Not enough panels

itchyfeet · March 2012

I finally got ac bugs worked out and system running . I am now faced with a new issue, I believe I over did the battery bank. I based my system on from a web site showing diagram for an 8kw system, I really don't understand the math they used, 16 100w panels( 5 hrs a day) for a 4000ah battery bank?, I have been learning more from these post in two weeks than the last two years and 3 systems later. I have 3040 ah 12v battery bank, if my math is correct 3040* 0.10 = 304 amps 304 * 12 =
3648 at 77% = 4738 watts of array needed? I have 2080 watts which I think is really good for about 1600 watts. I want to get the panel wattage up when I can afford it, and cut the bank in half for now, but can I add the other half in say 6 mnths from now? Or rotate the two banks in and out? Im regretting not going with 24vlt system, I could make every thing work except the two Vfx 12vlt inverters, im kinda stuck there.

Thanks for any suggestions

westbranch · March 2012

Re: Not enough panels

tell us the specifics of what you have, all of it, that way the options become clear

HTH

itchyfeet · March 2012

Re: Not enough panels

I have (32) 65watt sharp solar tiles, (2) flex max 80 (2) vfx 2812 Inv, hub4, mate2 (16) 6vlt L16xc usbat 380ah wired in series/parallel, 7500w gen, atkinson gscm

itchyfeet · March 2012

Re: Not enough panels

Its funny but not so much, when I started this thing a few years ago, I kept hearing about the cons of 12vlt system because of wire sizing, at the time I was making my own panels wiring them in parallel, so I was more concerned about the amps, so I opted for the flexmax80, I am now trapped with only 1250 watts per flexmax at 12 vlts. but it still seems the less expensive route, I need more pannels regardless, so I figure two more flex max 80's and breakers and I keep a large battery bank or two new 24vlt inverters and two battery vent fans.

westbranch · March 2012

Re: Not enough panels

Assumption, as you did not list the battery model:
it looks like you have a huge battery, too bad it is in 12 volt configuration. he wiring must be a rats nest, 8 parallel 12v batteries. you actually have, if I read it right 3,040 Ahr of storage (36,480Wh).

IIWM I would at least switch to 24 volt configuration to at least simplify the wiring and ensure equal charging of each battery. 48V would be better yet. You should not have to change up any of the wiring , just the Inverter. Charger settings for 24 v are simple with that CC.
HTH

Windsun · March 2012

Re: Not enough panels

Just as a really rough estimate, you can usually figure around 1 watt of panel per AH of battery, and even by that guesstimate you are way over-batteried.

BB. · March 2012

Re: Not enough panels

I like to start with the batteries as the "heart" of your system... Everything else should be sized/operated to keep your battery bank "happy" (unhappy batteries can have a very short and expen$ive life

).

Assuming these are 16x L16 6 volt * 380 AH arranged into a 12 volt battery bank:

16 * 6 volt * 380 AH / 12 volt battery bank = 3,040 AH @ 12 volt battery bank

The recommended rate of charge is around 5 to 13% rate of charge (we use the 20 Hour Rate for capacity estimates):

3,040 AH * 14.5 volts charging * 1/0.77 panel+charger eff * 0.05 rate of charge = 2,862 Watt Array (minimum)
3,040 AH * 14.5 volts charging * 1/0.77 panel+charger eff * 0.10 rate of charge = 5,725 Watt Array (good sized nominal array)
3,040 AH * 14.5 volts charging * 1/0.77 panel+charger eff * 0.13 rate of charge = 7,442 Watt Array (rough maximum cost effective array)

For parallel wiring of batteries, see the SmartGauge website. Improperly wired batteries can discharge/recharge unevenly (batteries with the least resistance in the wiring will cycle the most).

Note, not many people do this--but each parallel battery connection should have its own high current fuse or circuit breaker--If a tool gets dropped on the battery bank, or you have a shorted cell--These fuses/breakers can help reduce the chances of lighting your battery bank on fire. These Lead Acid batteries are more than capable of supplying thousands of amps into a dead short. More than enough to weld a wrench to the battery bus and/or overheat your wiring.

In your case, that would seem to require 8 fuses/breakers (one per parallel string). Not real cheap or easy to install/maintain (several hundred amp fuse/breaker per battery string):

High Amperage Inverter Fuses & Breakers
Neat Battery Mounted or or two fuse blocks

I don't know what you have for test equipment--but for large systems and lots of paralleled battery banks, I would highly suggest a DC Current Clamp Meter (400 amps maximum). Here is a "cheap one" that is good enough for most people. Also included other normal DMM type functions.

Besides the issues of heavy copper cable for 12 volt systems with high wattage/current requirements (and a maximum of 1 VDC wiring drop)... There is the issues of how higher voltage devices need less copper for basic DC functions--so larger inverters/charge controllers when operated at higher voltage battery banks.

For example, your Outback charge controllers are rated at 80 amps maximum, at 12/24/48 volts--So a single array supported wattage can be:

80 amps * 14.5 volts charging * 1/0.77 derating = 1,506 Watt array @ 12 volt battery bank
80 amps * 29.0 volts charging * 1/0.77 derating = 3,013 Watt array @ 24 volt
80 amps * 58.0 volts charging * 1/0.77 derating = 6,026 Watt array @ 48 volt

So, going with a higher voltage saves you $500-$600 a clip for charge controllers--Possibly covering the costs of your higher voltage inverters.

Your thoughts?

-Bill

niel · March 2012

Re: Not enough panels

i can't say he is too high on capacity unless i would know what he needs from his batteries every day. it may not be over batteried, but severely under pved.

the real problem he has in going to a higher voltage is that he opted for 12v inverters. he could cut wiring losses down putting the pvs at a higher voltage string, but the output is still at 12v and limited on the current it will output. now he could take out the inverters and buy new higher voltage ones allowing an expansion to the pv system without needing to buy more controllers to handle the extra power. add to that he can either return the inverters for a refund or sell them at a loss so he can get the higher voltage inverters. if one needs that much current capacity in a battery bank then go to a 48v system to cut some of the losses in efficiency. going to a 48v battery bank will allow each controller to handle up to 4x the power. roughly figuring 80a x 12v = 960w while 80a x 48v = 3840w.

vtmaps · March 2012

Re: Not enough panels

itchyfeet wrote:

....and cut the bank in half for now, but can I add the other half in say 6 mnths from now? Or rotate the two banks in and out? I'm regretting not going with 24vlt system, I could make every thing work except the two Vfx 12vlt inverters, i'm kinda stuck there.

You have 8 batteries in parallel. That is massively parallel and will cause you problems. Even if you cut the bank in half you will have problems.
Read: http://forum.solar-electric.com/showthread.php?14674

Even with the cost of new inverters you will save money in the long run by redesigning your system... your battery bank is worth more than your inverters.

--vtMaps

itchyfeet · March 2012

Re: Not enough panels

I really appreciate all the feedback, the knowledge on this is vast. I had alot to ponder at work today. Battery link makes sense, bill, I do have my bank feeding from different ends using 4/0, I like the idea of independent fuses and balancing the bank even further. as far as my situation, good point,niel, about (my needs from bank on daily basis), but thats just it I dont really know, lol, I am down to 1 teenager out of 5 still living at home, so my uses have changed dramatically, I really got this going to power a 30 x 30 moss rock garden waterfall pond I built and kept going with it, wife says I need pv anonymous. What I have, is the current system powering 3 bedrooms and 4 bathrooms, not really much heavy loads except occasional hair dryer, and jacuzzi tub, all which was originally wired into a sub panel at that end of the house which I just rewired into my own new ac panel (off grid). system was floating when I got home today, Its dropping to about 12.3 by morning and back up to 12.6 by 9:30 am, but I'ts just a light here and there and a 36" lcd on for mabey 2 hrs at night. I think the only thing I did right is not try to add to much right away. my main concern is keeping my banks happy at this point until I can fund the next phase. Now I agree with the larger vlt system but I did have a possible alt. plan. what if I cut the bank in half, max that system out at 2500 watts (1250 per controller). keep it as 2 phase

BB. · March 2012

Re: Not enough panels

I like to push Battery Monitors (and Victron too)--They allow you to understand your system from the battery bank's point of view--Many times, the only point of view that really matters (unhappy batteries tend to commit suicide--Or is that euthanasia by the owners?).

-Bill

itchyfeet · March 2012

Re: Not enough panels

for larger loads. I have a smaller mixed parts system, all the breakers, gfi, shunt ect... add a 24vlt inverter, cc, and panels for the other bank to run independently of that system, i know it would require a second ac panel and mate as well as another underground wire run.
Once again this greenhorn appreciates all the input.

itchyfeet · March 2012

Re: Not enough panels

lol, yea im starting to see that, I had plans to get the flex net as my next purchase.

Photowhit · March 2012

Re: Not enough panels

Sell your inverters on Ebay, and buy 24V or even 48V if you believe your system will expand. Rewire your bank at 24 volts (all like batteries of the same age?) Your 1500Amp hour 24V battery bank will likely want Close to 4000 Watt array to remain happy.

I'd say you could double your array with the same CC's but I think you have too much array for 2 flex 80's now, but I'd have to do the math and know your winter temps. But there will be some savings there.

When I ran the numbers I figured my @2000 watt array was pretty minimal for my 800 Ah 24V battery bank, though I live w/o a backup charging alternative, with the right inverter you have the option of charging from your generator.

itchyfeet · March 2012

Re: Not enough panels

well I think I'm sold on all the suggested path of selling the inverters and going to 24vlts, the only other piece I would have to replace is two battery vent fans. will the 4/0 battery cables be ok still? or will the over- kill be an issue? the voc on my panels is 10.9 I have 8panels * (65watts) in series for 87.2 vlts (2 strings for each cc) the isc is 7.90 amps which is 15.8 for each cc. The rating @ 24vlts is 2500watts. I believe I could run 4 strings of the above to each cc for a total of 2080 watts and 31.6 amps for each cc. that would give me 4160 watts for 1500ah bank. does that sound right?

niel · March 2012

Re: Not enough panels

itchyfeet wrote: »

well I think I'm sold on all the suggested path of selling the inverters and going to 24vlts, the only other piece I would have to replace is two battery vent fans. will the 4/0 battery cables be ok still? or will the over- kill be an issue? the voc on my panels is 10.9 I have 8panels * (65watts) in series for 87.2 vlts (2 strings for each cc) the isc is 7.90 amps which is 15.8 for each cc. The rating @ 24vlts is 2500watts. I believe I could run 4 strings of the above to each cc for a total of 2080 watts and 31.6 amps for each cc. that would give me 4160 watts for 1500ah bank. does that sound right?

i'm confused. you have pvs with a voc of 10.9?

stephendv · March 2012

Re: Not enough panels

itchyfeet wrote: »

that would give me 4160 watts for 1500ah bank. does that sound right?

If you already have the battery, then in my opinion the size of the PV should be sized based on how much energy you use every day- not on the size of the battery. So no need to buy 4kW of PV straight away, first start with what you have and check whether you're getting your bank charged properly. Add more as needed.

itchyfeet · March 2012

Re: Not enough panels

yea probably sounds wierd, They are sharp solar tiles. at first I was going to wire 2 in series to have 21vlts @ 7.9 amps per set. the amps were going to add up quick, so after talking to ob I didnt know I could wire as many as I want in series as long as I stay under 145 vlts, the flexmax would do all the conversion on its own. I had been basing all my panel shopping thinking I had to have 12 vlt panels for a 12vlt system. I wasnt crazy about the tiles (effiency) but im lucky to have to roofs to play with. I also couldnt find a better deal, at $69 a pc, thats just over $1/watt.

BB. · March 2012

Re: Not enough panels

Regarding solar array voltage... You have to also check the Voc cold operating point of your array. For cold areas, you probably cannot exceed Vmp of ~100 VDC maximum as the Voc cold will exceed 145 VDC in sub freezing temperatures.

-Bill

itchyfeet · March 2012

Re: Not enough panels

good to know, BB, This winter has dipped below freezing a few times, the coldest its been here in 10yrs is 16deg. I am concerned about the hot weather on my battery bank, i've been keeping the temp in my control room at about 80deg or below, summers come quick here and am guessing I will need to install a small window box unit, all my gear is inside a 12'L 6'w x 7 'h enclosed trailer. its actuallya pretty good deal if I move, Im partially mobile except underground work and panels.

BB. · March 2012

Re: Not enough panels

In general, there is a very handy engineering rule of thumb... Basically, for every 10C (18F) degree rise or fall in temperature, there is a factor of two change in its "life"...

For example, batteries are rated at 25C/77F--Increase battery temperature by 10C, then life is cut by 1/2. Increase by 20C, life is cut by 1/2*1/2=1/4

Conversely, cut temperature by 10C, life will double.

Of course, there are different parts of the life, Cycle Life, Float Life, Sulfation Life, etc... Some of which are very temperature dependent and others have different wear/failure mechanisms...

But, keeping your batteries below 80F is a good idea for long term life. I am not sure I would go as far as an A/C system--But insulation during the day and good ventilation at night may be "good enough".

-Bill

Not enough panels

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