Will these old commercial panels work for a small solar backup project?

[WYsolar]

List,

First time poster/newbie question:

I am trying to build a small solar backup project. I have two 6v 220Ah golf cart batteries I scrounged up and I hope to find a charge controller and a small inverter to finish the job.

Just recently I came across the following panels I can have for free:

Specs:
Solarex Corp, model SX2
approx. 12" W x 10" H

Voc 11.3
Isc 0.34
Pnoct 2.5
Inoct 0.29
Vnoct 8.8

Pnom 2.7 W@STC noct= 49deg C
Pnom 2.3 W@ 100mW/cm^2
0.35A @100mW/cm^2

My questions are: I have 15 of these panels available. Can I use them for my project? If so, how do I wire them and what kind of charge controller can I use to charge my 6v (presumably wired for a 12v system) batteries?

Thanks for your advice. I am hoping I can save $$ with these free panels.

J
Cheyenne, WY

[Cariboocoot]

Welcome to the forum.

If the panels are still good you could put two in series for a Vmp of 17.4 which would work on a 12 Volt system. However, they are only 2.5 Watts. All together (14, not 15 of them) you'd have a mere 35 Watts which is definitely not going to recharge those golf cart batteries. They would barely put out 2 Amps, or less than 1% charge rate. You need closer to ten times that much panel (about 180 to 360 Watts) for proper charging.

[WYsolar]

Thank you very much for the information.
It looks like this panel option will not work for me.

Do I understand correctly that to effectively charge my little battery setup I need somewhere around 200-300W of solar panel power? And that this could be accomplished with 1 large panel or a few smaller ones wired together somehow?

My problem right now is finding panels I can afford. I have around $150 right now for that part of the project. I suppose I could start with one small panel and add more later on....

[Cariboocoot]

Yes; 220 Amp hour at 12 Volts needs something along the lines of at least 11 Amps peak charge current to fully recharge, and 22 would be better. A 140 Watt panel (for example) puts out roughly 7 Amps, so you'd need two such panels: 280 Watts. Or whatever combination can be found that falls within the charge parameters for both Voltage and current. The cheapest (new) per Watt tend to be very large panels (over 200 Watts) with a Voltage too high for charging a 12 Volt system meaning you need a more expensive controller to make them work.

$150 isn't going to buy much new. You might find a used deal somewhere, but generally people paid a lot more for their panels then than they do now so the used panel prices tend to be out of line with reality. You could do a lot of shopping around before you find a bargain.
My advice is: Save your money.
One of the worst things you can do with any solar electric project is to buy a bit at a time. You end up with a bunch of bits that don't work together. If you build up the bank account to where you have enough to get the stuff all at once you'll be much better off. Take a look at the prices of new equipment, work out a target for what you want, and calculate a budget from there.

It isn't any fun, but it will produce better results.

[WYsolar]

Thanks for the info C.

I found another used battery option. It is a single 12v 120Ah deep cycle out of a UPS that is being surplussed.

If I have to downsize and use only this battery in my solar backup project, how do I now calculate the correct amount of panel wattage I need to correctly charge it? Is there a formula or something you used to obtain the :... 220 Amp hour at 12 Volts needs something along the lines of at least 11 Amps peak charge current to fully recharge, and 22 would be better" estimate you calculated above?

I would like to be able to figure out this type of thing given whatever battery bank I can piece together. Thanks again for your help.

[Cariboocoot]

There's a rule-of-thumb which has proven to be fairly accurate: deep cycle batteries want a peak charge current of 5% to 13% of their Amp hour capacity. AGM's can take higher amounts of current, FLA's not. You can use the 5% if there are no loads during charge time, but it's best to err on the side of caution around 10%. The idea is not that they will charge always at that rate (or perhaps even ever) but that the potential is there, which brings everything else in line so that they will recharge fully on a good day during the few hours of available sunlight. For most system 10% peak charge current potential works well with 25% maximum depth of discharge.

So the math looks like this:
10% of 120 Amp hours is 12 Amps. 5% would be 6 Amps, 13% would be 15.6 Amps.
12 Amps * 12 Volts = 144 Watts.
If you use a PWM type controller you want the Imp of the panels to add up to around 12 Amps. If you use an MPPT type controller you can get a bit more current from 'extra' panel Voltage (and do more math: 144 Watts / 0.77 [efficiency of panels + controller] = 187 Watt array).
Of course you have to adjust for what panels are actually available but as long as it falls within range it will probably be all right (barring unexpected site problems).

[niel]

it is a rule of thumb we do use and one has to watch that 5% minimum as that is what the battery(s) should get regardless of the source of the current and with no loads on at the time. for a 120ah battery this means a consistent 6a. now if you could deliver 5.9a the world isn't going to fall apart because of the rule of thumb, but it is best to stay at least as much as the minimum as that is the 20hr rate. thinking this through one has to realize that you should never drain a battery beyond half of its capacity so that 5hr of solar most may get in a good day would only replenish half of the lost 10hrs of capacity meaning at least 2 days to recharge. (the absorb charge stage is slower and more spread out necessitating more time) this means that even if you have a source delivering the 10% rate of charge that it will take longer than 5hrs to deliver the full charge into the batteries, but most batteries can take the 13% rate of charge that may just squeak the full charge into one day for that battery drained to 50% soc.

now back to the 5% rate from the pv as a charge source point of view. pvs are rated by stc conditions, which are ideal and not real world. your conditions may vary too, but generally we have settled on the 77% rate of delivery for a pv's stc rating. in the above 120ah battery example i determined it to have a 6a minimum charge rate to be required and to get a pv to consistently deliver this we take the 6a figure and divide it by .77 to get about 7.8a. this would be the minimum goal for the current of a 36 cell 12v nominal pv. the stc rating then would be that 7.8a imp x the vmp to give you the stc watts. the vmp can vary a tad and can be anywhere between about 17v to 18.5v and would be suitable for use on a standard pwm controller. the higher priced mppt controllers can better utilize the power when voltages go higher and are especially useful with many of the high powered pvs out there today, many of which have 60 cell configurations. the higher voltages going through a pwm may work, but would mean lost power as only the imp would be deliverable without being converted like an mppt cc can do. so in the 120ah battery example we could see a vmp on a pv of around 17.0v and with a 7.8a imp this gives about 132.6w stc as a good minimum goal.

this brings us to state that one roughly wants to be in the 5% to 13% charge range and divide that by .77 to get the pv's stc imp rating. the wattage seen is the imp x the vmp and the vmp can vary a bit because of the acceptable voltage range for pvs to charge a 12v battery and do understand that there are other factors that can influence these numbers such as resistive losses or battery efficiency as agms are more efficient.

twice that 132.6w would be better for 10% rate with 265w and could go even higher to 13% acknowledging that many batteries can take even higher charge rates. the max rate that they can take would depend on the battery in question and what the manufacturer states is ok to give to it. this can be important if one designs the charge system to deliver say a 10% charge rate while also under a load that may put it above the 13% (or the battery manufacturer's recommended max) when the load is removed.

note- stay away from gel batteries as they are very picky in their charge/discharge and can get ruined easily.

btw, those freebie pvs may be good to maintain a battery using 2 of the pvs in series.

also note- agm type batteries are sensitive to overvoltage, but are otherwise very very good batteries at a premium cost. some controllers allow a voltage swing that is not good to use on that type of battery. a c40 is one of those types.

[WYsolar]

Niel,

Thanks for you reply. The mods and knowledgeable folks here on the forum are fantastic. I am learning a great deal about the possibilities of solar power.

I am wondering about your idea whereby I can use my free panels in my backup scenario. I know they are way too small to do much with...but they are free and I would like to use them somehow.

Would you mind providing more details concerning your idea?

Thanks.

[niel]

j,
the maintainer idea will not recharge your batteries, but are good for batteries that are already fully charged and you may want to keep them up without dedicating larger pvs or periodically dragging out a battery charger. i'm thinking like you have another car battery laying around that is still good or even if you park your car for long time periods. you have much more of them than you will probably use, but they are good for tinkering and little projects too. you will still need the larger pv array to charge your batteries. you can keep what you like of those small pvs and if you want to you can either sell or give the rest away. we now have a thread for selling, but if you intend to give some away then do so in the same thread.
http://www.wind-sun.com/ForumVB/show...or-Sale-Thread