Battery bank sizing

Re: Battery bank sizing

Hi Cariboocoot,

First, I realize that working backwards was not ideal, I totally get that. But I bought all 6 of the Kyocera 135 watt panels for $200 each at an auction here in the desert and got the Xantrex Prosine 2.0 for $50 from a guy on Craigs list who thought it was broken. It wasn't. I hooked it up to a 12 volt deep cell marine battery and everything checked out. It was too good of a deal to pass up so I grabbed them while I could. But anyway, that's why I had the panels first. And you're correct. It's forcing me to work backwards.

Is this making sense?

Mostly. I just had to read it 13 1/2 times You have given me a lot of good info. Here's my takeaway from what you suggested. And please let me know if I'm on track here in a general sort of way.

It seems to me that using your recommended 450 Amp hours would almost ensure that my batteries would always stay charged using my panel configuration. In other words, limiting myself to 450ah would sort of keep me honest, in a way, by making sure enough power was always incoming from the panels to the batteries. Too many battery amp-hours would strain the charging ability of the panels.

That said, some of my options?:

1) I would need to, roughly, double the amount of panels I have now (which is 810 watt) to effectively use the 1040 amp hours that I originally calculated.

2) Or, I could just install 1040 amp hours of batteries and keep an eye on my charge controller to make sure that the panels keep up with the batts and maybe I'll get lucky and not use as much power as I thought. (Is that the sound of laughter I hear?) The only real heavy duty appliance I have is an AC 1/2hp water pump, and that will be converted to a DC pump someday. But it only runs for 2-4 minutes at a time @ 4-5 times per day to fill up my 80-gal pressure tank. Also, I can control when to fill it up by keeping the AC pump wired to a power strip and only turning it on when all other loads are off. My cooking and refrigeration will be propane, my heating will be a wood stove (not pellet), and my evaporative cooling in the summer is a 12vdc powered "swamp cooler" that only uses about 8 amps. I posted a vid here of that project: https://www.youtube.com/watch?v=gcWIlm1CZQA (Sorry for the shaky video)

3) Or, if I find that my batteries are not being fully charged by my panels, I can supplement my 6 panels with my Honda EU200 generator, correct? My Prosine 2.0 inverter is a charging inverter so this is an option for me.

Yes, I do have lot's of sun and, yes, it does get up to 112-115, but that's only for about 14 weeks. peak, then it's not bad. I live in the "high" desert so we're not like Palms Springs or other areas which get to 120+.

Someone on youtube gave me the good idea of using a broken refrigerator to hold my batteries - http://www.youtube.com/watch?v=dzufK2q9N-c
A larger refrigerator should hold roughly 8 (or less) Trojan 225 ah (T-105) batts. It would be insulated from extreme desert heat, water proof, lockable and in the shade. I can use a hole saw to cut vents in the back for charging ventilation and to route cables to my inverter and CC. I would also stack hay bales around it for added insulation, much like a miniature "Earth Ship" hay-bale enclosure. The temp inside would then much lower than ambient. I think this should work for a hot climate battery box. We'll see.

As far as the panels themselves getting hot and decreasing output there's not much I can do about that except wait out the summer, get more panels, or supplement with the generator. Or maybe I can put them in the shade

Questions for you:

Do some of those options sound ok?

How long does it take a 2000 watt generator to charge 1040 ah of batteries? Assuming the batteries are close to 50% discharge. Is there a formula for that? Do you use your generator to charge your batteries ever?

So working backwards, that 52 Amps from the array could support a maximum of 1040 Amp hours, for which it would be 5%.

What is the 5% part of that statement referring to? 5% of what exactly?

read why parallel batteries cause problems: http://forum.solar-electric.com/showthread.php?14674

There are solutions if you really need that much battery capacity. One solution is to use a higher system voltage... eight of those T-105 in series (48 volt) holds as much energy as eight of them in a series-parallel arrangement (12 or 24 volt). The other solution is to use different batteries with higher ah capacities per cell.

--vtMaps

I will read that link later today. Thanks for the link vtMaps.

I know I have lot's of questions, but my old high school band teacher once said, "Never be afraid to ask stupid questions". He would be so proud of me right now


Thanks for the help.

Hairfarm.

Re: Battery bank sizing

Greetings.

A couple of points first. If you live at high elevation your panels will work a little better than expected. The temperature is still an issue, as both panels and batteries 'generate' heat. They both work best at around 20-25 Celsius, so the higher the ambient the higher the operating temperature. Hyper-insulating the batteries may even work against you, trapping the heat produced by current flow. Sometimes it's tough to get the right balance of ventilation and insulation (just like with a house).

I'll get a thermometer to monitor the heat in the fridge. If it gets too hot, I guess I can continue to "vent" it with a larger hole/s and use hardware cloth to cover it to keep out the rodents. The fridge seemed like a good idea because the batteries can NOT live inside the cabin. I'm not comfortable with that. A neighbor of mine built a house made of hay bails and clay and it stays within a pretty good temp range in both the winter and summer. In the summer he doesn't need AC. His place uses convection cooling too. He also recommended that I stack hay bales around the fridge as well as my 40' container to insulate in the summer as well. The fridge will live on the shady side of the cabin too. In fact it will be situated directly up against the outside of the cabin wall. On the opposite side of that same cabin wall, inside the cabin utility closet, are where my inverter and charge controller will be installed. My batteries should be no more than 2-3 feet from my inverter and charge controller through the cabin wall. In any case, I'll make it a point to keep checking the thermometer during the summer and try to find the "sweet spot" temperature for the batteries. Might even consider a fan that comes on when charging is taking place.

How long does it take a 2kW gen to charge 12kW hours of battery?

What is the formula for calculating kW hours from my system?

The 5% charge rate is the bottom of the range recommended for most batteries: 5% of the 20-hour Amp hour rating. It is often expressed as C/20 where 'C' is the capacity. So 1040 Amp hours has a minimum peak charge rate of 52 Amps.

I'm struggling with that part. Are you saying that with my panel array @ 810 watts is only charging at a 5%? What should my charge rate be ideally? Am I thinking about this correctly? With my panel configuration, 22.89 amps is coming out at 35.4 v.

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If I were you I'd be giving serious thought to the load side of things before buying more batteries, panels, whatever. Expansion isn't easy, but at the same time you don't want to spend more $ than you need to.

Cariboocoot - basically, you've convinced me to pull out my Kill-o-watt and try to get an idea of what I might use at the cabin and test devices here in my home. I know it might be hard to predict 100% what my needs are but it will still give me a better idea of what to expect. Maybe I can try to stop working, as you said earlier...backwards;)

Also, there's another whole set of issues to consider. During the hot weeks, the cabin might be empty for a few days at a time so I'll need to set up timers to keep some loads on the batteries occasionally, because I know that batteries need loads and shouldn't stay "dormat" for long.

Read why parallel batteries cause problems: http://forum.solar-electric.com/showthread.php?14674

There are solutions if you really need that much battery capacity. One solution is to use a higher system voltage... eight of those T-105 in series (48 volt) holds as much energy as eight of them in a series-parallel arrangement (12 or 24 volt). The other solution is to use different batteries with higher ah capacities per cell.

--vtMaps

In regard to vtMaps link that he sent me. Is a string when two batteries are wired in series? If so, then six 225ah batteries wired in three groups of two (12volts) then wired in parallel has three strings in it, right? And from the research I've done it seems that three strings should be the max. And should each string be wired together so that they have a common positive and a common negative wiring point. Instead of a "leap frogged" connection. Will this allow all of the individual batteries to be charged and dis-charged the same. Is that a concern?

And, you should, with parallel strings, have a fuse/breaker per string to prevent the other strings from feeding excessive current into a shorted string.

What would a diagram of that look like?

thanks,

Hairfarm

Re: Battery bank sizing

Whew. I feel like I owe you a consulting fee Cariboocoot I've actually been printing out copies of these posts for my installation in two weeks. I really do appreciate all of the help you, and others, have given. I never really had "math" brain, so it takes while to for this stuff to sink in. Sometimes I wonder if I'm nuts for spending so much money and time to attempt installing this system myself. I know a solar consultant nearby, but he want $75 an hour. And he's kind of an a**. That said, your helpful posts and tons or internet research, and two dvd's have helped.

Simple formula for determining the 5% charge rate: Current * 100 / Amp hour capacity.
So 22.89 * 100 / 1040 = 2.2%
But if you use those panels with an MPPT controller (which you should because their Vmp is much higher than the system Voltage) you will be able to convert much of the 'extra' Voltage (that which is above battery Voltage after system losses) into additional charge current. It looks something like this:
810 Watts * 77% typical efficiency / 12 Volts system nominal = 52 Amps And so:
5200 / 1040 = 5%

I follow all of that except for this: "5200 / 1040 = 5%". Where did you come up with 5200. Did you simply add two zeros to 52 amps from the previous calc?

Smart Gauge's wiring diagrams: http://www.smartgauge.co.uk/batt_con.html (Shows individual batteries; think of a string as a higher Voltage battery in the same spot.)
Method #1; No, no.
Method #2; works best with just two strings.
Method #3; my preference for 3 or four strings.
Method #4; technically feasible, but difficult for most to understand or execute.

I actually was reading that link last night. I found it on another thread discussing why not to parallel more than three strings. I was originally planning to use #2, but from your post it sounds like #3 would be better.

About those panels + breakers. You'd be making panel strings like this:

(-)PANEL(+)---(-)PANEL(+)---BREAKER---(+)

And then the parallel connections are between all the first panel (-)'s and the breaker (+)'s.

Ok. I understand the diagram. And apparently all of my wire needs to be the same length and size too, correct?

I've deiced to downsize my intended battery system, and use 6 Trojan 225ah (675ah) instead 8 like I originally planned. I'll see how that setup works and it will save me more $$ too. They will be wired: 2-in-series, 3 strings, then paralleled which meets the "no-more-than-three-paralleled-strings Rule of Thumb".

Big question: what type of connectors would I need to collect all three positives and all three negative wire using the #3 wiring example in the link you sent me? I've provided a picture and added red circles around the two connection points I'm referring to. In other words, how do I convert the two wires (pos and neg) from the charge controller into 6 wires - 3 for positive and 3 for negative as shown in the image below? Seems like the wires should be installed after the positive fuse for the CC. I can't seem to find any pictures (not diagrams) on line showing a connection like this. Can you please provide a link to a product, if any? Hope that makes sense.

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Wife says I have to get off of the computer now and fix the fence.



Hairfarm

Re: Battery bank sizing

That would give a better charge rate (about 7%). Just be sure you have enough battery for the loads (about 4kW hours stored there).

Are you saying that 675ah (my total ah) translates into 4kW hours?

Well, back to that kill-a-watt device again. I do know we already average roughly 16.03 kWh a day...ugh. But then I have two massive UPS units and other various phantom loads, three desktop computers, etc. Time to really pare down if we're going to try living off grid. My wife and I finally convinced our respective employers to let us both work from home full time. They've finally agreed after months of asking (whining). I put together a proposal for them to show that it could be done. So we plan to live in the cabin during the cooler Fall/Winter/Spring months, and migrate to Big Bear mountain for the 4 months of warmer weather when it's 110 in the desert. The nice thing is that Big Bear town and the location of our cabin are only 1 hour apart from each other, so it's not a problem for me go check up on the PV system once a week or so. Down the mountain, up the mountain.

You run wires from the batteries to common connection points/bus bars. The charge controller and inverter connect to these points. The poor man's way is insulated bolts where the wires come together (mounted firmly in wood, use stainless bolts). It is important to have proper circuit protection on all these circuits. This would ideally be three Blue Sea battery post fuses to protect the runs from each battery string to the common point, a fuse/breaker for the wire from there to the charge controller, and another for the wire to the inverter.

So a charge controller and inverter can share the same buss, huh? I remember I saw a diagram someone had posted who had used buss bars. I suspect I would still need the 4/0 wire to connect my batteries to the buss bar instead of directly to my inverter and then from the buss bar to my inverter I would need two more 4/0 wires, all under 8 feet max (according to NAWS) I presume?

Btw, how does the charge controller know to only feed solar panel power into the batteries (and not the inverter) if they're all three (batts, inverter, and cc) hooked up to a common bus? The buss is just a dumb piece of metal. Why not just go straight from the batteries to the inverter with a 4/0 wire and use the buss to only feed the batteries from the CC? Does that make sense?

I found these updated documents showing the use of a buss bar. I'm pretty sure these have circulated on the forum before.
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It's headache time again.

Just a warning, I'm probably going to keep asking questions until the answers stop coming. But look on the bight side, I'll be the guy that helps push your posting number to 10,000!:D

Re: Battery bank sizing

Thanks for the feedback BB. I will add it to my ever growing information file on pv systems.

Yep--But we design to loads. So the exact wire diameter and maximum lengths really depends on your actual designed power needs and voltage of the battery bank (you have been talking about a 12 volt system, but at these levels, you probably should be looking at a 48 volt system--1/4 the DC amperage for smaller wires and less money spent on charge controllers, plus less voltage drop so easier to run longer DC wiring when needed).

How would I power a custom made 12 volt swamp cooler and LED's? And are there 24-48 shallow well pumps available? If I can still power the 12 volt swamp cooler and LED's with a 24 volt system then I'd sell my 12 volt Xantrex inverter on ebay and go 24 or 48 volt in a second. I know that would be ideal. My swamp cooler uses a 12 volt automotive radiator fan and 12 volt 400gph sump pump. I would only need this in the summer though. But I would still need a way to get 12 volts DC to it in the summer. What are my options for DC 24/48 volt shallow well pumps? I do know they're pricey.

If I did do a 24 or 48 volt system, the only way I know of getting 12 volts to my swamp cooler would be a device like this: http://www.amazon.com/Kendrick-Converter-Output-Supply-Controller/dp/B000B7A1W4/ref=pd_sxp_f_pt

Seems like a crappy way to make 12 volts though. DC>AC>DC again. I guess I would have to set up an isolated 12 volt system. One that would run my 12Vdc swamp cooler and LED lighting. Seems like I'm just back to square one with that plan.

The bus bars are nothing special. Generally it is just a large metal plate (or two plates for +/- connections) where it is easy to attach all of the various power cables (large diameter for battery bank and inverter, medium for charge controllers, and smaller gauge for light loads for LED lighting, etc.).

Are the buss bars one size fits all?

thanks so much!

Hairfarm

Re: Battery bank sizing

From inetdog: At least four different calculations enter into the balance between batteries and panels:

1. Your panels and CC should be able to deliver at least a C/12 charge rate to FLA batteries. If it cannot do that you may not be able to get good gassing to prevent electrolyte stratification, and you will not be able to charge the batteries quickly enough to make up for a few days without sun. The rule of thumb that goes with this is that the panel watts going into an MPPT controller should roughly match the battery bank AH number for a 12 volt system.

Can someone please tell me if I'm interpreting this correctly? For a FLA battery, the maximum charge rate should be C/8 and the minimum should be C/12 (according to the above quote.) I thought the minimum is 10% of a C/20 charge rate?

So now, using C/12 as a minimum it looks like my panel array should provide adequate charging power for battery bank. My battery bank is 675ah.

My calculation: 675/12 (C/12) = 56.25. So 56.25*120 (C/12 rate) = 6750 / 671 = 10%

This comes from the AWAS battery FAQ: Example: "The C/8 rating is the battery capacity at the 20-hour rate divided by C/8". So I simply replaced C/8 with C/12 for my calculation based on inetdog's C/12 minimum.

However, another member on this forum calculated my charge rate this way:
810 Watts * 77% typical efficiency / 12 Volts system nominal = 52 Amps And so: 5200 / 675 = 7.7%

So I'm confused a bit. If I use a C/12 rate then I'm in the ballpark for recharging my batts. Can I get by with a C/12 charge rate or do I use the C/20 @ 10% minimum rate? And why is .77 used as a overall efficiency rating? Is that just an average?

Btw, does C/12 convert to roughly 8 percent? What is the formula for converting a C/ rating to a percentage? I know I shouldn't be so confused over this.

FYI, I live in an area that is the sunniest in the US. Zone 1. So this should increase my power theoretically I'm assuming:
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Hope that makes sense.

Re: Battery bank sizing

C/'X' to "%" is a matter of reciprocals: 100/20 = 5(%), 100/10 = 10(%), 100/8 = 12.5(%) and so on.
Just as current * 100 / battery capacity = charge rate percent.

Cool. Everything just clicked!

Just as current * 100 / battery capacity = charge rate percent.

675 / 12 = 56.25, 56.25 * 100 = 5625, 5625 / 675 = 8.3%. (* This also happens to be exactly a C/12 charge rate too. 56.25 *100 = 5625 / 675 = 8.3%. That's weird.)

I realize that I didn't factor in the .77 efficiency rating though. In that case:
810 Watts * 77% typical efficiency / 12 Volts system nominal = 52 Amps And so: 5200 / 675 = 7.7%

In either case it's going to have to work for now. I'm out of budget and roof space:grr

Do you think a C/12 rate is too low a charge rate then?

thanks Cariboocoot