Battery Bank for my 12v system??

Re: Battery Bank for my 12v system??

Al, weren't you going to use a Tristar 45 PWM for your 12 Volt system? If so, 870 Watts is more than it will handle as it has the capacity for 55 Amps. If you want to be running that A/C drawing 50 Amps out of the battery, better go up to a 60 Amp controller.

Three 115 Amp hour 12 Volt batteries? Not a good choice, starting with three parallel connections. What would work minimally here is the common golf cart battery @ 220 Amp hours: two in series gets you 12 Volts, two such strings in parallel gives you 440 Amp hours. I just bought some East Penn PS2200's and they're 232 Amp hours: that would give you 464 Amp hours @ 12 Volts. Keep in mind my system is 24 Volts so there's four in one string. I also have only 700 Watts of panel, but it's a tad more efficient up here.

Small 'frige @ 290 Watts? What is the point? My big 'frige uses 120 Watts or about 1.2 kW hours per day.
CFL's are usually 13 Watts, unless you have two. Some have rather poor power factor so they'll actually use more Volt Amps.

Since you're nearly at a similar system to mine, I'll tell you I use 2.4 kW hours per day. NO A/C (millions of gallons of water in the front yard; who needs it? ), but full size 'frige, 1/3 HP water pump (about 6 minutes per day; if more is needed the generator is run), computer set-up (big power user even with laptops because it is on so long). My battery bank is sized to keep things going "overnight" and works well in that respect. If I were to plan on days of no sun I'd have to double or triple everything. For me that's too expensive; it's cheaper to run the gen a few hours a day and not have a lot of solar capacity doing nothing on days when the sun does shine.

The size of the wires between batteries depends on the maximum current you expect to draw from the batteries. The current draw will be the same on all connectors. 2 AWG is good for up to about 130 Amps, which is around 1500 Watts on 12 Volts. It's 3000 Watts on 24, btw.

I really think you should be going 24 Volt system here. 800+ Watts of panel, parallel batteries? You're nearly there. You've got practically all the ingredients except the inverter.

Re: Battery Bank for my 12v system??

ywhic wrote: »

I thought we decided the 7.75 IMP per panel was ok for upto 6 panels.. 46.5 amps into the MS TS-45 PWM controller??

I was guessing at the fridge numbers.. the one I was just eyeballing is .9 KW/day rated.. sorry..

Yes, you're right; I forgot your panels have a higher than normal Vmp.

Along those lines the Wattage you'll get won't be the 77% efficiency factor usually used. It will be more like Imp * battery Voltage. The chances of seeing 7.75 Amps @ 14.4 Absorb Volts are pretty close to nil.

It's a shame you can't afford the MPPT version of the MS because at 870 Watts the advantage would be there. Especially the ease of wiring the array advantage. I know: hundreds of dollars! When I bought my MX60 it was over $900. Still a good long-term investment.

Have you see the thread on small 'friges under energy use & conservation? http://forum.solar-electric.com/showthread.php?12272-Just-how-bad-a-small-frige-is There's a lot of good info in there.

Re: Battery Bank for my 12v system??

Okay, that was a mistake. Sorry, but with the number of questions (many of which are very similar, of course) I sometimes forget who's got what.

The 5 panels will still work in parallel, even on 450 Amp hours. Trouble is, if you want to up system Voltage and/or use MPPT ...

Maybe we can carve that one in stone someplace around here: always buy panels in pairs. At worst you'd have an "extra" panel waiting in the wings. Or you could disconnect it on sunny days and add it in on cloudy days, which was what you had intended to do anyway.

Remember what it says on the cover of The Guide: Don't Panic!

Re: Battery Bank for my 12v system??

What you "feed" to the battery bank depends not only on the size of the array, but also on how much the battery bank "needs" (state of charge). Once it is full, the panel power potential is not realized without additional loads.

This is why there's the quick Icarus formula for determining potential AC Watt hours based on array size alone:
Nameplate rating of array size * hours of equivalent good sun * 52% over-all system efficiency.
With 725 Watts and 4 hours minimum you get 1508 AC Watt hours.

This gets checked against the size of the battery bank that 725 Watt array can recharge @ the 10% rate (usually), which in this case is 387 Amp hours. That size battery bank at 25% DOD should supply about 1 kW hour AC. This is good to have more panel Watt hour capacity than battery Watt hour capacity: it means you'll be able to recharge from greater than 25% DOD and that you'll get something on cloudy days, even if not much. Then on sunny days you have a potential surplus of power.

You can also check to see what the maximum current works out to be in terms of percentage charge rate on different battery banks: peak current * 100 / battery bank capacity. For 38.7 Amps you get 8.8% on 440 or 4.4% on 880. So it would work on the 440 bank but not on the 880 one. You would have about 1.1 kW hours AC from the 440 bank @ 25% DOD, so it would still work for recharging.

Remember that all the numbers and calculations are relative guidelines, not absolute facts. The sun does not shine the same every day and that will have a big effect on how well your system performs. More so than almost anything else (other than completely mis-designing it).

Re: Battery Bank for my 12v system??

ywhic wrote: »

So at 725 watts of PV basically.. I could go with 660ah on the battery bank and be at 5.8%.. that wouldn't be good either..

No that would not be good. Too close to the edge to be comfortable. Even slightly cloudy day ... insufficient charging

The 870 watts of PV would be 1809 AC watt hours.. now I got lost after that.. would that work decent on a 660ah battery idea??

Through the PWM controller where you get 45 Amps peak current? 45 * 100 / 660 = 6.8% peak charge rate. Again a bit slim, especially after you consider that any loads operating at the same time will detract from that.

Now put that 870 Watts through an MPPT controller and you get 55 Amps (the MPPT advantage) * 100 / 660 = 8.3%. That one works better.

Again this is all just comparative. You have to always "round in the right direction". Assume loads will be greater than expected, battery capacity less than rated (they do go down over time; something that doesn't get mentioned often enough around here), and panel output will only be at its best when the sun shines fully.