Efficiency questions for 24v RV system install- Feeding current 2 different ways

[Randy540]

There is a similar question already posted but this one contains some different details (and I don't want to remove the existing RV converter box).

I have an RV with a 120v system and a 12v system. The 12v water pumps, fans and lights are not very power hungry but there is a 12v refrigerator that runs at about 125w. To operate off grid I will be installing LFP 24v 400ah batteries, a 3000w inverter and solar panels. If I am not running the AC or any kitchen or additional appliances there will be long times where there is nothing I need the inverter for. The refrigerator will empty the existing 12v AGM battery in 2-3 days.

I am thinking of ditching the 12v battery and running two feeds off the 24v batteries, one to the inverter, another to a 24v step down to 12v converter and then back into the existing RV converter box thus mimicking the feed from the 12v battery when there is no shore/inverter power. I am thinking this would be more efficient for long periods of time to convert from 24v to 12v and then right to the fridge and lights etc, versus leaving the inverter on and going from 24v to 120v then back 12v.

Does this sound like a reasonable approach, would there be any issues with the existing RV converter trying to charge what it thinks is a 12v system when I turn the inverter on? (I was just going to have the inverter feed the RV like it is shore power.) The existing RV converter is not tuned for LFP and also using a step down converter like: https://www.victronenergy.com/uploa...nd-12-24-DC-DC-converters-non-Isolated-EN.pdf . . . I am thinking would not accept current going the other direction.

Is this a good idea? (Thanks in advance for your replies.)

 

[Ep1phany]

In this setup I'd remove or disconnect the old converter completely and just step down to 12v for those appliances that need it. Just size the DC to DC converter to meet your 12v consumption needs.

 

[Tomthumb62]

Why such a large inverter? You can save a lot of battery by using only an inverter big enough for your needs.

We downsized our camp trailer from a 2000W inverter to a 1100W one and it’s greatly extended the use of our battery runtime. Plus the smaller inverter has an ECO mode that actually works cuts the idle draw of the inverter down to a third.

 

[Randy540]

I had planned on leaving the existing converter in place just to keep the installation as simple as possible. The larger size inverter is to accommodate the a/c unit on hot days.

 

[BobH]

Is your converter & 12v DC loads/fuse panel an all-in-one? If so, remove the Converter AC and feed the DC fuse panel with the 12v from a 24v-12v step down. My converter in my rig was separate, so I just removed it and replaced it with a 24v/12V 70A Victron Orion. Works like a charm. Typically the AC-DC converters supplied with the RV's are pretty inefficient. Replacing it with a 24-12v converter will be much more efficient (90%+).

 

[Randy540]

Is your converter & 12v DC loads/fuse panel an all-in-one? If so, remove the Converter AC and feed the DC fuse panel with the 12v from a 24v-12v step down. My converter in my rig was separate, so I just removed it and replaced it with a 24v/12V 70A Victron Orion. Works like a charm. Typically the AC-DC converters supplied with the RV's are pretty inefficient. Replacing it with a 24-12v converter will be much more efficient (90%+).

Yes, it's an all in one. I needed to hear another voice about efficiency, thanks. (You're avatar is great btw, funny!)

 

[joshjhayes]

Thanks for your post, Randy. I'm in the exact situation you were in. Any updates? In my case, I'm going to leave the onboard converter for now along with 12v SLA battery. I'll use the converter sparingly if I need to charge quickly in a rare portable generator situation. My main "converter" will be a victron 75/15 that will use 24v bank as source and send to 12v batt. I may add a dedicated panel to this controller down the road to charge the 12v, and I will likely replace the SLA with a cheap Chinese 12v lifepo4. My charger for the 24v is 1,660 watts of solar, 2 victron 150/35's, and a 24v 13 amp charger that I wired to the "solar on side" port.