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Author Topic: Most efficient conversion from 48VDC to 24VDC ?  (Read 4355 times)
stephend
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« on: May 29, 2009, 02:25:29 PM »

I'm planning on a 48V battery bank, but have noticed that so many DC devices designed for off-grid are available in 24VDC.  I'm thinking specifically of:
- Water pressure booster pump
- Ventilation fans
- LED lighting in house

What's the best way to connect 24 V devices to a 48V battery bank?  The batteries will be about 40m from the house, so will it be more efficient to invert to 230VAC and then use a single switch mode power supply to bring it back down to 24 vdc and run a 24dc circuit in the house?  Or use 48vdc and a dc-dc converter to bring it down to 24v?  ... or something else entirely?  Smiley

Another question, are the DC water pumps really that much more efficient than AC pumps?
« Last Edit: May 29, 2009, 02:35:24 PM by stephend » Logged

timbo
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« Reply #1 on: May 29, 2009, 03:54:15 PM »

I'm planning on a 48V battery bank, but have noticed that so many DC devices designed for off-grid are available in 24VDC.  I'm thinking specifically of:
- Water pressure booster pump
- Ventilation fans
- LED lighting in house

What's the best way to connect 24 V devices to a 48V battery bank?  The batteries will be about 40m from the house, so will it be more efficient to invert to 230VAC and then use a single switch mode power supply to bring it back down to 24 vdc and run a 24dc circuit in the house?  Or use 48vdc and a dc-dc converter to bring it down to 24v?  ... or something else entirely?  Smiley

Another question, are the DC water pumps really that much more efficient than AC pumps?

Sounds like you understand the principles  of DC-DC conversion efficiency and that cable losses are related to current not voltage...I think you just have to crunch the numbers (you also need to know the resistance of your cable btw)...that said, my own preference would be to get fat cable rather than have extra electronics.

Google "DC DC convertors" for lots of options including circuits to make your own.

timbo
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martin
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« Reply #2 on: May 29, 2009, 04:17:20 PM »

The dog's danglies - http://www.alfatronix.co.uk/ - UK company, top notch products - really nice to deal with too! Wink
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stephend
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« Reply #3 on: May 29, 2009, 04:29:26 PM »

The dog's danglies - http://www.alfatronix.co.uk/ - UK company, top notch products - really nice to deal with too! Wink

Interesting, their 48vdc -> 12vdc is between 85-90% efficient.  And their AC -> DC12V is 85% efficient, so really I'd only be saving on the losses through the inverter if I went AC between batteries and house.  ...and I'd save on some thick DC wiring too.  Will have to work out cable sections as it may be cheaper to invest in more solar panels to cover the inverter losses, than to stick do DC and buy thicker cabling.
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timbo
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« Reply #4 on: May 29, 2009, 04:40:43 PM »

The dog's danglies - http://www.alfatronix.co.uk/ - UK company, top notch products - really nice to deal with too! Wink

Interesting, their 48vdc -> 12vdc is between 85-90% efficient.  And their AC -> DC12V is 85% efficient, so really I'd only be saving on the losses through the inverter if I went AC between batteries and house.  ...and I'd save on some thick DC wiring too.  Will have to work out cable sections as it may be cheaper to invest in more solar panels to cover the inverter losses, than to stick do DC and buy thicker cabling.


Many people quote efficiency at or near maximum output power ....running at well under maximum (which I would recommend) you may find that efficiency %age is quite a bit worse.

This is because some of the power losses in a switcher are related to power output but others are fixed regardless of the output level. It could go either way.

Worth a careful read of everyones datasheet.
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rob26440
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« Reply #5 on: May 29, 2009, 05:43:17 PM »

Quote
I'm planning on a 48V battery bank

Might be a daft question, but what's stopping you from having a 24v battery bank if you are still at the planning stage?
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stephend
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« Reply #6 on: May 29, 2009, 05:53:57 PM »

The size of the system at 900Ah batteries and almost 3kW PV would make 48V a better choice.  And the inverter/charger I want to use only comes in a 48V version.
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johnrae
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« Reply #7 on: May 29, 2009, 08:34:35 PM »

If the 24 volt loads are generally motors and the like then you don't need a convertor.  Simply get something like a 4QD motor controller designed for 48 volt supply.  This can then be set to provide a 24 volt output using pulse width modulation.   Efficiency of their units are extremely high and they are rated to at least 300 amps - and they are reasonably priced.  See their website 4qd.co.uk for loads of info on their devices and control of DC volts.  No connection, just a very happy customer

Jack
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knighty
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« Reply #8 on: May 30, 2009, 12:35:34 AM »

ahhhh.... why not cut yourself twoi 24v lines out of the battery bank ?


easy to do.... take a begativ from your 48v negative conection, and a positive connection from half way through....
this half way through point is also the negative for the second 24v line, which uses the 48v positive for your second 24v positive...

in essance.... you're treating it as two 24v banks that are charged by 48v

ok... the down side is that if you run one of then 24v banks right down... you'll also lose some of your 48v power.... but as long as you pretty much balance out your loads on the 24v lines you shouldn't have a problem.... advantace is.... NO conversion losses !


thinking about it some more.... I'm sure it wouldn't be too hard to have some sort of change over switch.... one made to switch between two 24v systems when the power one of them starts to drop ?

or.... I don;t see why it wouldn't be possible to join the two 24v lines together using some diodes or something like that ?  - I don;t really have any experience in this area... but it sounds pretty simple to me.... would mean you'd take an even load from each 24v bank... :-)
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martin
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« Reply #9 on: May 30, 2009, 08:26:35 AM »

probably NOT a good move - classic advice is not to mix batteries of different types/capacities or states of charge in the same bank, it is the precursor of some or all of them knackering out fast, particularly if you "run some right down" Wink
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knighty
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« Reply #10 on: May 30, 2009, 02:29:55 PM »

I think the only problem would be if you ran one of the 24v banks down.... and then triend to run the 48v bank down even further....

if somebody in the know knows how to combine the two 24v banks, and still charge them / run the invertowrs from 48v then this wouldn't be a problem... as all the batteries would be in the same condition all the time ?
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Justme
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« Reply #11 on: May 30, 2009, 02:41:38 PM »

Dont do it.
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scott
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« Reply #12 on: July 02, 2009, 10:54:51 PM »

use 24vdc battery and double the Ah ratings
to get required storage
24vdc battery systems are made to over 6000ah
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