Using 24v panels to charge 12v battery bank

[dave123]

It would seem that, if you wired a 24v solar panel, through a charge controller, and to a 12v battery bank, you would get the benefit of a chargeable voltage in low light conditions.  Assuming the peak open circuit voltage of the panel is not above the max input to the charge controller, would this not be correct?

[ghurd]

Not correct.

They make panels that way for a reason. It gets the most power the way they make them.

If wiring them different would increase the output, don't you think they would do that at the factory, and brag about it in the brouchures?

Simplified.  If a 17w 12v rated PV makes 1A max, a 34w 24v rated PV makes 1A max, and 2 series 17w 12v rated PVs make 1A max.  So for 12v...

Would you rather have 1A or 2A?

G-

[Victor]

36 cell panels already reach "chargable voltage in low light conditions" If you wan't to use 24 volt panels on a 12 volt system, use a MX 60 to get the full wattage of the panels into the battery.

[somemathguy]

I have to say that what ghurd is saying doesn't seem quite right...if your panel is rated at 24V it should running at about 34V. If your 12V charge controller can handle that kind of input volts, it will drop it down to 12V and increase the amps internally - so you will get more power from the 24V panel than the 12V panel with the same amp rating (1 amp or whatever).

It's like with batteries...say you have four 100Ah 6V batts. You can wire them in series for 100Ah at 24V, or in parallel for 400Ah at 6V. The apparent difference in amps does not mean that there is more 'power' in the 6V configuration. Similarly, 1 amp coming out of 24V will have twice as much power as 1 amp out of 12V. Think in terms of water coming out of a hose...Volts is pressure and amps is the volume of water. If you double the pressure (Volts) by squeezing the hose, but the same volume of water is coming out per unit time (Amps), then the water will shoot out farther (twice as much Power or Watts).

[pyrocasto]

Ghurd should be right. UNLESS, you use a MPP CC or what not. Just like a wind genny. If  you have 50 volts into a 12 volt battery, it wont boost your amperage and give you all the power. It will slow the genny down to the battery's voltage.

[troy]

More voltage does not always equal more power.  Solar panels function pretty much as constant current devices (assuming constant illumination intensity).  So theoretically, if you had two 12V (nominal) panels that made 2 amps in parallel (12V) and one amp in series (24V nominal), the parallel 12V arrangement would put more watts into the 12V battery than the "24V" series arrangement.  

The reason is the 24 volt string will only make one amp no matter what, and the battery will pull the voltage down to 13V or so.

1A x 13V = 13 watts.

The parallel arrangement will make 2 amps at 13V:

2A x 13V = 26 watts

Running a 24V panel into a 12 volt battery is a little bit like stalling a windmill, it can never make its rated wattage unless allowed to spin up to full voltage.

Number of cells in series controls open volts.  Number of cells in parallel along with light intensity contols amps.

If you have a mppt controller, ignore all that, as it does all the matching for you to maximize wattage into the battery.

Good luck and have fun!

troy

[ghurd]

Controllers are not transformers.  Power in doesn't equal power out.  Amps in equals amps out.

Solar panels put out amps, more than watts.

Like 2, 12v, 3a panels in series put out 3a.

If you connect them to a 12v battery, you get 3a.

The same panels in parrallel get you 6a.

If you want a few/several hundred dollar MPPT controller, it will shift things around a little, but not that much.  Most MPPT controllers for 12v out should still have 12vs of panel in.  Independant testing of these do-hickies show a much lower gain (3-8%?) than the brochures from the guy trying to sell them.

Most normal/smaller solar installations would get a MUCH MUCH bigger benefit using the MPPT money on more panels.

As far as the hose example goes...

When you double the pressure, with the same volume of water, it shoots out farther.  BUT, it still takes just as long to fill up your swimming pool.  The pool holds gallons, not PSI.

Back to batteries...

You double the V, with the same amps. It still takes just as long to fill the batteries.  The 12v battery holds amps at 12v, not 24v.  BUT the cost is double for twice as many panels.

G-

[Ungrounded Lightning Rod]

troy hit it on the head.

Note this particularly:

Number of cells in series controls open volts.  Number of cells in parallel along with light intensity contols amps.

Dim sun means lower current and doesn't affect voltage at all.  So you don't gain anything - and lose half your current - by wiring your panels for 24V.

A max-power-point controller will cover for that.  But it will also eat some power in its own inefficiencies.

IMHO the only time you want to wire your panels for 24 when charging 12 is when you have a MPPT controller AND you need to run your power a long way from the panels to the batteries.  Then you cut your current in half (and your line losses for a given size wire by a factor of 4).  This lets you get away with slightly smaller (and significantly cheaper) wire, or lower losses in the same wire size (depending on which way you want to do it).

[boB]

>>Number of cells in series controls open volts

Number of cells in series also controls the Max Power Point Voltage, since MPPV

is a percentage of the open circuit voltage.

MPPT is also worthwhile at close PV to controller range because at cool temperatures

at least, the 5% or so of efficiency loss in the MPPT controller is much less than the possible 10 to 30 percent power you are throwing away by operating the PV array at battery voltage.

I hear all the time about people that have wired their array for higher than nominal

battery voltage (at close range) getting more usable charging in the early and late hours of sunlight while using an MPPT controller.

Since high temperatures can make the max power point at or below the battery voltage,

wiring for higher nominal voltage ensure that the power point voltage is always higher than the battery voltage.  And the current in is always less than current out with higher input voltage. Even same nominal PV and battery voltage wiring. Wiring for a MUCH higher input voltage may not be the best idea if the PV to battery wire length is real short, say, 10 feet, but at least one nominal PV voltage higher than battery voltage helps the hot panel situation as well as early morning and late afternoon performance.

Ideally, power in is equal to the power out of a MPP controller, but of course nothing is ideal

Have a day and 1/2 !

boB

[Psycogeek]

I had the same question. these are some thoughts.

i am not an expert and do not play one on tv, but often things are designed for one purpose that suits that purpose best, and not nessisarily everyones purpose under the sun :-)

and the answer (like many answers) is yes and no

Yes

there IS a place that the solar cells will output a minor ammount of amps at 1/2 thier voltage.

No

you dont really have a lot (or even usefull) amps untill there is enough sun that it is very close to the voltage for the cell.

SO

Overvoltaging a Bit, so you are charging a battery , instead of at its same voltage is a good thing. overvoltaging a LOT is not usefull, as there is not much power even comming out of them when at 1/2 thier voltage.

some figures i got. 3a e-bay cells

open voltage when there is enough light for .1A .43v

Open voltage when there is 1A of power .45v

because this is kinda important to my project, which is simple charging of some li-ion cells, i will test this further, after all i am just testing anyways.

--------------

the THEORY

when CHARGING something with unregulated charging  , as the voltage differential gets lower, the flow of current will reduce.

IS there a place where a much higher voltage differential provides for a much higher current?

-----------------

this would be true in other unregulated things, if it is true for solor cells i will test for its potential use in charging circuits, and get back to here.

with regulated charging , its all going to be totally dependant on the circuitry stuff that is used. some regulated charging curcuits would turn your overvoltage into total waste heat, or even ruin them.

You see with that same question in mind, i was trying to figure out a (simple) way to switch to more series than parrellel when the potential differential is so low between the cells and the battery during a cloudy day, that there is 0 charging instead of .01 charging :-).

like a relay that dropped out when the voltage is to low, and switched the pannel from parellel to series, to increase the output voltage.

It does not look practical or usefull to do that.

it looks more like a 1.5v battery when its at 1.0v so much of it is gone at that point, putting 2 of them together is not going to help enough, because there is not enough current no mater how you stack it. (like not enough when its cloudy)

Newbie

[ghurd]

You got it.

But, a good panel will still produce when it is cloudy. At least some.

G-

[TomW]

G;

Most normal/smaller solar installations would get a MUCH MUCH bigger benefit using the MPPT money on more panels.

Amen to that, or add tracking which is actually cheaper than either.

Most stuff I have seen claim tracking returns something like 30% gain but I think it is much higher but no data to back it up. I can, however, show when my panels are facing due south and morning sun clears my pine tree at 9 AM that I get maybe 5 or 6 amps but if I track the panels to face that morning sun I get my full 18 amps. Just observation from here. Your mileage may vary, of course.

Cheers.

TomW

[pyrocasto]

Plus, you can build your own for cheap(<$100) and have it work just as good.

I'm thinking of getting several of these satilite mounts I see sticking from the ground. Heck, I would sell them if I could find someone around here to buy them, but no one in my town is interested in RE.