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Old 10-18-2017, 04:07 PM   #1
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Input on DC system thoughts?

Hello Everyone,

As I putter aimlessly on my systems details I am now pondering DC loads. As I am planning on something in the neighborhood of 1.5-2kw of solar I will definitely be running a 24v house battery.

How am I going to support 12 volt loads? I see a few options:

1) 24v to 12v converter (no 12v battery) rated a bit higher than my largest expected 12v load.

2) 24v-12v battery charger and a (t1275) battery.

3) Renogy 20a MPPT charge controller charging a battery (t1275) from the 24v house battery.

4) Run 12v lighting (led) in series pairs so that I can power them directly from 24v.

Option 4 would not be a stand alone solution as I will have loads that require 12v.

Options 2&3 offer the most flexibility as far as larger loads go.

Option 3 is the least expensive.

I am thinking a combination of 3&4. Configure all of my 12v lighting in series pairs so they will run from 24v and run my other 12v loads from the single 12v battery being charged by the MPPT controller.

Do you all see any challenges with my ideas? Any better ideas?

Thanks

S.
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Old 10-18-2017, 05:07 PM   #2
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Looks like a good set of alternatives you're considering. The "have a 12 volt sub-system" philosophy is common to the first three, with the variable being how that system will be energized. One is batteryless. Two concerns I can think of, if they rise even to that level, are these:
  • what is the step response of the dc-dc converter when you turn on a new load. There might be a droop on the 12v rail which might be visible as flicker in lighting or even audible in an audio system. I suppose the overshoot when a load is turned off would be comparatively less.
  • what happens when a load with a converter at its input is powered from this converter-supplied 12v rail. I haven't studied this sort of thing at all; probably results (good or bad) depend on the nature of the specific converters.
For option 4 (loads such as 12v lighting) wired in series pairs, good balancing of the power consumption of the devices would be important. The current through devices in series be the same, so the voltage across the devices will divide in proportion to their power consumption. Single LEDs aren't necessarily exactly equal in their power consumption, but two equal-length sections of the cheap LED lighting tape with many discrete LEDs probably would average out to be fairly equal (they're kind of crummy for efficiency though because so much power is dissipated in current-limiting resistors). If the voltage didn't divide evenly then the inherent slight variation in light output from one LED to another could be exaggerated even further. If they're near one another such variation would be really obvious visually, but if they lit opposite sides of the bus (for example) the eye would be less likely to notice the difference in brightness.


This probably comes across as being pessimistic on the 24v idea. I don't intend it that way at all; I think it makes a lot of sense to run a higher-than-12v house battery. I'd like to do it too. If you're willing to hack at things a bit you could make it work pretty well. The not-so-efficient but really cheap LED tapes, for example, come off the shelf as series-parallel combinations of LEDs and resistors. I've thought about modifying one to delete the resistors and make longer series chains, then power it from a higher voltage supply with a current-limiting LED driver circuit. I haven't yet worked through the idea enough to figure out how much it would save in terms of heat produced nor how much work it would be to modify the LED tape.
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Old 10-18-2017, 06:15 PM   #3
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Thanks for the thoughtful reply Family Wagon,

I agree with your concerns regarding how the converter will deal with varying loads. I think it would be be noticable but not sure it would be a problem.

Regarding pairing 12v loads in series for 24v operation, I would be using identical loads in pairs. Two each of identical lights.

Any thoughts about using the MPPT controller to maintain a 12v battery from the 24v bank?
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Old 10-19-2017, 08:16 AM   #4
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Hi Steve, your probably familiar with my 24 volt setup but just in case...

I went with #1.

I invested heavily in my 24V house battery and wanted all of it's power available to the entire house (so didn't want a separate 12V battery). I tried to purchase as many 24V components as possible (water pump and such) but still ended up with many 12V components (lights, fans, window & shade motors, CPAP machine, etc.). I elected to go with multiple 24-12V DC converters. Currently there are two of them, eventually three. They are each 30 amp units strategically located to provide power to a 12V fuse block/panel that powers "local" components. I haven't done any analysis but I've never noticed anything odd from them (power fluctuations or ???). The thing I should do, but haven't, is measure their power consumption in a no load state.

Oh.. should mention. The reason for multiple converters was using smaller wire for the long runs (@24volts). It is a tiny difference but makes me feel good. If I had spent more time deciding exactly what would be powered by each one, I could have probably selected lower power units.
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Old 10-24-2017, 10:05 PM   #5
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I am going with a multi-layered approach

220V for Oven, Hob, Electric heaters/ roof A/C
120V for microwave, dishwasher
24V for solar transfer efficiency
12V , just because ??
5V USB- charging phones, etc.


As usual, I am coming from the military angle. They employ 24 V systems for majority of their systems. You can get many accessories that are extremely well-built & quiet for reasonable in surplus. Impact guns to ventilation fans to lighting to fuel pumps. Packard connections rule here.

I use the PSWNV720 Pyle step-downs for the 12V supply- 85 % efficiency, .2 amp no-load draw, so a few rocker switches to kill when not in use help. $37/ebay delivered

Screen Shot 2017-10-24 at 8.41.11 PM.png

Many LED lights are rated up to 30V, so no need to convert from the 24V.

USB DC/DC 12V to 5V 3A 15W Dual Female Panel Mount USB Step Down Module L1I9 $4/delivered from China, again, rocker switch when not in use.

usb.jpg

DC motors are voltage dependent, so a 24V DC motor just spins slower at 12V.
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