I'm mixed up on wattage from my panels....help

Blindowl1234Blindowl1234 Registered Users Posts: 43 ✭✭
edited February 2016 in Solar Beginners Corner #1
Still experimenting with my little solar setup for some Led lights. I've got a 100 watt panel and a 25 watt panel hooked to an old 12 car battery for now. Battery is old so I don't care what happens to it right now lol. I got one of those in line digital meters that measures watts, volts, amps etc...I thought I had it working right but something doesn't seem right. In full sun off both panels I get max watts of maybe 30. Battery is fully charged in about 3 hours which is fine. It runs about 20 watts of lights for 10 hours. I added another old 12 volt battery into the mix today, and noticed the watts shown on the meter doubled to just over 60 watts. How can that be if the watt meter is reading watts etc from the panels? I went back to one battery and the watts are showing 30 on the meter. As far as I know the meter is hooked up correct...maybe not. I'm sure the wattage reading is from the panels but why did adding another battery affect the reading? Doesn't make sense to me. Thanks in advance for what to do next

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 5,658 ✭✭✭✭✭

    Do you have a charge controller involved?

    Regardless, batteries approaching fully charged will only accept so much current.

    Typically a battery will accept as much current as it can when deeply discharged, as it approached fully charged 80-85%, the voltage will 'run away' a charge controller will limit the voltage to a set point while the battery slowly reduces the amount of current it will accept. Once it is sending very little current to the battery the charge controller will reduce the voltage to another set point only allowing the current to rise to compensate for loads.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    PW has a point. For me, id say that what youve written is hard to understand possibly explains why you are mixed up.
    Still experimenting with my little solar setup for some Led lights. I've got a 100 watt panel and a 25 watt panel hooked to an old 12 car battery for now.
    So both panels are 36 cell mono or poly panels? And they are connected in parallel?
    Battery is old so I don't care what happens to it right now lol.
    Thats fine, just the usual caveats apply.
    1. Old or new, any battery has the potential to bite you. Take care and ensure safe wiring and fusing at all times, and work in a non flamable area.
    2. Car batterys are not designed for more than a couple dozen deep cycles. So expect to kill them.
    2. An old battery can sometimes be difficult to learn on, as it behaves strangely. But sounds like it has some capacity so prob all good there.

    I got one of those in line digital meters that measures watts, volts, amps etc...
    So we are talking about a DC wattmeter, along the lines of those made by RC electronics? Where is it connected, between the PV and CC or the CC and battery, or elsewhere. Being specific, helps us help you.
    I thought I had it working right but something doesn't seem right. In full sun off both panels I get max watts of maybe 30. Battery is fully charged in about 3 hours which is fine.
    You need a charge controller, which one are you using? Under ideal conditions during the bulk stage youd expect to see: Power delivered to the battery =125Wp*0.77= 96W. But talking into account: voltage mismatches, season, wiring, panel angles etc  being non ideal, then expect less, maybe 2/3s.
    It runs about 20 watts of lights for 10 hours.
    Do you mean:
    a) thats what i run every day and its all good
    b) under a load test situation, i can do this before the battery dies on me
    c) something else?
    I added another old 12 volt battery into the mix today,
    Have you done the design math to determine the correct ratio of PV to battery? Random actions = random outcomes. For example: for a 0.1C charge rate, 125Wp/12V*0.77 = 8A, thus aprox 80Ah battery required.

    and noticed the watts shown on the meter doubled to just over 60 watts. How can that be if the watt meter is reading watts etc from the panels? I went back to one battery and the watts are showing 30 on the meter. As far as I know the meter is hooked up correct...maybe not. I'm sure the wattage reading is from the panels but why did adding another battery affect the reading? Doesn't make sense to me. Thanks in advance for what to do next
    The short answer is that you do not PUT power into batterys, but batterys accept power, when they need it and how much they need at a given point in their charge cycle, and life.

    As a rule, a battery will suck up all you can throw at it during bulk, and require less and less as the battery exceeds 80% full. Thats assuming a CC is limiting the current post bulk. If you are playing around without a CC, then what will happen is:
    - when the battery is still low SOC, the battery will hold the PV down below its peak Vmp, and current be will somewhat less than peak watts.
    - as the battery voltage rises you reach 14.8V or so which is the normal safe end of bulk, and as there is no controller, it just keeps climbing, rising along the way to the Vmp of the panels, usually around 18V for a 36 cell panel.
    - at this point you will have some serious bubbling going on, and the battery will get really hot.
    - if left unchecked, the voltage will keep on rising until it nears Voc, usually around 23V. The current will finally reduce to a low level because the panels can no longer supply much power once they exceed Vmp. At Voc the current is by definition zero, but in practice itll hover just below, massively overcharging a 12V battery to 22V.

    Now, thats a healthy battery. A sulphated, abused, or otherwise old battery will behave slightly differently. It will appear to charge quickly, come up to voltage just fine, then taper its current acceptance back to very little. Youll go, great its charged. But when you try to load it the voltage drops just as quick as it came up, and its time to trade the core.

    Aint it fun.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • Blindowl1234Blindowl1234 Registered Users Posts: 43 ✭✭
    Thanks for the replies. Yes I'm using a typical $20 PWM 30 Amp charge controller which seems to be working ok so far. Zone Blue asked "So we are talking about a DC wattmeter, along the lines of those made by RC electronics? Yes that's correct. I guess my confusion is that I'm trying to see the total watts the two panels are producing at any given time of day. I realize that I'll never see a full 125 watts so I figured maybe 90 or so...sorry a wild guess there. The most I've seen on the dc watt meter is 49 watts. I guess in the long run so long as a battery is charged and the lights are doing what I need that's a plus lol. So long as I get at least three to four hours of halfway decent sun the lights run 10 hours no problem. If the next day is cloudy well I get about 6-8 hours out of the battery. I've not noticed the battery drop below 12 volts as yet. So if I'm understanding correctly as the battery reaches full charge then the watts showing on the dc watt meter display are reflecting that? I thought the watts shown on the watt meter reflected the watts being produced by the panels. That's what I was hoping the display was showing anyway. Yes all this solar stuff is interesting but a real learning curve is involved that's for sure. Also panels are 36 cell mono. The 100 watt is a Grape solar panel. The battery which is actually five years old has done well for over a month except when temps got to zero degrees outside. The set up is in an unheated garage as well. Since everything appears to be working at least as good as I expected in keeping the lights charged, I'll purchase a deep cycle battery soon.  Guess the main question is what I'm seeing on the watt meter is that the actual watts coming off the panels themselves. Thanks again for all the help. Sorry for the confusion too
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    edited February 2016 #5
     Yes I'm using a typical $20 PWM 30 Amp charge controller which seems to be working ok so far.
    Getting info outa you guys is like pulling teeth :) Which one?

    Zone Blue asked "So we are talking about a DC wattmeter, along the lines of those made by RC electronics? Yes that's correct. I guess my confusion is that I'm trying to see the total watts the two panels are producing at any given time of day. I realize that I'll never see a full 125 watts so I figured maybe 90 or so...sorry a wild guess there. The most I've seen on the dc watt meter is 49 watts.

    We still dont know where you inserted it. But being PWM, the current on both sides of the controller is the same, thus either side the power throughput would be the same. Anyway the answer to your question is twofold. First  to understand how current reduces post bulk digest this (real actual pencil) graph (couldnt find a decent version of this online, and im bedbound with appendicitus):

     

    See how once you go past (about) 80% SOC the battery accepts less and less current? Thats the first possible explanation. Thus adding a second battery, provides two lots of current suckers, allowing the controller output to rise.

    The second possible reason is to do with the the real world reality of PV in winter, not facing the best possible angle, not at the best possible time of the day, etc etc etc. For the FAQ version of "why does my 100W panel not produce 100W" see http://forum.solar-electric.com/discussion/comment/354388#Comment_354388
    I guess in the long run so long as a battery is charged and the lights are doing what I need that's a plus lol. So long as I get at least three to four hours of halfway decent sun the lights run 10 hours no problem. If the next day is cloudy well I get about 6-8 hours out of the battery. I've not noticed the battery drop below 12 volts as yet.
    But what do you mean by "i only get 6-8 hours"?  Does the load disconnect on the controller kick in? Or you just turn it off when you think its getting low? If so how do you assess that? Being clear = being clear.

    But if the battery never drops below 12V then youre not going too low SOC really. Low for 12v battery is like 11V. But your wise to try to keep the battery cycling in the band between 75% and 100%.
    So if I'm understanding correctly as the battery reaches full charge then the watts showing on the dc watt meter display are reflecting that? I thought the watts shown on the watt meter reflected the watts being produced by the panels. That's what I was hoping the display was showing anyway.
    The wattmeter is measuring the current flowing through the itself, and the voltage on its input side and multiplying them. Its a circuit right? Its in the same series circuit as everything else. If theres no current in one side of the circuit it wont be on the other side either.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • Blindowl1234Blindowl1234 Registered Users Posts: 43 ✭✭
    edited February 2016 #6
    Zone Blue, It's a Chinese charge controller SL02 which I determined today isn't all that great. Time for a real charge controller with a nice display lol. If I get a decent day of sun lights run 10 hours no problem. If the second day is cloudy, then that night after 6-8 hours the lights go out the load/low voltage disconnect on the charge controller kicks in at 11.98 volts. So it appears that the system and charge controller is doing what its supposed to. I just wanted to be able to see want it was doing...Some kind of readout. I got out my DMM today and total watts off the panels was 74 watts. I'm happy with that reading. Evidently the DC wattmeter won't read and or show the amps or watts, the panels are producing. I'd like a charge controller with a display that shows battery volt's, panel amps, panels watts etc...So more or less problem solved the panels are doing what they're supposed to do at 74 watts on a sunny day in winter.
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