Micro CoGen.

what i have is a 48 volt dc battery bank and charging system, along with all the inverter i need for AC production

i also have a 12volt dc buss, for all my 12volt loads

what i need is a source of 24volt dc, and would like a dc/dc converter to drop the 48volts to 24 efficiently to run
some intermittent loads such as a water transfer pump.

my question

could one build a converter that takes the 48vdc, and powers a mosfet driver and use pwm control to get down to ~24vdc
and have good efficiency?  i don't need to boost amps so to speak, and i think i only need maybe 20amps max output at 24vdc
perhaps even half that would be sufficient for my needs.

failing that i could setup a third buss for 24volt loads, which really is not a bad thing, in that i can use the better used batteries out
of the 48 volt string to make up a suitable 24 volt string as time goes along, effectively getting full use of the batteries that make up
the 48volt string.. we all know how it goes with batteries, at some point they begin to fail or loose capacity, and one finds himself with
half of better of the string still usable but can't because it is not a good thing to add in a new battery to an old string.

so getting back to the dc/dc 48/24 converter, perhaps it should also be regulated to charge the 24volt bank as well?
more complication ?

thoughts?

this is just something rattling around in my head, and haven't really explored my options yet, but figured someone here might have already
addressed this issue and has worked out a viable solution

bob g

I think your idea of using a buck converter for the intermittent 24VDC load (20A) is a good one.

Rolling your own buck converter might be a bit much, especially starting at 500 watts.  While simple in concept, in practice they can be a glitchy nightmare.  You would want a bit more experience in power electronics than you have, unless you are willing to spend a lot of time on learning curve and are willing to look at it as an ambitious learning experience.

You have a lot of other stuff to build, I would think buying the 48v-24V buck converter would be wise, if you can find one at a non-industrial price.

Found you one that might meet your requirements:

http://www.zahninc.com/sd11.html

model:  
DCDC10028-24  $305

28 amps output, 24V  

Here's one for $188.  48-24V, 500watt
http://www.trcelectronics.com/Meanwell/sd-500l-24.shtml

Some other sources:

http://www.solarconverters.com/eqlist.html   

http://store.solar-electric.com/12to24or24to.html reliable vendor

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=454-1190-ND  roll your own, I have PCB layout and parts list if you want.

Those are interesting sources, Mike, but none are 48V to 24V at 20 amps, which is what Bob wanted.
Or did I miss something?

the first link has a 24 to 48 that works in either direction, up or down
which looks like a nice fit

bob g

i think i found my solution

the linked pdf file gives the spec's on this unit

http://cgi.ebay.com/24-VOLT-DC-POWER-SUPPLY-ASTRODYNE_W0QQitemZ140373531785QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item20aeea1c89

i can't even think about building anything for this price, even though i have to feed it with my inverter AC output
it will suffice for my needs

or at least it looks like it will

bob g

http://www.solarconverters.com/equal3HP.htm

This unit is an up or down voltage converter whose output is proportional to its input. It functions both ways, producing 24 V @ 30 Amps from a 48 V input or can be connected backwards to produce 48 V @ 15 Amps from a 12 V input.  >95 % efficiency
order thru dealers  http://www.solarconverters.com/dealers.html

Thanks Mike,  I've never seen a reversible converter before, and I didn't look close enough.  I couldn't find a price at online dealers for the 24/48V 30 amp unit, but it looks like it will be around $350. or so. 

Bob, That ebay unit is only 13 amps. Is that enough to get by with?  Can't beat that price.

13 amps should be adequate, my pump is rated at 4.7amps at 24volts, so i think it is an option
even if not as efficient as the linked converter.

thanks guys
bob g

Hello
Newbie first post with similar problem - dual 48v banks with possible 24 v loads.  Unless I missed something that eBay item is 96-264 VAC input so would not run from 48vdc bank.  I am still sorting out neutral issue etc. to run 5 1.5kw pure sine inverters with zero crossing input synch from 48vdc bank to get 120/240 vac.  Not off grid nor grid tie just concerned. 

Dick

ProfG- I think no one has responded as your comment was clearly answered in the text above:

"i can't even think about building anything for this price, even though i have to feed it with my inverter AC output
it will suffice for my needs"

Quote from: PROFG on February 23, 2010, 11:57:54 AM
  I am still sorting out neutral issue etc. to run 5 1.5kw pure sine inverters with zero crossing input synch from 48vdc bank to get 120/240 vac.

If you are thinking of connecting several inverters to create 240VAC, you must look at "Stackable" inverters.  They either have a sync cable, or a common control box, to enable them to stay 180 deg out of phase with each other.   I don't think there is any other way.   
If you need 7.5KW of power, you might consider the Xantrex XW6048.  One box, 6KW, 10 sec of 100% overload capacity (12KW) and runs from 48V. 120/240 split phase output.  They have a XW4548, and you can stack two of them or mix 4548 & 6048 to get capacity you need, but generally the discount price is only about $100 difference between the 2. Also has integral 240vac charger to recharge the battery bank from grid or genset.
http://store.solar-electric.com/nexaxwseinan.html

To Entire Forum
Thanks Bruce, just when you think you read it carefully  - I didn't.  I stand (or sit) chastened.  MBob, in an off forum email, answered me without pointing out my gaff so I ordered couple of the "rectifiers" mentioned elsewhere by Yellowhead.  Pretty sure I can bump up output to 57v - will let forum know how it goes.
Dick

sorry if you got ignored, i assure you it is not you!

i get into obscure stuff most folks don't care to read about, so it is easy to get overlooked.

:)

anyway welcome to the forum, tell us what your working on, or have planned
we can always make use of some fresh meat to chew on around here.

ideas that is, not you literally!  :)

(although i am not sure, there are a few guys here stirring the stew pot, adding taters and carrots, they might have
other ideas)

only joking

bob g

Thank you Bob / Bruce, I do feel welcome.  My reference to off forum was to point out that I would not have known about my gaff without rereading previous posts (which I often do anyway) or having it pointed out .  The "rectifiers" are on the way as I think that is quickest way I can get bulk charge into my 2x48v banks - present charger is one 48v gulf cart type rated 17 A max which I never see because never allow SOC to go really low. Have tested the 5 rack mount 1.5Kw inverters with Dranetz and they do transfer bumpless since inverter runs full time charging battery if line power available (double conversion).  Plan on stacking 2 or 4 for 240vac deep well pump until I can find reliable 48vdc one.  Like to go play with the Yanmar now.