12v for RV: DC-DC converter or AC-DC converter

[whatismouse]

So when I designed my system I actually removed the converter that came with the RV and went with a 48V>12V step-down converter. This works fine but there are a few issues. One is that because the converter is upstream of the inverter, it is not in communication with the LVD of the inverter. In other words I can run the batteries down to the LVD on the inverter and then my 12V loads can continue running the battery down until the BMS trips. This is not ideal. My big 12V load is the furnace fan, which if running all night on a cold night, is not an insignificant amount of power consumption. This means in order to avoid running my batteries all the way down I have to keep track of where my battery SOC is and stop using AC loads well before the LVD. This introduces human input into my system which I'd rather avoid, especially as I rent out my RV.

I remember reading about a perpetual power loop necessitating that the converter must be disabled upon adding an inverter. And so I removed the converter and the 12V battery and added the 48>12 converter. But I'm not sure this ever applied to my system, as the inverter is drawing upon the added 48V bank; I could have left the 12V battery in place.

I'm considering adding the 12V battery back, as well as the original converter, and removing the DC/DC converter. I don't see why the old AC>DC converter would cause a perpetual power loss loop, as the inverter is drawing from the 48V bank, and the 12V loads could draw from the 12V battery.

Then all my loads are downstream of the inverter.

Maybe this creates a new problem. When the LVD hits, my 48V battery bank will not have any draws. But the 12V battery still would supply the 12V panel and allow for loads, and with no way to keep it charged now, it will die. But maybe I don't even need the battery, if the converter itself is sufficient to supply loads?

So I'm confused on what to do and how others have solved this issue.

 

[mikefitz]

my 12V loads can continue running the battery down

BatteryProtect - Victron Energy

Disconnect the battery from non essential loads before it is completely discharged with Victron Energy's BatteryProtect. Find a dealer near you.

www.victronenergy.com

Used for any load except inverter.

Mike

 

[12VoltInstalls]

don't see why the old AC>DC converter would cause a perpetual power loss loop, as the inverter is drawing from the 48V bank, and the 12V loads could draw from the 12V battery.

The ‘converter’ is an inefficient transformer that will constantly attempt to charge the 12V battery. I don’t like converters unless all you do is hop and run between shorepower poles. Taking it out was smart.

My choice would be to put the 12V back in and keep the 48V setup to run the 48V/inverter system as you have been and buy a windyNation or something 100W panel/pwm kit for ~$135 and use that on the 12V battery.

In other words, run an independent 12V charging system OR use that 48V-12V getup; either way will work I’d just like to have both. 12V led lights are inexpensive and easy to install.

Ymmv

 

[HRTKD]

It sounds like you don't have enough Ah in your 48v battery bank.

I have 560 Ah in a 12v LiFePO4 battery bank. I can go for a couple days in single digit weather with no charge from solar. Otherwise, my PV has my battery bank charged by early afternoon in the same single digit conditions with me not doing any power conservation.

 

[John Frum]

You can put one of these https://www.victronenergy.com/battery_protect/smart-battery-protect
on the dc2dc converter circuit but it seems the real problem is your battery is not big enough.

Please provide product links to your inverter and to your buck converter.

 

[whatismouse]

thanks all for the suggestions so far.

this is my buck converter: https://www.amazon.com/gp/product/B07ML5ZML2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
this is my inverter: https://watts247.com/product/pip-3048lv-mk/

my battery is only 5Kw, I am considering doubling to 10KW. Even so, in the winter months, any capacity beyond 5Kw would simply be excessive because there isnt enough solar energy to charge more than that. And I use the fully capacity running my mini-split in heat mode, until I need to switch to the propane furnace. In the hottest parts of the summer I'm using the AC well into the evening and/or at night and so again I'm using all the capacity. However in the summer, I could make use of a 10Kw battery and then wouldn't be running it down all the way. 5Kw is absolutely sufficient during the fall and spring when I'm not using the mini-split so much. Adding capacity would be very expensive for just some additional functionality in the summer. And, in the summer, I have no risk of killing the battery from DC loads because I'm obviously not running the furnace. (lights are already LED, water pump and other 12V loads are negligible)

the 48V battery protect from victron is $150. probably worth it to avoid possible bms trips in the winter months.


I was also wondering what would happen if I connect a 12V battery to my DC panel, in addition to the buck Converter? Would they play nice together?

I'm still unclear as to whether I ever needed in the first place to remove the original AC/DC converter.

 

[John Frum]

I was also wondering what would happen if I connect a 12V battery to my DC panel, in addition to the buck Converter? Would they play nice together?

That buck converter will likely output no more than 13.6 volts.
Not enough to fully charge a lifepo4 or lead acid battery.
If you want to add a battery then you need a 24->12 dc2dc charger.

I'm still unclear as to whether I ever needed in the first place to remove the original AC/DC converter.

You didn't really need to remove the ac/dc converter but they are brutally inefficient.
You already have a 15% penalty to convert dc to ac you would compound another 25% on top of that by running an ac2dc converter off of the inverter.
Do you have an automatic transfer switch?
Do you have a generator?
What ac2dc converter do you have?

 

[Porch]

What kind of battery? Generally, the LVD is set low enough that the battery is 0% SOC, if not lower. This is not always good for the battery.

 

[whatismouse]

yes automatic transfer switch in inverter. So thats another possible solution would be some sort of auto-start for the generator. although I'm not always in places where I can run the generator, and most times Id rather just have loads turn off at a programmable SOC, as my AC loads do.

The LVD for the inverter is programmable and I have it well above 0%. In fact I take into account some hours of additional DC loads by setting the LVD higher.

 

[Porch]

yes automatic transfer switch in inverter. So thats another possible solution would be some sort of auto-start for the generator. although I'm not always in places where I can run the generator, and most times Id rather just have loads turn off at a programmable SOC, as my AC loads do.

The LVD for the inverter is programmable and I have it well above 0%. In fact I take into account some hours of additional DC loads by setting the LVD higher.

Cool. The built in original converters tend to be cheap and inefficient at best, so I would stick with your DC-DC system. If you want the DC-DC converter to shut off when the inverter hits LVD and shuts off, get a 120V AC relay and have it powered by AC, and between the DC-DC converter and the 48V battery. So when the inverter is on, the AC turns on the relay and the DC-DC converter is given power.

 

[John Frum]

yes automatic transfer switch in inverter. So thats another possible solution would be some sort of auto-start for the generator. although I'm not always in places where I can run the generator, and most times Id rather just have loads turn off at a programmable SOC, as my AC loads do.

The LVD for the inverter is programmable and I have it well above 0%. In fact I take into account some hours of additional DC loads by setting the LVD higher.

You can rewire the ac side of the ac2dc converter in parralel with the generator input to the all in one.
You should have a breaker in a junction box in front of the ac2dc converter.

 

[whatismouse]

You can rewire the ac side of the ac2dc converter in parralel with the generator input to the all in one.
You should have a breaker in a junction box in front of the ac2dc converter.

not sure I understand this. what would this do?

 

[whatismouse]

Cool. The built in original converters tend to be cheap and inefficient at best, so I would stick with your DC-DC system. If you want the DC-DC converter to shut off when the inverter hits LVD and shuts off, get a 120V AC relay and have it powered by AC, and between the DC-DC converter and the 48V battery. So when the inverter is on, the AC turns on the relay and the DC-DC converter is given power.

oh cool. I hadn't thought of this before. I've never used a relay so I've got some research to do, but this sounds even cheaper than the victron battery protect and would essentially do the same thing.

 

[John Frum]

not sure I understand this. what would this do?

The ac2dc charger would supply 12 volt power to your legacy dc loads.
It would only do this when the generator was running.

 

[whatismouse]

The ac2dc charger would supply 12 volt power to your legacy dc loads.
It would only do this when the generator was running.

Oh I see. I pretty much never use my generator

 

[Porch]

oh cool. I hadn't thought of this before. I've never used a relay so I've got some research to do, but this sounds even cheaper than the victron battery protect and would essentially do the same thing.

Assuming your DC-DC converter does not draw more then 30 amps on the 48v input, this relay would work.

https://www.amazon.com/dp/B087G6D24G/ref=cm_sw_em_r_mt_dp_XTG5RG1PYVN2PEYQWAD9

Or for bigger loads.

https://www.amazon.com/dp/B07FCJFGL9/ref=cm_sw_em_r_mt_dp_2C3B8C91XKTGJ8DSWAXS

 

[whatismouse]

Assuming your DC-DC converter does not draw more then 30 amps on the 48v input, this relay would work.

https://www.amazon.com/dp/B087G6D24G/ref=cm_sw_em_r_mt_dp_XTG5RG1PYVN2PEYQWAD9

Or for bigger loads.

https://www.amazon.com/dp/B07FCJFGL9/ref=cm_sw_em_r_mt_dp_2C3B8C91XKTGJ8DSWAXS

Thanks!!