Having a little trouble charging battery @ max Amps...

[gambit1982]

Hi all, I'm new here to solar panel. I've just recently build my own panel using the cells I've gotten from Ebay. My panel puts out about 18.5 volts open voltage and around an average of 2.75 amps short circuit. I'm using an RV deep cycle battery that I picked up at WalMart, the Everstart brand (does anyone know if this battery is good to use with a solar panel?), which is rated at 75aH and cost about $43. My question is, why is it that when I hook the solar panel to my charge controller, and from the charge controller to my battery, it's only pushing 900 milliamps at best? Why can't the battery take the entire 2.75 amps? Even when I hook the solar panel directly to the battery (minus charge controller) with a DVOM in series to the battery, it still only consumes 900 milliamps. What I noticed is when my solar panel gets awfully hot, the voltage drops all the way down to 15.5 volts. Would this be the reason why it's not able to push the entire 2.75amps into the battery? But during the evening around 4.30PM or so, the panel cools a little and the voltage shoots up to 16.3 volts or so, but I'm still getting 900 milliamps at best to the battery. I'm stumped. Can anyone give me any insight on how to solve this problem? Could it be the battery itself? I duno, just guessing here. Well, any help would be greatly appreciated. It takes too long to charge the battery at 900 milliamps.

Oh, one more thing. When I monitor the battery voltage level while being charged by the solar panel, I noticed that the battery voltage would max out at 12.60 volts. It would not go any higher. This was done both with and without the use of my charge controller. Shouldn't the battery peak out at roughtly 14 volts or so? My charge controller is a sunsei 10A, 120watt controller, if it matters. Please help. Thanks.

Steven

[willib]

hi gambit

in my opinion you are not going to see the full short circuit current into a battery , because the voltage difference with a resistive load is greater than when charging a battery .

if you boost your open circuit voltage that will help when charging a battery

[jimjjnn]

I think I would put a load on the battery and discharge it a little and see what the chg control puts out. Also, I hope you aren't using battery clips to the battery as they are poor connection. If Batt is new, I would also clean the batt terminals and use standard battery cable ends.

[Flux]

I am not a solar expert but I think commercial panels have an open circuit of about 22v and the maximum power point is about 17.5v so your voltage is rather low and as you say it falls when hot.

Even commercial ones only deliver about 70% of the short circuit current into a battery.

As Jim has said, you may see a fair increase in current when the battery is below 12v.

I am not sure why you didn't see an improvement with the panel cold but the insolation level may have been lower later in the day. Did you tilt the panel directly at the sun in each case.

Flux

[ghurd]

How many cells in series?  Standard is 36, some brands use 40 cells, and anything less than 36 is going to be a lower output under a lot of conditions.

I have a feeling you have used 30 cells?

The configuration was sort of common for a while, sometimes called 'self regulating', used 30 to 33 cells. One of the larger websites refuses to sell them any more just for the reason you discovered. Most brands don't make them any more.  "Self Regulating" because they don't need a controller, because they about can not overcharge a battery... but they don't work worth a hoot.

The small voltage drop of the controller can nearly stop current flow under less than ideal conditions.

Most factory panels will put ~90% of the short circuit amps into a battery, but only 70% of the rated watts.

G-

[scottsAI]

Hello Gambit1982,

Here is the Data sheet for BP Solar: BP 340U 40W Polycrystalline PV Module:

http://shop.altenergystore.com/pdfs/bp/BP%20340_4029_v1_Sep_04.pdf

This panel is similar to what you were trying to make.

Second page has the output of the panel at various temperatures (IV plots).

Plots show the current with resulting output voltage.

Panel is rated:

Electrical Characteristics: BP 340

Maximum power (Pmax)3 40W

Voltage at Pmax (Vmp) 17.3V

Current at Pmax (Imp) 2.31A

Warranted minimum Pmax 36W

Short-circuit current (Isc) 2.54A

Open-circuit voltage (Voc) 21.8V

Open circuit is 21.8 volts almost 6 volts more than yours.

Max at rated current is 17.3 v.

Yours is 16.3 when cool. Vs 15.5 when hot.

Your loading it with the battery, your operating on the vertical line of the IV plot.

You should be operating on the horizontal line of the plot to get better power out of the panel.

Max power is at the point of the horizontal line starts to curve down.

Since the performance of the panel drops so much over the hotter temperatures you need to give yourself some space to work. More to the right so with a hot panel your still in the horz line.

The Max power can be had when using a MPPT controller.

The MPPT expects a higher voltage input than the battery. The MPPT keeps the load on the solar panel at the point of max power, it does this by converting the voltage to the lower voltage needed by the battery yet keeping the IV as seen by the panel at the max power point.

Have fun,

Scott.

[gambit1982]

Hey, thanks everyone for the helpful reply.

I woke up this morning at 6am and went out to wipe the moisture on top of my glass off. I then went in and turn on my dvom and monitor the amperage to the battery. It was still early, the sun didn't come out till around 6.30am or so. By then, the current produced by the panel was weak, so I had breakfast and surf the web for a while until the sun was bright. I kept monitoring the amperage level until it reaches 900mA at about 9.30am or so. I kept my eyes glued to the monitor and I was surprised to see the amperage slowly increasing, but not enough. It peaked out at around 960mA and then it just stood there. So I swap the battery with a used one I had laying around for a while now. I hooked everything up and was surprised to see it taking in exactly 1A. I let it sit for awhile but it wouldn't increase or anything. One Amp was the best I got out of it. During that time, the panel open current was about 1.32 amps if I remember correctly. Open voltage reading was about 17.35 volts or so. So I guess only about 70% or so of the open current made it to the battery like some of you have stated.  

Then I rechecked again in an hour or so, and the amperage to the battery dropped to 900mA. So I thought about upping the voltage like some of you had advised. I was a little lazy, so I only used one solar cell i had laying around, and I hook it up in series to the negative side of the panel and the open voltage jumped up to 17.70 volts. I also noticed the amperage to the battery jumping up 50mA. So I'm assuming my open voltage is too low to begin with? I do have 36 cells in series. My panel is slightly tilted to the sun, maybe 5 degrees or so??? It's sitting fixed on top of my roof at the moment.

Scott, I noticed from the pdf file that you posted, how is it that the panel is making 21.8 open voltage, while mine is only making roughly 18.5 open voltage? I'm using the same amount of cells as that company. Is it because they have some kind of antireflective coating on the cell to minimize reflection of sunlight? Next time I'll try 40 cells instead of 36.

I made a mistake of using wood as my backing. Well, I used aluminum sheet and silicon it to a piece of wood. So when I found out that it was overheating and dropping voltage, I cut out a square size, about 8 inches by 8 inches in the back of the wood to expose the aluminum, and then I stuck a 120mm (about 4 3/4 inch) computer fan there to try and cool it. Guess that didn't work. lol. I called around to get quotes on aluminum plates but they were way too expensive.

At the moment, I'm running I believe it's 14 gauge wires from the solar panel to the charge controller, to the battery. I know, I need bigger wiring. I ordered some 8 gauge off Ebay for cheap and they should be here by tomorrow. Maybe it's the wiring that's causing the low amperage??? Well, I remember using 10 gauge wires in the beggining before I even put the glass on, and I think it didn't push that many amps to the battery either. I duno, it's been a while so I might have forgotten the exact numbers. I guess I have a lot to learn before I decide to build another panel. I guess I can't really do anything about it right now since it's sealed up. But thanks again everyone for your advice. If I ever fix my problem, I'll reply back and let you guys know. Sorry for the long confusing paragraphs. Thanks again..

Steven

Here's a picture of my homemade panel. Not the greatest looking thing, but it'll do for now

[willib]

i hope some of the cells are not shorting aginst the aluminum backing?

[gambit1982]

Nope, that's why I used strips of paper where the contacts meet. It was probably a bad choice, but I wasn't sure what else to use to prevent the cells from shorting out.

[commanda]

The cells from eBay may be seconds or rejects, with slightly lower output (volts and/or milliamps) per cell. Therefore may need slightly more cells in each series string to get the volts up.

And no, the short circuit amps is not the same as amps into the battery; line losses aside.

Also, measure the volts across the battery, and the volts across the terminals of the solar cell (at the other end of the cable run). Ideally shouild be the same. If battery volts is 12.6, and the cell volts is 17.6, you are obviously dropping 5 volts across your cable run. If the current is 900 milliamps, then line resistance is 5/0.9, or 5.56 ohms for the return circuit. Ordinary figure 8 speaker cable would be about about this.

And to the obvious question; what exactly is the cable from the solar cells to the battery?

Amanda