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DC to DC upconverter

DC to DC upconverter

DC to DC upconverter

(OP)
For reasons that make zero sense to me my new house has 2 solar power systems. The main one is nominally 48V, and feeds a 3 kW inverter at a bus voltage of 48-54 V approximately, depending on SOC and power draw.

The second is a rather uncontrolled 12V system with some huge ancient accumulators. Its voltage sits at around 12.2-14V, and at the moment it does nothing useful. It is about half the size of the other system, so if I could switch it in it would extend the main system's battery life considerably.

So, I'd like to buy or build a 12V(ish) to 50V(ish) DC to DC upconverter. Max current would be say 40A.

Any ideas?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: DC to DC upconverter

Charge in parallel at 12 discharge in series at 48?

RE: DC to DC upconverter

(OP)
OK, it gets worse.

The 12V system comprises 6x2V glass acumulators in series, and 8 12V sealed lead acids, in parallel. So the only way I can make 48V will make no use of the glass accumulators


This is the sort of thing I was thinking of, and frankly at that price it is cheaper to do than think about.

http://www.ebay.com/itm/Boost-DC-DC-Converter-Powe...

that's only 150W, I just bought 3

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: DC to DC upconverter

(OP)
Thanks, that was one of my concerns, and for the sake of 3-10 watts and a dollar a pop for even the most over specified ones it seems a good precaution. Obviously my main criterion is looking after the inverter, lead acids are nigh bulletproof, and the PV cells are pretty safe.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: DC to DC upconverter

(OP)
Incidentally MintJulep, I wouldn't want to reconfigure the batteries on the fly as the biggest load actually occurs during the day when the irrigation pump kicks in. It draws about 30 A from the bus, of which 10 or 15 come straight from the panel typically, so leaving 15-20A which is not really abusing the 400Ah batteries at the moment, but every little bit helps extend their life. Anyway, a hundred buck solution is nothing for a 50% increase in battery capacity, now let's see if it works!

I do have the option of switching an immersion heater on which would gain me a lot of brownie points, that'd draw 60A for hours at a time, that's just pushing everything too hard. So my other job is to find the /least/ powerful standard immersion heater element.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: DC to DC upconverter

Greg,
It sounds like you inherited somebody's "starter-kit" along with their "grown up" system. Are you convinced the old batteries themselves are worth using? Mating the banks together implies they have some compatibility. The fact that the previous owner didn't do so hints that they are not. You should confirm that all of these batteries have the same chemistry and construction. Flooded lead-acid is not the same as gel-cell lead acid, nor the same as valve regulated lead acid. Each requires different voltage setpoints on the charge regulator. Error = acid puddles. If you introduce a DC converter, then the parallel battery bank(s) will not be regulated according to the charge controller's program, but rather to some variable factor of the program, that will drift around 1/4 dependent on temperature of the room, temperature of the cells (not the same) and the steadiness of the DC converter's own setpoints. And who's to say that once the main bank has completed its bulk charge cycle that the little bank is ready for float charge, too?

Maybe with a lot of effort it can be forced to work.

What are the Amp-hour capacities of these banks? (estimate what you can for the glass cells, I have no help for you there). Can the array of 12V panels be re-arranged to complement the 48V array? Are these grid-tie solar panels or are they designed for battery charge? What charge controller is running the show on the 48V battery bank?

STF

RE: DC to DC upconverter

Greg,
Are the large glass accumulators open top style, contained in a " Battery room". If they are, they may be a relic of an even older "Generator set"
B.E.

You are judged not by what you know, but by what you can do.

RE: DC to DC upconverter

Why not just put a second inverter on the old system? They don't cost much as long as you don't need a true sine wave.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: DC to DC upconverter

(OP)
dgallup - I'd have to set up a separate 240v system in the house if I had a second inverter. That's a major project as the cables are buried between the solar shed and the house.

The 6x2V glass accumulators are old telecom ones, 560Ah, which were in vogue 20 years ago for home solar. The 8 SLAs are 40Ah each, giving me 12V and (560+320) Ah. The main battery system is 2 strings of 200 Ah at 48V.

So far as I can tell this is the third major iteration of solar power in the house, which is 30 years old. We ripped out the original pV controller/inverter setup before we moved in, and there is another old one in the shed. As such, yes there is an accumulation of bits and pieces.

I agree it is a nasty mess, but like I say for a hundred bucks (the price of one SLA) I don't have to see much improvement to be ahead.

Sparweb, yes, I think splitting the two types of battery into two strings with separate pV might be a better approach. There's several alternatives there. I'll have a ponder.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: DC to DC upconverter

Quote:

... there is another old one in the shed...

I've been running an "old" system for a few years now. Do you mind quoting a few manufacturer names and models so that I can guestimate if it's still worth anything? There is quality RE equipment from the 1990's still running, showing no sign of quitting, and still of use in systems like mine. You don't have to throw it away just because it's old (perhaps why the previous owner kept those glass cells).

It is possible to "slave" an inverter that is in phase with another. This requires some trickery if the equipment is old, or a lot of trickery if the inverters aren't identical. I can't remember if you're in the UK or USA. I can only talk about 240V split phase at 60Hz, if that's what you have.

I get away with the things I do because I put my system to use powering out-buildings, barns and keeping vehicle batteries at float, etc, not as a grid-tie in the house. But it's always ready to go in case of an outage, as a backup. You may be able to employ a similar philosophy, taking advantage of the extra batteries and panels to supply power to a different building that doesn't have it (greenhouse, garage, shed, whatever). Depending on how much effort you want to put into wiring them, of course.

STF

RE: DC to DC upconverter

(OP)
Testing the upconverters was more interesting than testing the immersion heater, my 12v->48V converters are little demons, happy to pull 80A from the 12V lead acids, which causes the solar controller to bomb out, as it thinks there is a load fault. Ultimately I'll have to switch via a relay rather than the solar converter if I really want to pull big numbers . So this weekend I'm going to measure their negative output conductance (d Iout/ d Vout) and try and figure out how to set them to feed the main battery bus. Caution suggests setting them to say 48V but then nothing much will happen most of the time, I'm inclined to use a little more, but with the caveat that I don't want to upset the main battery's voltage cycling. I'll have a read of the manual, if push comes to shove I may have to disable the 12V system occasionally, and run the 48V battery through a 4 day conditioning cycle. Having said that, at the moment, at C10 discharge the 48V strings are nicely balanced, I didn't even writing down the individual battery voltages they were so close.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: DC to DC upconverter

I'm listening, reading, learning. Thank you.

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