged-into the 110V outlet sockets. When no shore power is available but I want to use 110 VAC in the camper, I unplug the cable to the power distribution panel from the shore power socket and plug it into the inverter box instead, which has AC sockets on its front panel. I usually leave the converter plugged-into the shore power box socket, because there's normally no reason to unplug it, and it will remain configured and ready to recharge the battery the next time I hook up to shore power.

Even if I forget to move the distribution panel cable back to the shore power outlet box when shore power is reconnected, it's no big deal; the converter will be charging the battery and running the inverter. I'll get reminded to move the cable the first time I try to run the 1500W microwave and the 600W (continuous) inverter starts beeping at me!

jje1960 · Jul 23, 2016

Gary RV Roamer said:
If you have a separate breaker for the converter/charger, yes it works. There may be other things on that breaker, though, so check it out first.

The issue here is that sooner or later you WILL forget to turn off the converter. Or somebody will notice the breaker and helpfully turn it on for you. The charger-battery-inverter loop will cause an immediate heavy drain on the batteries. It's a runaway condition, because the more the charger strives to replace the lost amps, the greater the load on the inverter and thus increased amp draw.

Let's say you have a charger capable of 60A. It pulls 7-8 amps from the inverter to do that (around a 10% overhead). To supply that,the inverter pulls about 75-80A from the batteries. See where this is going?

Well, this is pretty humorous..... When you posted this months ago..... I said to myself... 'Self, you will not/not forget to turn off the converter....' During our Myrtle trip, here I was sitting in a Walmart parking lot.... Wondering why the heck the voltage rates were jumping like Mexican beans..... Yea, I forgot to turn off the converter before turning on the inverter...... Bet I don't forget again..... Yea right!!!!!!!!!!! :

Nothing but the odd voltage indicators happened, however it was funny to me and I actually thought of Gary's post! Live and learn.... However the battery bank and inverter did perform awesome!

michaeldroot · Jul 23, 2016

I can't plug my 2014 Trail Runner into the house system using an 30 amp adapter. Keeps tripping the GCFI in the garage. traced the trailer wiring and can't find a short. I've checked all fuses and circuit breakers. I can't get power to the converter.

calinb · Jul 23, 2016

Jim,

I hate to admit it but I did manage to run my fridge off my Samlex inverter twice, pulling my expensive Odyssey 2150 battery down to 10v (which I think is pretty close to the ~9.8v where the fridge is supposed to shutdown, IIRC)
The inverter started to beep at me and I remembered to switch the fridge out of "auto" mode to use propane. Luckily my Odyssey still takes a full charge but 10v is a bit hard on them as the bottom of a deep(ish) cycle, I think.

I have two smaller Odyssey batteries that I got for my motorcycle and homebuilt airplane. They are both over 10 years old and still running strong. The airplane Odyssey has been sitting around for about 1-1/2 years since I last charged it (no "float" or "trickle" charger even) while my airplane is torn apart. I checked it yesterday and it still reads fully charged on my Fluke DVM! It must have an amazingly low self-discharge rate.

-Cal

Converter - Battery - Inverter - Converter Loop

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