Trying to find the problem in the system...

js_70h
js_70h Registered Users Posts: 5
So I'm living at a friend's place right now, on their off-grid system. This system has been in place for about 5 years, and never really seemed to run properly from the beginning. It turns out it was installed by someone with less expertise than they claimed, but we haven't been able to figure out what's wrong.

The problem is that we don't get nearly as much power out of it as we should.

The system is composed of:

780W of solar panels
5kW backup gas generator
Xantrex 4024 inverter
Xantrex C45 charge controller
14 Concorde PVX-2580L batteries (255 Ah each)

The batteries are wired in series/parallel as seven 24V units. After a routine two-hour charge via the generator (during which the inverter went properly through the bulk, absorption, and float stages, the voltage dropped immediately from 26.8v (generator on/float stage) to 25.0v (the moment the generator was turned off), then within 10 minutes to 24.4v, then within half an hour to 24.2.

We took all the batteries out and drove them to the local car parts place to be tested on their battery tester, and they all passed as good batteries, each sitting at about 11.95V and 130Ah (we pulled them out in the morning, before any significant charging took place).

We have noticed that according to the inverter, the generator only seems to be putting 13-15 amps into the battery bank while charging, and yet claims to have the batteries fully charged in two or three hours. The charge controller is set to bulk of 28.0 and float of 27.2, the inverter has bulk at 28.4 and float at 26.8 (though we have never in memory seen it above 25.6, and that's while the generator is running.

Can anyone assess what is wrong with this system? We think the problem lies in the generator not effectively charging the batteries for some reason. There isn't enough PV input to recover from heavy usage, and the generator just doesn't seem to be able to put much in either - the inverter claims that it's full after just a couple hours, after having the bank at 23.8 volts or less.

Any help would be much appreciated (and I would be happy to supply any necessary numbers or run any tests on things)!

Comments

  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Trying to find the problem in the system...

    Do either of the battery chargers have temperature sensors?
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Trying to find the problem in the system...

    At first glance this sounds like a case of "too many batteries, too little PV". As a result, possibly all the batteries have suffered. If I read your numbers right, your total bank capacity is 1785 Amp/hrs. You'd need a lot of wattage to charge that. If I did the math right you'd need more than 2000 Watts of panels minimum. A 700 Watt array would be good for about 600 Amp/hrs max. (The experts here will be glad to do the accurate calculations I'm sure - they're very helpful! :D )

    AGM batteries are very picky about being recharged properly.

    A 12 volt battery should test 'good' at 12.5 V, not 11.95. I suggest you re-charge each battery individually, let it sit 24 hrs, and check the voltage again. If there's something wrong inside (like a collapsed plate) then that battery would appear to charge yet start losing voltage immediately. One bad one in the bank will draw the rest down.

    Did the system ever work right?

    Maybe you want to check the rest of the system: gen output, wire/fuse sizes, et cetera.
  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    Re: Trying to find the problem in the system...
    We took all the batteries out and drove them to the local car parts place to be tested on their battery tester, and they all passed as good batteries, each sitting at about 11.95V and 130Ah (we pulled them out in the morning, before any significant charging took place).

    I had the exact same symptoms with my old "BAD" batteries. Each battery should charge up to the charge voltage then after a while the surface charge will bring them down to around 12.6 volts in a few hours with new batteries and no load. The real test is to put a load on them and follow the voltage curve down. This is the only effective test of battery capacity. Check this against the manufacturers curves.
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Trying to find the problem in the system...

    I suggest you read the following links,,, I think you have been chronically undercharging and now they are done!

    http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries
    http://www.batteryfaq.org/

    Good luck,, welcome to the forum and keep us informed.

    Tony
  • js_70h
    js_70h Registered Users Posts: 5
    Re: Trying to find the problem in the system...

    I read those links, but didn't see anything definitive in them.

    I don't understand what dictates the relative size of a battery bank and its associated PV setup. We don't use much power here; I would think that a big battery bank would be better, since it wouldn't be drawn down much, thus extending the life of the batteries.

    I will go ahead and do a more formal load test on one of the batteries.

    There is a temperature sensor on one of the batteries which is plugged into the inverter.

    The one thing that makes me not suspect the batteries necessarily is that the PV panels seem to do a much better job charging the bank that the generator. A good sunny day produces noticeably more power than several hours of running the generator, which doesn't make sense to me. The system has never worked like it should, but early on the owners noticed that if the system was filled up by solar, it could run for most of a week through clouds, like it was supposed to. Charging with the generator would barely keep things running overnight.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Trying to find the problem in the system...
    js_70h wrote: »
    The system has never worked like it should, but early on the owners noticed that if the system was filled up by solar, it could run for most of a week through clouds, like it was supposed to. Charging with the generator would barely keep things running overnight.

    That's the first clue the PV side is underpowered. You need more PV, you aren't getting a full charge in the batteries. I have no idea what the inverter charger's settings are (I'm assuming the 4024 is an inverter charger combo) What's it charger rating?
    If it's Power Factor is less than 1, it will be wasting the generator power, and maybe overloading it.
    A 5KW genset can deliver 4KW for extended periods, 4KW is about 140A @ 28V, so there should be plenty of power there, if your charger can handle it.
    AGM batteries are somewhat resistant to sulphation, so if you can reduce the loads for a couple of days, and let the sun and genset charge the batteries, you may be able to bring them back.
    You also are likely to have a wiring mess with all the series/parallel connections, and this site may have some insight on optimum wiring practice by helping share the loads better:

    http://www.smartgauge.co.uk/batt_con.html

    Also some other links there you may find useful too. (battery charging)
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • mountaintop
    mountaintop Registered Users Posts: 16
    Re: Trying to find the problem in the system...

    It sounds like you have 2 problems the batteries and the settings in the inverter when the generator is running.

    With a battery bank of 1785 you should have around 2000+ watts of PV. I don’t see how your batteries got to float very often and this caused then to sulfate. I would put money on the batteries not being equalized properly.

    The second issue is that the generator and the inverter. It could be the wiring or the charge settings. IMO this should be the first thing that gets fixed. A lot of inverters have issues with generators under 7 kw (I’m not talking about generators the use inverters).

    I would then use the generator to equalize half of the batteries and then let them sit over night to see if this helps with holding a charge.

    Mike posted when I was typing :roll:
  • js_70h
    js_70h Registered Users Posts: 5
    Re: Trying to find the problem in the system...

    Quick question - in the Xantrex inverter settings, what (roughly) should the max charging amps (from the generator) be set to? Ours is set at 15, which seems pretty low.

    (Edit)

    Second question - when calculating total battery capacity for the sake of setting things like max charging amps, what capacity do you use? Rated capacity from the manufacturer, 50% of that to allow for safe discharge levels, or something else entirely?
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Trying to find the problem in the system...
    js_70h wrote: »

    14 Concorde PVX-2580L batteries (255 Ah each)
    The batteries are wired in series/parallel as seven 24V units.


    Quick question - in the Xantrex inverter settings, what (roughly) should the max charging amps (from the generator) be set to? Ours is set at 15, which seems pretty low.


    Second question - when calculating total battery capacity for the sake of setting things like max charging amps, what capacity do you use? Rated capacity from the manufacturer, 50% of that to allow for safe discharge levels, or something else entirely?

    255A x 7 = 1575AH
    Battery chart http://www.sunxtender.com/xtender_main.php
    From our host's battery FAQ:
    http://www.windsun.com/Batteries/Battery_FAQ.htm#Battery%20Charging
    The Concorde AGM batteries are a special case - the can be charged at up the the Cx4 rate, or 400% of the capacity for the bulk charge cycle. However, since very few battery cables can take that much current, we don't recommend you try this at home. To avoid cable overheating, you should stick to C/4 or less.
    C/4 on your entire bank would be : 390A for the Bulk part of the charge. Yow !
    C/8 is nearly 200A. I don't think your inverter/charger can handle even that.
    780W of PV should yield 24 Amps, (after de-rating). You are WAY under charging.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • nigtomdaw
    nigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Trying to find the problem in the system...

    Pls assuming your inverter is a SW4024 by Xantrex a 5 kw generator should easily cope with 70 80 amps dc output to the batteries, at 24 volts nominal system voltage I have a 5kw gas gen that can sustain 90 amps at 24 volts plus Idont know what reading your getting from the inverter to say only 15/17 amps going into the batteries as it doesnt have a dc amps out read out on its display.

    Invest in a Dc Clamp meter and start taking some reading when the inverter is charging the batteries .

    If your inverter is going from bulk to absorb then float and you never seen the batteries above 25.6 volts on your batteries when the generator is running check your bulk absorb settings

    Also check DC connections for good contacts no loose terminals corrosion, usually manifesting as heat at terminal joints.

    I have a battery bank twice the size of yours run a American Fridge freezer and with a similar inverter SW3024E can replace a no sun days energy use in 3 hours with a 5kw genny putting in 80/90 amps initially.

    Nigel
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Trying to find the problem in the system...

    js-70

    Bigger batteries are better,, in that you discharge them less. However,, unless you are able to bring them to 100% every couple of days, they will slowly discharge faster than you can charge them if you don't have enough Pv/charge capacity.

    Remember that all batteries have a self discharge,, the bigger the battery the larger (net/net) self discharge.

    For example if you have a huge battery and you use say 1kw of power/day,, but you have .5kw of self discharge since the battery is so big, you have to recover a minimum of 1.5 kw, plus charging efficiencies just to stay even.

    If you had a more reasonably sized battery, used the same 1kw,, but only had a self discharge of .05 kw, you would only have to generate 1.05kw plus system loses to stay even.

    On an other note. I down sized my battery bank from 4 L-16 batteries to 4 t-105 to keep them in the right range. The short term benefit was cheaper cost of buying the batteries,, but also way cheaper in the long run. I calculated if I got 5 years from the T-105's as opposed to 8 years for the L-16 I was way ahead on a per kwh basis. Time will tell if I was right. My original string of 2 T-105s is still going on standby service after 10 years.

    Icarus
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Trying to find the problem in the system...

    WELCOME , you have a good puzzle.
    According to Concorde

    "Initial charge or recharge: 2.37 to 2.40 volts per cell at 25 C (77 F). Float charge: 2.20 to 2.22 volts per cell at 25 C (77 F). Equalize charge: 2.40 volts per cell at 25 C (77 F). Temperature compensation = ±3.75 mV. per cell per degree C [Reference to 25 C (77 F)]. This is for battery temperature (not ambient temperature) and is useful for battery temperatures from 0 C (32 F) to 40 C (104 F). Contact Concorde Battery Corporation for temperatures that exceed this range"

    I assume that your batteries are similar to mine (AGM) in that they have 6 - 2v cells so then charging should be done @ 14.22 v to 14.4 V per 12 v battery, with the EQ charge at 14.4 as well.

    So for your setup you need to be at 28.44 V the 28.8 V

    your charger should be able to handle the amperage needed for 24 v charging as this - http://www.xantrex.com/web/id/43/p/124/pt/4/product.asp - indicates an output of 110 amps on AC

    "Continuous Charge Rate @ 120 VAC input 110 amps DC"

    and all you wanted to know, and more, is probably here
    http://www.sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf


    AND at the bottom of section 5.1 they give an example that almost fits your case to a T,

    258 Ah battery at 75% SoC will take 6.3 hours to charge at 15 AMPS
    more heavily depleted battery will take much more time.

    Do you have a battery monitor ? A very useful tool to have.

    there is also another little known problem with sealed AGMs the bonding of O2 molecules with the Neg plate to dePOLARIZE the neg plate(s).
    found here:
    http://www.battcon.com/PapersFinal2008/DeMarPaper2008PROOF_7.pdf


    happy reading8)
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • RCinFLA
    RCinFLA Solar Expert Posts: 1,484 ✭✭✭✭
    Re: Trying to find the problem in the system...

    From original,
    "We have noticed that according to the inverter, the generator only seems to be putting 13-15 amps into the battery bank while charging, and yet claims to have the batteries fully charged in two or three hours."

    On the XW there are settings for maximum AC current draw from AC1 (grid) and AC2 (generator). This setting supercides the charger current settings if lower.

    Make sure neutral line from generator is connected to inverter. You did not mention exactly what generator you have and some manufacturers wattage statement is not a continuous load rating. Assuming you can deliver 5 kW continuously, the AC2 current limit on the XW should be set to 20 amps AC.

    The inverter might prematurely go to float due to imbalance in the state of charge of the batteries. One of the two batteries might reach full charge and its voltage goes up significantly before the other causing the charger voltage trip to revert to float.

    I am not super familar with XW but believe the criteria for exiting bulk are voltage, current and time. All are settable. Time should be used as a fail-safe feature. When batteries reach the bulk voltage (28.8vdc) the inverter goes to voltage regulation. During this period the current will gradually drop as batteries become more charged. If you set the bulk termination current too high it will drop to float too early. Problem is you have seven parallel strings with likely variable state of charge between batteries. One string may initially take a large percentage of the inverter output current. Fortunately AGM's can take a very high charge rate. Anyway, you should set the bulk abort current to about 2-4% of total A-H rating of batteries. For your seven batteries this is quite a bit of current (about 25 amps dc). You can lower this even more to see if you can get all batteries fully charged.

    Here is where my lack of knowledge of XW comes in. On SW series, all current settings were referenced to AC input current because they had no DC current sensing so you had to compute AC input charge current settings from the desired battery DC charge current. The best way to do this is battery voltage times battery charge current (watts into battery) then divide by inverter efficiency (0.92) to get AC input watts, then divide by AC input voltage to get the AC current necessary. This AC input current will still hold true for XW series. A 5KW generator should be able to produce 150 amps of battery current at 28.8 vdc battery voltage. (5kW*0.92=4.6kW, 4600 watts/28.8vdc = 159.7 amps)

    One other point, you did not mention what AC load might have been on inverter during your measurements. The AC2 current limit will be distributed between battery charging and inverter load. Charging is backed down if inverter output load takes up more of the AC2 input current. This keeps the generator from getting overloaded.
  • nigtomdaw
    nigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Trying to find the problem in the system...

    RCinFLA the system was installed five years ago that excludes the XW system sos :cry:
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Trying to find the problem in the system...

    This is way over my head,, and perhaps off the wall,, but if you have a floating neutral generator,,is it possible that the ac charger "thinks" it is putting out more DC current than it is because it can't reference itself to ground,, ergo going to float way too soon?

    Tony
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Trying to find the problem in the system...
    icarus wrote: »
    This is way over my head,, and perhaps off the wall,, but if you have a floating neutral generator,,is it possible that the ac charger "thinks" it is putting out more DC current than it is because it can't reference itself to ground,, ergo going to float way too soon?

    Tony

    Shouldn't do. The charger should be referencing (-) for voltage (positive isolated from the actual battery connection). My system works fine this way, although there may be a difference in inverter/chargers.

    Seems the two obvious problems are: too many batteries for the array and gen/charger settings wrong. So check everything, and see if the batteries can be salvaged.