Lost in the Battery Numbers off-grid

System

Hello Again, Been a while since I made my first few posts and since that time have been doing some research and gathering the necessary $$$.

Everything was going along just perfect and I thought I had a fairly good grasp on what was going on, until I ran across an on-line battery bank sizing/wiring tool.
http://www.freesunpower.com/battery_designer.php



After reading a bunch I was hoping to go with a new Xantrex XW4024-120/240-60 inverter/charger. Seemed like it was big enough, had all the features and would allow for detailed monitoring etc. Also from the reading I had done, 24V seemed to be the way to go. The only loads that we will have in the house will be limited for now to a 7cu ft chest freezer 1KWH per day and a bunch of 120v CFL's. The Cabin is wired for standard 120/240V power and appliances. Future needs will grow to include a small energy star fridge and most likely a pressure pump. Right now we get water from a 125Gal tank, so I would have plenty of options for a small pump just to provide pressure... the well is 800ft and any LARGE pump for that would likely just run off the generator to fill the 125 Gallon tank. 5K Genset that is super quiet and plugs directly into the existing pannel, supplying electricity to every circuit we have in the house. 30Amp When the time came, it would power the deep well pump when necessary and charge the bank if we go to long without PV charging.

Ok, so all that being said, I figured 24V was the way to go with the inverter and would allow the cabin to grow over time and include other supplemental power sources like Wind and possibly even micro-hydro along with the PV. Next came some decisions on the Battery bank. I was hoping to start with 200 useable amps in the bank (so really thats a 1000Ah bank with only the top 20% being used).

This is where things went awry....

Option 1, the 24V inverter:
According to the tool listed above (click 6v, 350ah, 24v, 1050) and it shows a diagram of 3 groups of 4 (6V) batteries in series @ 350AH which produce a 24 V bank with total storage of 1050 Amp Hours).
So, thats 12 batteries

Note: According to this wiring, the only option to grow is to add another set of 4 (6V) batteries for a total storage of 1400 Amp Hours) after that, looks like you are pretty much done.

Option 2: The 12V inverter, (click on 12V, 210Amp hours, 12V, and 1680 Amp Hours storage).... This is where I REALLY got lost. This diagram shows a total of 8 batteries in parallel @210Amp hours each to produce a 12V bank with a total storage of 1680 Amp Hours. The other advantage it looks like is that this config can expand to 2520 and 3360 amp hours respectively by adding one or two more sets of 4 batteries

I am now officially lost... Why would I want a 24V system with 12 * 6V batteries that appears to have limited expansion vs a 12V setup that uses only 8 * 12V batteries and provides 50% more storage.

Where does the advantage of a big 24V Inverter like the Xantrex mentioned above come in? Seems to me the 12V batteries like say... the SunXtender AGM's from Concord like the PVX-2120L's at 212AH and 12 volts are pretty reasonable... 8 of those would be approximately $3600.00 and provide a bank with 1680AH (360 usable).

Am I all wrong? Have I incorrectly assumed that 24V and such a high end inverter was NOT the right way to go?

Any advise or direction is greatly appreciated. I have not purchased any hardware yet, so there is nothing lost by changing the plan now.

Thank You

mike95490

Re: Lost in the Battery Numbers off-grid

I'm partial to 48V banks, if you have PV arrays at over 65V on a hot day. (equalize voltage)
You can series batteries much more reliably than you can parallel them. (parallel, even with diagonal wiring, does not always share all the current to all the batteries equally)
48V is also half the amps that 24V is, and so therefor, half the copper losses (wire resistance)
You can use the same 6V batteries, just by hooking up the wires differently, you get the benefits. Most large inverters & charge controllers have 48V as an option.

The only "gotcha" can be the local inspector, looking at a 48V bank, will not realize that while charging and maybe equalizing, you will have 60V floating around there. Some will take voltage, and apply 125% and flunk you. Maybe float a 48V system for approval at the plan check desk first.

Also, of note, the "diagrams" at the battery sizing tool site. They show ONE way to wire the batteries. Connecting on the "diagonal" will extend the life of the batteries by helping share the loads.
Here's a great writeup on how to properly wire your battery bank (diagonally)
http://www.smartgauge.co.uk/batt_con.html

System2

Re: Lost in the Battery Numbers off-grid

I do like the idea of using a sort of bus bar to connect the multiple batteries and have read that it has advantages. Equal load etc. In fact, ran across someones page who did just that. I can definitly see the advantage to that part. http://www.wagonmaker.com/newbatt.html

What really gets me is the AH capacity and the number of batteries required. Sure, I'd probabably be fine with a 1000-1200 AH bank to start, but looking at the configurations, to get a really large storage capacity would fill the basement with expensive 6v's


The 12V system seems (at least on the surface) to offer both the least number of batteries/interconnects as well as offering the highest possible AH bank size and to boot it seems MUCH cheaper. Guess im back to square one in the 12V vs 24V question.

All in all, I would rather have extended run times on the bank, vs the rapid charging by a huge array. Our time spent at the location is only on weekends. So having two days of available power on weekends leaves the system to replenish itself during the week (at its leisure). That being said, the fewer batteries required the better. Conditions are not optimal as far as temp goes (in winter) nor would it be easy to perform monthly maint which is why I was focusing on the AGM's. The cost of either does not bother me.... but when looking at the configs.

$3,600.00 to have a 24V bank with at least 12 6V to give a bank with 1000AH (all estimates)

vs

$3,600.00 to have a 12V bank with 8 12V to give a bank with 1680AH (estimated)

Seemed like a no brainer. 50% more storage and 1/3 less batteries for all about the same $$$. Thats where I am struggling.

Appreciate the info you sent on the wiring... it does help.

Solar Guppy

Re: Lost in the Battery Numbers off-grid

I think your missing the part of how many watts are in the battery bank

Your 12V ah and 24 ah are not the same value in total watts and thats where I think your losing it.

1000AH * 24V = 24kWh
1680AH * 12V = 20kWh

In general, you want a higher voltage for sure. I'd suggest 48V also.

I have 440 amps on my XW-6048, that two stings of 6V, 220 ah AGM's for about 21kWh of storage and cost about 2400.00 for the battery from a local battery shop

System2

Re: Lost in the Battery Numbers off-grid

Thank You... That may be exactly what my problem is... I did not do any conversion on the 12v vs the 24v, so I was simply looking at total amp hours, regardless of voltage.

So all that being true, a 48V system with 1000AH bank is really 48kWH of storage?

Using this batt as an example, Concorde PVX-12255 (8D) 255 amp-hour, 12 volts (4 of these cost about $2,200.00 maybe a more given shipping etc.)

Tieing 4 of those together gives 1020AH and 48 volts for 48960 watts, and further, taking only the top 20% its really 9760 usable (under ideal conditions). Or put another way... running a 1KWH chest freezer for almost 10 days without dipping below an 80% charge.

Is that basically the "high level view" ?

System2

Re: Lost in the Battery Numbers off-grid

Oh, BTW, I don't mind at all stepping up the Xantrex to the 48V model, just was REALLY confussed about where the advantages were. Sorry, may seem like a very basic thing but I've been looking at numbers for a week and apparently missed a KEY conversion of the V numbers while in that process. Since nothing has been purchased, I'm really happy have knowlegable folks out there who have dealt with this stuff for a while.

Its greatly appreciated.
Thanks

mike95490

Re: Lost in the Battery Numbers off-grid

roadrash1970 wrote: »

So all that being true, a 48V system with 1000AH bank is really 48kWH of storage?

Ding ding ding !!! A winner.

As your voltage goes up, your amps go down, and wire size gets smaller. (for the same watts)

Start with 4, 12V 100A batteries.

series - 48V, 100A (high voltage, lo amps)

parallel - 12V 400A (hi amps, lo voltage) but same KWH, and heavier wires. (400A)


Yes. wiring to a common BUSS bar is good, but each wire needs to now be a matched length, or else you get un-equal currents again.