Need to run 24 volt loads off a 48 volt bank

openplanet

I'm upgrading my system to 48 volts but still need to run some 24 volt loads: led lights, ceiling fan, small (SunDanzer) fridge, pressure pump).  From what I can see, my options are

1: 48-to-24 volt converter
2: small mppt charge controller charging small 24 volt bank from the 48 volt bank

Interested in knowing people's thoughts.  Thanks.

Estragon

My first choice would be option 1, but I've read where converters can fail in a way that exposes lower voltage loads to the higher voltage.

Horsefly

48VDC-to-24VDC converters are not common enough to be a commodity item, so you will probably pay more than you think you should. Have you attempted to total up what amps or watts all your loads add up to? I only did a quick Google and found a few, but the power output was surprisingly low for the price on most of them. I would guess that the items you list would add up to at least 15A / 360W, and that may be tough. 

On top of that, I'm not sure how dependable these buck converters are. I'm already somewhat dependent on a 24VDC-to-12VDC converter in our cabin, but at least nothing critical is on the 12VDC circuit. 24VDC to 12VDC is pretty common, so there are lots more choices out there (at lower prices).

In your situation, you may looking at a single point of failure in this converter.  If I had a fridge, I'd consider looking for a new one.

openplanet

Thanks for that input.  The 24 volt loads add up to surprisingly little, because the fridge is super efficient.  And the Meanwell converters, which have a good reputation, are relatively inexpensive, such that I might buy two to hedge my bets.  But using a small MPPT controller to charge a small 24 volt bank from the 48 volt bank is still an interesting option to me.  Hoping to get some thoughts about that approach.

jonr

I see new 48->24V 15A buck converters on ebay for < $25.

I'd only explore a second battery bank if you need high surge amps.

mike95490

I'd use a small MPPT controller from Morningstar, and have it set to FLOAT SERVICE for a smallish 24V battery. Use the right size fuses and a 28V crowbar circuit.

patel

Actually I am little bit confused... My total load of house is 9kw then if I install on grid system then what will be inverter ratting? And if I install off grid system then what will be inverter ratting as well as battery battery capacity . If I want 7 hrs battery backup. Please guide me ..thank you

bill von novak

patel said:

Actually I am little bit confused... My total load of house is 9kw then if I install on grid system then what will be inverter ratting? And if I install off grid system then what will be inverter ratting as well as battery battery capacity . If I want 7 hrs battery backup. Please guide me ..thank you

If you want to do an on-grid system your maximum power doesn't matter.  If your grid connection supports it right now then it will support it with solar added.

If you want to do an off-grid system and your maximum load is 9kW then you will need a inverter good for 9kW or better.

Your battery capacity will depend on you ENERGY not your POWER usage.  (Energy is kwhr; power is kw.)  If, for example, your average use is 1kW (so 7kwhr at a time) and you want to operate down to 50% (pretty standard) then you will need 14kwhr worth of batteries.  This will cost you around $4000 for decent lead acid batteries.

However saying "I want 7hrs battery backup" is fairly meaningless if you are off-grid. In that case if your average draw is 1kw then you will need a ~144kwhr battery bank costing around $50,000.  (3 days autonomy down to 50%.)

Dave Angelini

Here is another meaningless statement, I have been battery backed up for 25 years.... It definitely did not cost me $50,000. The Utility told me long ago I would be begging for them to connect me and they wanted about $100,000. Other than the yearly fire season preparation, it is a joy to be remote from all the hustle and bustle! We love it!

jonr

In the on-grid case, the inverter rating should ideally be a little less than the rating of the panels - loads don't matter.

_OS_

I have good experience with the Victron DC/DC converters. They are more expensive than the cheap ones from eBay but hopefully more reliable. I have had a 24/12 on for two years now in temperatures from -20C (-4F) to +30C (86F) with no issues. Victron has 100W and 250W versions. Datasheets here:
https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-DC-DC-converters-isolated-100W-EN.pdf
https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-DC-DC-converters-isolated-250W-EN.pdf