Author Topic: 0.1V difference between panels costing me 0.24A :S (Read 3973 times)

terramir · « **on:** April 10, 2007, 04:11:43 PM »

I got two panels on my roof and I did some measurements today, Inline

current for one panel was 1.26A for the other about 1A however when

they are in parallel the total power is 2.02A at a certain time. the

open voltage for the panels was 18.98V and 18.88V respecitvely,

however of course the panels both get dragged down to the battery bank

voltage when they are connected.. one note the diode to prevent

reverse charging at night is connected to the line near the battery

and they both share the same diode. so each panel does not have a

seperate diode. kinda worried that that juice that is disappearing is ending up in the lower voltage panel and slowly distroying it.

Let me know

terramir

ghurd · « **Reply #1 on:** April 10, 2007, 10:39:28 AM »

"kinda worried..." Doubt it.

Try a separate diode for each PV. The bigger, the better.

A very minor change in solar or battery conditions, between measurements, could cause it.

G-

terramir · « **Reply #2 on:** April 10, 2007, 10:50:00 AM »

well looks like I'm gonna have to solder on the roof. Yikes also I measured this twice in sucession first one panel A then the other A then total amps then voltage on both seperatly and then from the beginning again, would be an extreme fluke if this wasn't something systematic. Gonna go on roof today I'll come back with some measurements. Also I'm right now using 6A like 200V diodes 2 in parallel to handle a max 12A load there rectifiers but I choose them over shlotsky because I rather lose the 0.2 V then having the reverse flow at night and I was having with the shlosky's I was using before. I was losing 1A hr over the whole night.

terramir

Flux · « **Reply #3 on:** April 10, 2007, 10:56:46 AM »

Don't forget line resistance, it has more effect with both panels.

Flux

Opera House · « **Reply #4 on:** April 10, 2007, 11:41:45 AM »

Make your current measurements shorting the panels out at the battery location. I've seen this happen from the battery voltage increasing over added charge current and added voltage drop. Panels are rated at 19V and more for a reason. As you stack up the voltages beyond the power point the current will drop.

terramir · « **Reply #5 on:** April 10, 2007, 12:22:06 PM »

Yeah I just figured that out on the roof LOL soldering the dam diodes into the panel box I checked the currents and well I'm screwed, I got 1 panel at 1.7A one at 1.4 A and together there like only 2.2 A my wires are costing me over an amp right now and I suspect by the time it's noon it will be costing me another 1.5A - 2 A for a total loss 30- 40 W hrs

My line can only handle around 2A- 2.5A max it seems.

Must be an age thing as well as the wire size I was using.

yikes I've been producing way more power than I got all along, might be one reason my battery died so quickly lol.

Hmmm Ohm's law I= V/R R= V/I

13.5V / 2.2A = 6.13 Ohms hmmm that dun make sense even with aged wire maybe not all the strands are conducting well I did a test and well one strant on the negative wire is giving me a funky voltage like it's not connected properly but it just seems like the wire itself has increased in resistance I'm gonna go upstairs and fiddle the wires together down here and see what the resistance value is.

terramir

terramir · « **Reply #6 on:** April 10, 2007, 12:41:56 PM »

ehh I was measuring inline currents not short currents. and at the battery in inline current right now is 2.3 A the panel short at the battery area is 5.67A and well I went on the roof and measured the line resistance it was 1 ohm I wonder does the resistance increase that massively with 2A and more flowing through it. What you said about the mpp doesn't make sence to me since I measured the current for each panel seperately in line hence connected to the batteriesand both mpp should be around 16 to 17 V and now since I put the diodes on each panel seperatly this will prevent the panels from charging each other. so there should be no interaction there.

Question one can wire with a 1ohm resistance measured with a few milli amps increase to 6 when about over 3 amps is applied and only let 2.2 A though and when 4.5A is appllied only let about 2.5A max through.

terramir

Opera House · « **Reply #7 on:** April 10, 2007, 02:46:43 PM »

13.5V / 2.2A = 6.13 Ohms hmmm that dun make sense.......

I couldn't have said it better. Panels are current sources and at th panel you would measure about 3V if you short at the battery. I doubt that you have an instrument that can measure around one ohm with any accuracy, most meters are not very good in this range. Battery 14.5V + .7V + 2.2V = 17.4V That is why panels have to be over 19V open circuit!

terramir · « **Reply #8 on:** April 10, 2007, 03:55:56 PM »

but still I gonna do one more experiment right now I took a mustang headlight (had one laying around from my old mustang, measured 2.9A amps on one side the other beam 2.0 A. At the 2.9A side I got about 14V BTW my voltage drop on these rectifiers is about 0.5V not .7 like you calculated. Anyways I'm gonna go to the roof that light should be able to handle up to 6A short term and do some voltage and current measure ments to ensure that it is mostly the wire that is giving me problems. If I get a much higher voltage and current I'll know then that I have to get myself at least another round of this wire to half the resistance. I was figuring that this wire should be enough but heck it's between 14 and 16 gauge 8 strands of 24 gauge each way that adds to 2mm^2 16 is 1.5 and 14 is 2.5 mm^2. didn't figure it limit the current that much :S because the resistance I calculated was below 1 ohm.

Results laterz

terramir

terramir · « **Reply #9 on:** April 10, 2007, 04:29:33 PM »

dam same figures seems like the internal resistance of the head light is killing that one. what would the internal resistance of 2 115 amp new lead acids be? could the battery itself be dragging the current down?

or do I have to find a light bulb with lower internal resistance to check this out some more.

BTW 14.5 V (battery) + 0.7V (diode mine is actually 0.5) + 2.2V (what is this figure)= 17.4 V well 17.2 for me explain that figure please.

terramir

ghurd · « **Reply #10 on:** April 10, 2007, 05:21:57 PM »

I'm not sure where to start.

I believe you are over-thinking it. A lot.

We do not know how far the wire runs.

A light bulb won't tell anything. They can do funny things used like that.

AWG #15 is listed as 3.2 ohms per 1000'. Don't forget wire goes out and back.

So figure how many feet of wire from the PVs to the battery, double it, calculate the resistance. Most connections have resistance too.

Quick thinking, I'd say you dont want to go more than about 25 feet from the PVs to the battery, if both PVs are going through the same wire you said.

The math is not as straight as it seems at first. It's not linear. I'd say forget about it, read and check the chart in this link.

http://www.windsun.com/Hardware/Wire_Table.htm

Then...

run down to the hardware store, get some sunlight resistant 12-2 w/g UF-B, and 3 water tight strain relief fittings for 12-2. That wire is a bugger to work with, but worth the effort. Run out of one PV, into the next, then out and down to the battery... so 3 fittings.

My self, I would move the diode to the battery. A big diode, like use 2 terminals of a 25 or 35 amp bridge rectifier. Or 3 terminals if you can figure it out so it uses 2 diodes in parallel. It will be easier when you decide to change over to a charge controller like an SG-4 (regular readers knew that was coming).

A lot of this is a gut feeling and experience.

I will post a sketch of the bridge using 2 diodes if you want it.

G-

terramir · « **Reply #11 on:** April 10, 2007, 05:41:56 PM »

yeah I can move the diode back down, lol it is actually two diodes both 6 amp 200V in parallel so 12 Amp max I can add another one for 18 Amp capacity

. And I can get more of these, I like these particular ones because they only have a .5V drop instead of .7 like most others. Think there made of germanium instead of silicon and they have no back flow at night like the dam shotky ones. (and well I get them from a place that sells cheap

. As for the wire I might have to beg a friend for additional wire right now and just parallel it no money right now me poor and all.

that would add up to 12 gauge being 4n mm^2 total my distance is about 50 to 60 ft total.

so yeah according to the table I should go with 12 gauge yeah and if I look at the table I am exceeding the length easy right now.

After I go to the friend and borrow some wire and run it in parallel I will be able to tell if that is actually my problem which I suspect it is.

terramir

PS: still would like to know where that 2.2V comes from

RP · « **Reply #12 on:** April 10, 2007, 05:49:57 PM »

"8 strands of 24 gauge"

When running parrallel strands to make a large conductor you run the risk of having one or more bad connections. This won't be obvious since the other strands will still conduct but with higher resistance.

I might be worth checking and/or resoldering the connections at the end of your cable.

ghurd · « **Reply #13 on:** April 10, 2007, 06:17:58 PM »

I do not understand where you measured the missing 2.2V. It's probably from the PVs being drawn down to the battery voltage, wire losses, both, or most of it doesn't exist.

Thats the way it works. It's not linear, and it gets worse working near the peak power voltage of the PV, which you certainly are.

The PVs can not be 2.2V above the battery V, with only 0.5V diode drop, and 0.8V line loss.

See Kirchoff's 2nd Law.

Schottky diodes should have no significant back flow at night. Like 'none'.

The 0.5V forward drop (my belief- Not everyone agrees), is because the diodes are operating far below their rating.

That's why I suggested 25 or 35A bridges. Plus they are cheap.

G-

Here's how to use bridges as diodes.

(This is not a substitute for somehow doubling the amp rating of the bridge.

Keep the amps below 1/2 of the bridge rating, and 1/4 is better)

TomW · « **Reply #14 on:** April 10, 2007, 06:22:31 PM »

G-;

Good points. Too little hard information and a good dose of over thinking the plumbing.

Nothing personal to the poster but these posts happen a lot. No data on what the setup is hardware wise and confusion by the poster on what constitutes "a problem".

Math is how we define things in the electricity game but getting hung up on it providing you exact figures for a given setup is folly. In my humble opinion the math only serves to get us in the ballpark on values and reading too much into non matching numbers that are slightly off will just consume energy. I seriously doubt if many consumer grade meters can be relied on to be within 10% and if you have 2 readings that are off 10% low your apparent error will be 20% [i think] so just keep that in mind when evaluating these things.

It is also very common for the newbies in electric power to not understand how much poor connections or small cabling can kill the collected power before it even gets to the battery.

Just the view from here.

To the poster: Good luck getting a handle on this stuff. It is not hard just a bit confusing at first. Be sure to check out the wiring tables posted in this thread for good insight on your cabling needs.

Cheers.

TomW

terramir · « **Reply #15 on:** April 10, 2007, 07:34:56 PM »

I was missing 2.2A inline not volts at a point in time, that's why I was asking where he got volts not amps. Anyway doing a little experiment that I won't get a result from till tomorrow since my panels at this time a night only deliver about .5A together. (Unless I build a platform that rotates with the sun well you get my drift).

I remembered I had two 50 feet rolls of cheap as crp speaker wire 24 gauge double stranded I added them in parallel I know that shouldn't help much however it should be significant enough to tell me for sure if the wiring is the problem. Since I can just add or subtract those 0.5 mm^2 while doing inline current measurements at the battery. I know that won't give me the full power by a long shot, however even a .2A increase consistently during high sun times (where the 2A mark is exceeded which seems to be when the trouble starts) during the day in measurements will confirm your and my growing suspicion that my wire is the problem. BTW that would bring me up to 14 gauge LOL.

Anyways grateful for the responses

terramir

Ungrounded Lightning Rod · « **Reply #16 on:** April 10, 2007, 07:43:32 PM »

Tomorrow at noon (or whenever the panels are getting lots of light) short (WITHOUT an ammeter in it) the drop wire at the battery end, climb onto the roof, and measure voltage at each of the panels. Then post that.

It will tell us a lot about the wiring voltage drop, which may help diagnose your problem.

terramir · « **Reply #17 on:** April 11, 2007, 03:36:11 PM »

with the wire shorted I get about 1.3-1.4V total (both panels) And About 0.68V- 0.75v and 0.60V- 0.65V with the diodes inline or removed didn't make a difference. Another observation, after I fiddled around with the wires I get about 3A down here it is a gain of one amp maybe when I fiddled with the wires I improved the connections a bit, Although I'd say that I'm still not getting what I should considering it's only one hour 30 min past solar noon in Los Angeles. and I suspect if I climb up there right now adding the panels up mathematically would get me 4 to 5 amps.

Thanks for having patience with me

terramir

PS: those 2 speaker wires (24AWG x2)X2 of those do make a difference albeit a very small one like +.02A - .05A consistently seems like that stuff is pretty crappy. Also letting ya know due to me putting those on there I got a real estimate of the wire length now it's like 80 ft. round trip.

terramir · « **Reply #18 on:** April 11, 2007, 06:04:56 PM »

Help I really dunno Why I got those numbers please let me know What that will tell you and how it will help me,

terramir

Opera House · « **Reply #19 on:** April 13, 2007, 05:19:01 AM »

What more is there to say, you have thin wire and poor connections. The math does add up. Solar panels are basically current sources, but the currents will not usually add up in parallel. As the panels approach the upper voltage the current will drop off. Print out a typical panel voltage current graph. And guess what, as the panels get hot the voltage also drops. Panel specifications are at conditions that almost never exist. If you want to get the maximum power the panels must be put in series to make 24V and do a PWM conversion at the power point to 12V.

terramir · « **Reply #20 on:** April 13, 2007, 06:47:34 PM »

this guy asked me to do those measurements, well I did and confusingly nobody has talked about those, Also from what I know of electronics currents are supposed to add when in parallel. so if I just hook one panel up and get 1.8A (inline not short circuit) and then seperatly hook up the other panel and get 1.5A and almost identical voltages ( well actually identical because the battery will keep the voltages constant), then the total should equal 3.3A

Now at times I get more from a single panel (one panel is capable of 3A inline the other 2.5A as far as I know), these are just examples.

I know I got wiring problems, but is that really all that is wrong considering the amount of loss really doesn't make sense to me I know it's not linear but it seems like the curve goes really steep after 3A (was 2A connection got fixed now it's 3A) so if the panels are together like 3A it gives me 3A but if there 5.5A I might get 3.10A A curve that goes that steeply up without noticable warming of the wires dun make sense to me.

terramir

ghurd · « **Reply #21 on:** April 13, 2007, 08:16:47 PM »

The power (amps) curves show a sharp drop. Like walking off a cliff.

You are operating in this area.

Meaning... Like 1/10 of 1V will make a large drop in amps. Very large. Huge.

Meaning amps at 6V is very close to amps at 10V. And 15V.

But amps at 19V is near nothing.

Maybe $500 worth of wire would save $20 in more PVs?

That's your call.

Find some charts for the max power output of PVs.

All decent PV makers supply them.

Look at the Difference between amps at 16V and 18V.

Compare that to whatever basic math you are using.

Your calculator is trying to make sense of nonsense.

I have installed 100's of PV systems like yours.

Use some big-ass wire. Then forget about it. That's the best you can do.

Trust me.

G-

terramir · « **Reply #22 on:** April 13, 2007, 09:39:57 PM »

ehhh that is my point the amps I was talking about is either one or the other panel connected to the battery through the drop wire, which means because the battery drags down the panel voltage to it's level the voltage was constant it was the open circuit voltage that is 0.1 different,

I was thinking at the time that one panel is (reverse) charging and damaging(heating) the other that's at first what I thought was going on I can't believe at 12 to 14 volts the difference between the mpp in the seperate panels to the charging voltage would make such a difference in current, especially because they were built with the same type (BRAND IF YOU WILL) cells. Unless the resistance in my drop wire is pushing the voltage up beyond the mmp I can only theorize that the drop wire is swallowing my current, however that is a point I need to look into but I highly doubt it.

I spent about 200 bucks on these solar cells plus about $150 in materials so far and about $140 bucks for two batteries, and I have two working panels. Can make two more full cell panels in good time. Then it will be time to see if I can build a patch panel with all those half to 2/3 cells I have left over. However I need materials I cannot afford at this time.

Until I can get about 50 feet of 12 gauge double stranded wire, I'm stuck with stop gap measures. I just added two more speaker wire pairs 18 gauge(cost 4 bucks at 99 cents store), just to see if this might improve things, Until such time where I can get my hands on some decent wire.

terramir

BTW: Anyone know WHY the dude asked for the short circuit voltages?

ghurd · « **Reply #23 on:** April 15, 2007, 06:24:15 AM »

How many cells per panel?

terramir · « **Reply #24 on:** April 15, 2007, 09:46:31 AM »

36 cells per panel one pael is like in two pieces so to speak at 32 cell plus an additional 4 cells in series, (that's from the time when I didn't know what I was doing). So 2 panels at 36 cells each.

terramir

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Author Topic: 0.1V difference between panels costing me 0.24A :S (Read 3973 times)