Older 12 volt system upgrade ideas

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KU9L
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Older 12 volt system upgrade ideas

Post by KU9L »

I greatly appreciate all of the varied roles that Outback has the adaptability to mold itself to. I'm still working on a plan for a larger 12-volt cabin upgrade for an off-grid passive solar homestead we are buying in the near future. While 12 volts is obsolete in many respects for a home power setup, it still has its micro-niche where applicable.

In our case, the solar home has a 12 volt 2,000 A-H battery bank with an old Trace SB2512 modified Square wave inverter, Trace C-60 charge controller, a collection of old Arco PV modules totaling 735 watts, and an assortment of Square D breaker boxes for AC & DC loads.

While this system served one person well for eight years, it is clear that it needs upgrading to serve a family of three with slightly different needs. My first thought was to go 24 or 48 volts, however there are a number of things making that option very costly & not the best choice including:

1) The 2000 A-H battery bank is 2 years old, in excellent condition, and is a collection of 2-volt cells with a 20-year expected lifespan (owner worked at an off-grid commercial FM radio station). There is not going to be enough room in the battery room to double the bank size for certain(!).

2) All of the lighting is 12 volts already. The previous owner went to great lengths to double-wire the house and did a great job of properly sizing the DC wires (inspected by an ÔÇ£impossible to pleaseÔÇØ inspector) and the owner has about $1,200 invested in very high quality 12 volt DC CF, halogen, & DC halogen track lighting already. While it would be possible to replace all of the 30+ DC lamps with the AC versions and move the wires from the DC breaker box to the AC box, I'm thinking this should be a last resort due to the cost & waste involved as the lighting functions very well as it already is).

3) Every outlet in the house is wired as a Quad, 2 AC & 2 DC outlets (horizontal spades on the DC side). The water pressurizing pump, the solar hot water pump, the refrigerator compressor, the laptop computer(s), stereo, the TV's with built in DVD, etc. are all 12 volt DC loads and make no sense to replace them with AC or even consider buying a 24 volt DC version.

4) The AC loads are fairly small, a washing machine, a 600 watt microwave, a blender, a kitchen-aide mixer, one AC porch light (required by local code), a small vacuum cleaner, & a hair dryer, etc.

5) Then my own added loads require 12 volts in the Ham Shack where I plan to recycle my old Solpan Power Center with 200 amp pull-ring fuse disconnect (as a load center) and Square D breaker box to run my medium and high power HF/VHF/UHF ham radios & amplifier loads. The ham shack is right next to the power room & battery box where 4/0 cable would run about 10 feet maximum to the old Solpan. These really big loads are much more efficient when left at 12 volt DC instead of converting them from AC through a real big inverter or through a 24 volt to 12 volt regulator (very expensive!), etc (being I'll need peak current draws up to 100 amps(+) at 12 volts sometimes, etc).

6) The Air-403 wind generator is now 12 volts, providing about 30 - 50 KWh/mo to provide supplemental charging and would have to be replaced or at least figured in to the new scheme in the case of the battery bank being replaced and the Air-403 wired in to the split battery bank (with a line balancer).

After doing a detailed load budget, we came up with about 3 KWh per day for our family of 3, which is on the shy side of output current from the PV array (mainly in winter) considering system losses. The PV array, while fairly large for a 12-volt system, consists mainly of the older Arco 5.5 volt modules. When 'hot', three modules in series produces only 14.4 volts, which isn't enough overhead voltage to fully charge the large battery bank on hot summer days considering the DC wiring losses and voltage drop through the Trace C-60.

This is where I figure the MX60 comes in big time with MPPT. The problem was that I had an odd number of modules, 21 to be exact for 7 strings at 12 volts nominal and couldn't just cut it in half for 24 volts. Then I realized the MX60 could take any odd voltage input and efficiently convert it to 12 volts, so I came up with three strings of 7 modules in series (7x5.5=38.5 volts) that would be a win-win scenario at least to get us started toward making some PV current at all temperatures.

Then there was the nasty BUZZ from the Trace SB2512, plus the crappy power factor & THD that the washing machine, microwave, & my power tools see & complain about. I also couldn't run my cordless power tool chargers (although I have the 12 volt DC versions too, it would be nice to run them both at the same time during heavy power tool use). I figure the VFX2812 will be the perfect drop-in replacement for the old SB2512.

My next choice is to determine whether to just swap the raw components out or to consider also a PS2. The existing setup is a collection of hardware mounted to plywood that serves fairly well, but doesn't have the organization and neatness of a power center. While I don't have a need for 220 volts AC & other fancy features, it would be nice to have the extra 250 amp breaker in the missing 2nd inverter breaker slot as a load disconnect going to the Solpan load center in the ham radio shack, etc. The decision to go with the PS2DC in this case is a tough call as it is waaaayyy short of extra DC breaker space to handle the DC loads for the whole house the way it is wired now, yet it makes sense to use the PS2DC for the Air-403 input, the PV input, the shunt, & the safety features, etc. while also feeding an external Square D DC load center box, etc. It is a tough call, yet I like the fact that should I upgrade to the PS2, I still have the option of going to 24 volts in the future by trading in only the 12 volt inverter or using it elsewhere (so I am not out anything actually).

The main question I have about the PS2 is expansion capability. The schematic drawing shows two MX60 charge controllers, but the PS2 photo's only show one actually hardwired. The MX60 manual says it is only okay to block one vent, but not both sides, indicating that two MX60's can't be located side by side connected to each other on the PS2DC. Maybe the 2nd MX60 can go on top of the PS2DC(?), however neither the PS2, nor the MX60 literature says anything about this.

My ultimate goal is to move the old Arco 735 watt PV array down to a future garage PV system and replace it with a 1420 watt array on a Wattsun tracker wired for 24 or 48 volts, which is the full output of two MX60's when down-converted to 12 volts nominal (and output adjusted to 65 - 70 amps with the appropriate PS2DC breaker for MX60 output). We would do this further upgrade to provide power to a chest freezer we plan on adding later in addition to some other small loads we'll be adding at some point.

As one can see, all PV installations are very unique and what works for one setup certainly doesn't work for another. My friends keep arguing for me to go 48 volts, yet when I add up the total cost to make that transition, I still have a lot of 12 volt loads I need to power, so in this one case it makes sense to stay at 12 volts considering how all of the existing loads & appliances are set up, etc. It does of course make sense to raise the PV input voltage for certain. The great thing is that no matter which direction I am choosing to go, the Outback products seem to have the flexibility to meet those needs.

This turned into a full-blown article instead of a quick question, but does anyone have any comments on what I may have overlooked or not considered? I've spent a lot of time studying this scenario in great detail and it is great to see that Outback has the flexible equipment to make most any situation work and work well. It is clear that the PS2 was not designed for two VFX2812 inverters while running an existing 500 amp shunt, mainly because the 500 amp shunt shouldn't be allowed to run anywhere close to the full output of these two wonderful inverter appliances. However, in my case the 2nd inverter is surely not needed and the 2nd extra breaker slot already has a happy home to service the high current 12 volt office & ham radio loads ... a win-win situation for sure.

Thanks,

Dave Knapp - KU9L (soon to be in G.S. DN60)
Robin Gudgel

Older 12 volt system upgrade ideas

Post by Robin Gudgel »

Dave,
I would like to commend you for your research. You should work for us!
The VFX2812 should work very well in this application. The PS2 will also work well, although as you found, it does not have enough DC breaker slots for your application. The 500 amp shunt is actually adequate for this application for two reasons. If two 12v inverters were installed in series or in parallel, the current draw for them will rarely reach 500 amps except for transient conditions. You just won't be running that much power on a continuous basis. We run 500 amps through those shunts continuous although they get hot. The other thing in your favor is the fact that you could also be using the DC12-fan kit. This not only blows air over the inverters, but also the shunt. It is normally not required for vented inverters, but there is no reason it can't be added. Although the VFX2812 does not have the same surge capacity as the 24 and 48 volt units we have found it to be impressive. When running 2000 watts of lights, it is still able to start and run a 13,500 BTU air conditioner. We are pretty sure it will be able to hold its own with the Trace U2512.
Give it a whirl.
Robin
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KU9L
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Older 12 volt system upgrade ideas

Post by KU9L »

Robin,
Thanks for the flowers, I pride myself in becoming an educated consumer, but as a test equipment engineer working in aerospace, knowledge of details pay off in health & safety as well as a pretty face. ;-) Many of the aerospace inverters I fabricate test equipment for run at 28 volts DC, 270 volts DC, and even 400 Hz & 1200 Hz AC, from 20KVA backup systems to 90KVA main systems on military stuff. As much as I love electronics, there is nothing like the happy energy I feel when attending a hands-on renewable energy weekend workshop with Richard Perez or one of the many energy fairs we attend ... there is no comparison, ha.

Thanks for the heads-up on the DC12-fan kit, I hadn't thought of it and that should help ease my mind when it comes to it.

I'm sure I will be blown away by the surge capacity of the VFX2812 compared to the little RV size PowerStar I now power shop vacs & tablesaws on, ha. Motors on seem to run at about 80% speed with those digital waveforms, so my appliances will be much happier with more power with the Outback inverters. I will design our shop PV system for 48 volts from the very start though!

Also, someone pointed out to me that if I have room in the power room of the house to install a full size PSDC if it will fit. There just might be enough breaker combinations to fit my "drop in place" needs and it will nicely clean up the appearance of so many wires and small boxes.

Someone asked me why I don't become a PV installer and I suppose it is because I live in an area where there are few PV systems in the middle of the nuclear grid country in the Midwest. I'd rather spend hours tinkering with designing my own load budgets & installing my own PV equipment rather than trying to scrape out a basic living as a part time salesmen, part time electrician. I'd rather be settling an example rather than pushing hardware just for a paycheck. I do however seem to "give away" a lot of my personal free time on helping friends troubleshoot their own PV systems remotely, a skill I seem to be quite good at. Who knows,living in remote NW Colorado might find me in dense enough PV territory to keep me quite busy at it? Of course working for myself for low pay in PV is still much more exciting that being downsized in aerospace! ;-)

Dave
Christopher

Older 12 volt system upgrade ideas

Post by Christopher »

---can you mix voltages like this and still stack inverters?

Yes - we did consider this in the design - and it does work... but... its pretty odd. Keep in mind that the batteries will discharge at different rates.

The battery chargers will not work optimally however - in fact there is a chance that the battery on the SLAVE inverter will overcharge since it is really controlled by the MASTER
inverter. There are ways to solve this issue though I beleive.

So - consider it a strong "yah, but..." and lets talk more about it before you try it.
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Older 12 volt system upgrade ideas

Post by wd8cdh »

Hi Dave,

I too have a large 12V system. I have 6 3000AH 2 volt cells. I primarily use the system for grid failure backup since I also have a grid tied inverter for sell-back.

I don't know if this is possable but could a 12 volt FX be stacked with a 24 or 48 volt FX? The 12V inverter would stay on the present battery bank and panels with a MX-60 feeding it and the other FX would be on a new battery bank with it's own array and MX-60.

Robin----can you mix voltages like this and still stack inverters?
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