panel output problem/question

[defed]

i have a pair of 175w panels, rated at 35v @ 5 amps (loaded). in the late day sun, on a single panel, i get 120w...approx 35v @ 3.5 amps (into mppt). this is the same for both.

when i series them, i get 2x the voltage and half the amperage (equal wattage). something is wrong here, but i can't understand what. the leads from the panels go into a combiner box, which at this time, they are the only panels in use. from the box, it goes to a fused switch (+ side), then to the controller.

if each panel is correct on its own, what am i missing when i series them?

[riahserf]

If you have them in series, the voltage will double, wattage will double... but the amps should stay the same. According to the figures you gave, in series you should have 70volts @ 3.5 amps... so your wattage should be 245 ish....

[defed]

correct, that's what i expected to see...

after some more testing of each panel vs series...i think what it is displaying is probably correct.

xantrex xw-mppt

each panel on its own:

input (from array) = 32v @ 1.9 amps 61w
output (after mppt) = 26.4 @ 2.4 amps 62w

series:

input = 72.3v @ 1.4 amps 104w
output = 27.9v @ 3.7 amps 103w

i was expecting to see 120w and 1.9 amps in series.  at 72.3v, 1.4 amps is 101w and 1.65 amps is 120w.  i don't get home from work until my prime sun is passed, so the readings on the controller tend to not maintain rock solid stability.  a change in .25 amps isn't much, and accounts for 20w...

later, i tried it again.  series panels had 60w @ 1.1 amps, while one panel was 30w @ 1.1 amps.

i could swear i was getting almost the same wattage from one panel as in series earlier...hopefully this wkend, i can test them during prime hrs, where there may be less fluctuation in power and see what i get.

[Volvo farmer]

That that controller limits the power going into your batteries, especially in the late afternoon when they are likely through the absorb stage and into float.  The only time you can expect to see full panel power being utilized is in the bulk stage.

Do you understand the difference between bulk, absorb and float?

[defed]

i more or less understand the charge stages, but what does confuse me is that it stays in absorb mode when the batteries are quite low.  they are some old batteries that i am using on my test turbine, so i figured i'd connect them to the solar as i experiment w/ the system.

the small turbine might charge once a wk in the calm summer, so the batteries tend to drop to about 19.5v (i told you they were bad!).  i figured that would be bulk charge, but it evidently it is enough to trigger absorption.

also, i have to take the panels/controller inside when i'm not home (at this point) so on wkdays i only get to have them out well after prime sun (4-6pm).  hopefully this wkend we get some good sun so i can leave them out for the good sun.

[SparWeb]

Hi defed,

You will see something similar to what I do - I have solar panels and wind and they are on separate controllers.

On a normal day with no wind, in the morning the sun hits my panels for the first time.  The charge controller measures battery voltage and sees some drain-off of energy overnight has taken them down to 25V or so.  The controller allows full current to flow from the panels.  As the morning progresses, the current rises as the sun lines up on the panels, so it goes rapidly from 0A to 4-5A in an hour, then creeps up to 7A or more by noon.  Depending on the state of charge of the battery, the controller will find that this much current raises the battery voltage upwards of 27 volts or more.  Once it reaches 27.4 or so, the charge controller has to start doing its job, pulsing the current on/off so that the voltage doesn't rise any higher.  Once it starts doing this the full-sun power on the panels will be "regulated" down.  The average current will get lower and lower.  For the rest of the day the fully-charged batteries will stay at 27.2V and need less and less current to stay at "float".  By about 6PM when I get home from work, there's usually only about 1A going through.  That's partly from the controller regulating the current and due to the low angle of the sun.  In total about 20-25 Amp-hours are required a day for this, so it's a cumulative sum of the peak current in the morning, then low current for the rest of the day (less than 10 hours long now).

Batteries really like to have a sustained float charge on them all the time.  Basically that's the reason I got my solar panels - too many days with no wind when I would start fretting about the weak batteries.  Yours would be happy if you left the panels to charge them every day, too. 

[Ungrounded Lightning Rod]

i more or less understand the charge stages, but what does confuse me is that it stays in absorb mode when the batteries are quite low.  they are some old batteries ...

Perhaps they are somewhat sulfated.

Sulfation raises the resistance of the batteries.  That causes their voltage to be higher when charging, lower when discharging.  The higher voltage on charging may be fooling the controller into treating them as nearly charged even when they're quite low.

Let 'em float for a month or so and perhaps cycle 'em a couple times and see if it clears up.  Or get new batteries and see how the controller does with 'em.

[defed]

i'm sure my batteries are shot.  they were in a hand cart at work used to push heavy loads...and the idiots there would basically run them dead before recharging.  they aren't even true deep cycles (marine)...they've been abused badly, but i was hoping to get a cpl of decent ones out of 6.  i was just using them as a stop gap for the turbine, and decided to put the solar on it for now too.

at this time, i can't leave the panels on charging all the time because it's a temp setup and i need to make an enclosure for the controller.

let's presume that some of the output weirdness is due to funky batteries....this should have no impact on the input readings?  or...is the input amps actually affected by what the batteries can take?  i was under the impression that the input was the input...and the output varied on what was needed...but after reading and re-reading some stuff...i could be wrong.

[Ungrounded Lightning Rod]

let's presume that some of the output weirdness is due to funky batteries....this should have no impact on the input readings?  or...is the input amps actually affected by what the batteries can take?

Very much the latter, as far as amps are concerned.

Energy is conserved.  If the controller ate all the power the panels could provide and put out only the power the batteries can take, the difference would be turned into heat in the controller.  This is not cool (pun intended).  So they run as switching regulators and (in this case) drop the average current taken from the source.

(For solar panels they could also short the panels intermittently as an alternative.  Then the average voltage would be what dropped.  Either way the unused energy ends up heating the panels and (in the "don't pull the current" case) causing them to emit infrared photons you can't see.)

[defed]

Perhaps they are somewhat sulfated.

Sulfation raises the resistance of the batteries.  That causes their voltage to be higher when charging, lower when discharging.  The higher voltage on charging may be fooling the controller into treating them as nearly charged even when they're quite low.

this seems to be true. i put the controller into equalization mode, and sure enough, my 2 panels went from 120w @ ~ 1.5 amps to 280w @ ~ 4+ amps.  this resulted in a controller output of ~ 30v @ ~ 9 amps into the batteries.  so, the controller is cutting my panel output because it thinks the batteries don't need the power!

i had my parents stop at sam's today when they were out and they got me some battery prices.

[Ungrounded Lightning Rod]

i had my parents stop at sam's today when they were out and they got me some battery prices.

When you go to pick up the batteries, take a digital volt meter with you.  Get a set with the highest voltage you can find.

(No point in replacing sulfated batteries with new ones that were allowed to sit idle, self-discharge, and sulfate a bit before you even got there.  B-b  )

[defed]

i think i am going to get some soon.  been messing w/ my old junkers today with GOOD sun.  did some short circuit checks on the panels, checked out at 5+ amps like they should, so then i put the controller into equalization mode again at around 12:30pm and was kicking out over 300 watts at 5 amps from the panels.  a tad lower wattage than they guarantee (159w guarantee per 175w panel) but there was some wispy clouds so that cut it down a tiny bit.

then i was adding some loads to the batts.  a pair of 12v garden tractor headlights.  either 35w or 50w not sure.  this let the controller stay in float but still allow the panels to actually output some power.  they would drop down to less than 100w @ 1 amp in full sun on float.  i added a second pair of lights and still plenty of juice to hold in float.  i disconnected the panels from the controller to do the short circuit test and left the lights on.  after i hooked it back up, the charger went into bulk mode and put over 10a into the batteries.  i think one battery is fairly good, but the other has 2 bad cells...they barely even bubble in equalization mode while the other 10 percolate very nicely.  i guess this is why the bank usually 'holds' at 19v when not charging (~2.25v per cell?).

i do need to calculate how many batteries i want.  i think right now, with 700w solar and the small 4' turbine, 24v @ 220ah is probably more than enough.  6 hrs insolation in summer = ~3800wh output = 136ah (636w x 6hrs / 28v), 2 hrs in winter (BLAH!) = ~1300wh = 45ah.  this also not counting inefficiency losses.  the turbine should actually do quite well in the winter, but 'good' for a 4' isn't much!  plan to get a 10' up next year for some real output.  if my math (and method) is right, 220ah is still probably more than i can reliably charge in winter...so 440ah (2 sets of 4 batts) will be way too much.

[defed]

i guess i made an error in my battery calculations...a 220ah bank won't need 220ah to recharge as it would not be drained 100%.  i think i could maintain a 440ah bank if discharged 20%-30% (88ah-132ah) with just the solar panels let alone w/ the turbine.  if all works well (which it never does), and this winter is like all the others since i've lived here, the turbine will be cranking most of the night which should leave the solar to top off.  in the summer, the turbine doesn't run as much but there is alot more solar.  granted, this would all change quite a bit w/ a 10' mill on a 50' tower vs my 4' test model on 18' tower.

[B529]

i think i could maintain a 440ah bank if discharged 20%-30% (88ah-132ah) with just the solar panels let alone w/ the turbine. 

A 440AH bank @ 24vdc would be a little big for two 175 watt panels. Say you are getting 10 amps out of your PV, that's only a 1-44 charge ratio. I shoot for a ratio of 1-20 to "stick it" to the batteries.

 

[Ungrounded Lightning Rod]

  i think one battery is fairly good, but the other has 2 bad cells...they barely even bubble in equalization mode while the other 10 percolate very nicely. 

Or maybe the battery is out of equalization.

I'd try putting it in equalization mode until I'd pushed about twice the rated amphours into the battery and then check whether the weak cells are gassing.  If they get to the same amount of gassing as the others they may be not all that bad.  Then I'd let it sit for a couple days in float mode then switch to equalization again.  If the questionable cells aren't gassing after a couple amphours they're self-discharging excessively and the battery is dead in the "flakes from the plates make a high-resistance short that discharges the cell" mode.

(However, given the age of the cells and the refusal of the controller to do the equalization automatically I still think the problem is sulfation.  Even if the original difference in leakage was minor and wouldn't have been an issue if the battery had been equalized regularly, the lack of equalization will have left the low cells low for a long time and sulfated them.)

[defed]

A 440AH bank @ 24vdc would be a little big for two 175 watt panels. Say you are getting 10 amps out of your PV, that's only a 1-44 charge ratio. I shoot for a ratio of 1-20 to "stick it" to the batteries.

i have (4) 175w panels, i am just using 2 in my temp setup because i have to move the stuff in and out when i use it.  there will be 2 strings of 2 rated at 350w (about 320w  on mfg guarantee) @ 5a, so total panel output would be 350w @ 10a when 2 strings are paralleled.  thru mppt, this can be up to 25-30 amps into the batteries under ideal conditions.  also have the 4' turbine, which in summer hasn't provided much, so i can't say what it will add in the windy winter.

i just didn't want a 220ah bank and have it be too small and try to add another 220ah next year (with 10' turbine) but i also don't want 440ah now and not be able to maintain it.  for now, there will be no real draw on the batteries.  i plan to build an insulated box for them and just run a bulb or some resistor to 'heat' it w/ the power.  and maybe a small vent fan.

ran out of time to wire it up for actual use, but i still want to get the stuff up and running to monitor production that can be expected, especially the turbine.

[defed]

Or maybe the battery is out of equalization.

I'd try putting it in equalization mode until I'd pushed about twice the rated amphours into the battery and then check whether the weak cells are gassing.  If they get to the same amount of gassing as the others they may be not all that bad.  Then I'd let it sit for a couple days in float mode then switch to equalization again.  If the questionable cells aren't gassing after a couple amphours they're self-discharging excessively and the battery is dead in the "flakes from the plates make a high-resistance short that discharges the cell" mode.

(However, given the age of the cells and the refusal of the controller to do the equalization automatically I still think the problem is sulfation.  Even if the original difference in leakage was minor and wouldn't have been an issue if the battery had been equalized regularly, the lack of equalization will have left the low cells low for a long time and sulfated them.)

actually, the controller never does equalization automatically, strictly manual override.  but, the controller rarely ever goes into bulk mode automatically, only absorption then float...even tho it should bulk at 20v!  like you said earlier, the higher charge voltage required must be fooling the controller.  i would do as you suggest, but i can't at this time because i can't leave my equipment out yet until i get some other things finished.  but, i did pre-charge the batteries w/ a regular charger before i put them out.  if i recall, all of the cells in both seemed equal in gassing at that time (i cycled them as you suggest, some good gassing charging and extended low amp float charge), which is why i used them.  granted, the specific gravity readings were pretty bad, which is why i figure they are just plain shot.

[Volvo farmer]

i have (4) 175w panels, i am just using 2 in my temp setup because i have to move the stuff in and out when i use it.  there will be 2 strings of 2 rated at 350w (about 320w  on mfg guarantee) @ 5a, so total panel output would be 350w @ 10a when 2 strings are paralleled.  thru mppt, this can be up to 25-30 amps into the batteries under ideal conditions.  also have the 4' turbine, which in summer hasn't provided much, so i can't say what it will add in the windy winter.

i just didn't want a 220ah bank and have it be too small and try to add another 220ah next year (with 10' turbine) but i also don't want 440ah now and not be able to maintain it.  for now, there will be no real draw on the batteries.  i plan to build an insulated box for them and just run a bulb or some resistor to 'heat' it w/ the power.  and maybe a small vent fan.

I'm going to disagree with that other fellow who says 350W of solar isn't enough on 440Ahr of battery, especially since you say you are not planning on having many loads right away.

Everything I have ever read about batteries says that it's not a real good idea to mix old and new ones.  If I were in your situation, I would try and buy my batteries all at once.

Just a data point for you. I ran a whole off-grid house with four 170 watt panels and 800Ahr of battery for a year.  Of course things work much better now with eight 170 watt panels, but I was getting a 2 hour absorbtion and seeing the charger drop back into float on every sunny day.

Lastly, I would nix the battery box heater idea. The only thing that suffers when batteries are cold is their capacity. In my opinion, it is a waste of your precious electrons ruining a resistive heater in a battery box, just so you can realize an extra 20% capacity.  If you are really cycling batteries hard enough that you need that 20%, they will self heat to some degree through the charging and discharging process.  

[Rover]

I agree with Volvo Farmer.

I have ~400W of Solar, and a cpl of of wind turbines (don't add much). I have a ~ 690 A/H battery bank composed of 6 matched G27 batteries (el cheapo) . My average daily usage is ~ 350 W/H ,  average daily charge is ~ 450 W/H .  I also have a 40 amp marine battery charger that I use on occasion.  The bank is about 2.5 yrs old, and shows no sign of deterioration .

Its just a plan of usage based on charging capacity
 

[defed]

Lastly, I would nix the battery box heater idea. The only thing that suffers when batteries are cold is their capacity. In my opinion, it is a waste of your precious electrons ruining a resistive heater in a battery box, just so you can realize an extra 20% capacity.  If you are really cycling batteries hard enough that you need that 20%, they will self heat to some degree through the charging and discharging process.  

i should have mentioned that the controller also has to be in an insulated box, and that, i think, definitely needs a heater.  i would like to keep the controller compartment somewhere around 50F.  the specs say it will operate in the range of -4F to 113F, but i'd rather keep it 'warm'.  i also want to have SOME load on the batteries so i can monitor performance of everything.  if they don't have a load, they won't discharge much and there will be no output, especially from the solar.  the controller has a aux output than can run a diversion load when the batts are charged.  i think i could use this easily enough to run a 35w bulb for heat and an exhaust fan for the batteries.  it can be set for various triggers, but only one can be used at a time.  not sure how i would run the 'heater' when the batts are not diverting.  possibly a timer for set time on/off, or a thermocouple to open a relay.

[defed]

Its just a plan of usage based on charging capacity
 

right, that is one thing i forgot when i made my initial comments on the bank size...i was using the full amount of capacity needing to be generated every day, not just the amount used once fully charged.

[wooferhound]

I think you can have a really big battery bank even when you have very small charging capacity. My little solar power system is only 80 watts (6 amps) feeding a 225 amphour battery and it's working wonderfully.

[defed]

this is probably true as long as you don't draw off more than you can produce to keep the charge up.  if you draw 50% of your 225ah battery, you would have a hard time re-charging it w/ the 80w.

[ghurd]

We ran the solder shop with 85W and 220AH.  It was OK, but not enough reserve power during winter (>0.5H/D).
Added another 220AH and it was a lot better.

Later added another 85W and another 220AH, for a total of 170W and 675AH, and that works great 11 months a year.
Later added MPPT.

In the winter, usually add another 110AH, for a total of 170W and 785AH.
Might add another 32W or 60W panel if available.

The Big battery bank can take all the power when it is available, and has the reserve to get past that 4~6 weeks without going LVD.

85W and 220AH would get to regulation voltage pretty quick, even with low batteries, so a lot of the available power on the rare sunny day was not being stored.

All 12V numbers.
G-

[defed]

got my new battery bank installed finally, and the panel output was far more to my liking.  was getting over 500 watts @ 8 amps (700w @ 10a rated) from the panels which was over 17 amps (mppt) into the batteries in bulk charge.  this was early morning sun, and then the rain rolled in, so that pretty much put an end to the output.

just goes to show bad batteries have more ill effects than just poor storage capacity!