Battery Bank mysteriously losing voltage at night

System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
I have a dual Xantrex SW5548 power panel and 24 KC120 panels. I just purchased 16 new Rolls S-530 batteries wired in two parallel sets of 8 for 48V.

I purchased the new batteries to replace a bank of 3 year old L-16's when I noticed that the L-16's were not holding a charge. Very quickly after dark they were dropping to 44V within a matter of a few hours even with the inverters reporting AC amp usage of only 8 amps AC (which wold only be 20 amps DC). With a capacity of 700aH, that obviously should not happen. This would happen even after a solid equalization charge.

Now I have the new batteries installed, and the same thing is happening! Obviously I am alarmed. Where is the energy going!!!

I did not have a DC amp meter to measure exactly how much charge was going in/out of the bank - I purchased a meter from Northern Arizona and just received it. It is daytime, and it it reporting that I am charging at 7 net amps (it's cloudy). My generator is out of service until Monday, so I can't check the charge amps when I start the generator at this time. I am very curious to see what will happen tonight, but I suspect that it will report far more discharge than the inverters are putting out. We'll see.

I have a couple of ideas about where the power might be going.

1. Could I be losing power through the solar panels due to a bad diode?
2. Could the inverters be working at a extremely inefficient level due to some internal problem?
3. Could I be losing energy due to a bad ground? Loose wire?
4. My negative battery cable was not long enough when I switched banks, so I have two strung together for a total of 12 feet. They are solidly connected with a bolt through the cable terminals. Could this have any impact?

I intend to turn off the inverters to see if there is still a negative DC amperage. That should rule out the inverters. Then I will disconnect the solar panels to see if power is being lost there. Does that sound like a good diagnostic strategy? What else can I do?
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  • System2
    System2 Posts: 6,290 admin
    #2
    Re: Battery Bank mysteriously losing voltage at night

    One note: I have an Outback MX60, so I would assume it would disconnect the PV via a relay at night, so that would take away the panel leakage idea... More likely an inverter problem?

    Or... something else wrong with my battery bank?
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #3
    Re: Battery Bank mysteriously losing voltage at night

    Sounds like its a charge in vs load out issue

    How many watt/hours in do you put in the battery bank during a day ( MX60 can display rough amount ) vz night time load?

    8 amps is about 1kw+/hr which is a pretty hefty load, is this per inverter?

    What is the highest voltage the battery bank gets during the day would be the first clue.

    24 KC120's is about 12kWh/day ... your night time load looks to be more than this so this is why I would think your never getting the battery fully charged and going to near 100% discharge at night
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  • halfcrazy
    halfcrazy Solar Expert Posts: 720 ✭✭✭
    #4
    Re: Battery Bank mysteriously losing voltage at night

    Agreed with Solarguppy the 8 amp draw if it is continuis is a big draw all night and it may be all day also? if it is all day it is using darn near 1kw of the panels before thy even get to the batteries. more info needed to help you on this one
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  • System2
    System2 Posts: 6,290 admin
    #5
    Re: Battery Bank mysteriously losing voltage at night

    The draw is about 1-2 amps all day. 8 amps is only during the evening hours, then virtually zero from midnight to daylight. I estimate that I use 8-10kW-hrs per 24 hour cycle.

    What I am saying is that when I get a full charge (even after equalizing and running a generator for a solid 8 hours), and letting the batteries float for several hours, the batteries drop in voltage quickly.

    For example, if I stop the generator cycle at nightfall, and I am using 8 AC amps (total - as reported by the inverter AC output meters) the batteries will fall to 44V within about 5 hours. 8 amps AC should only be 20 amps DC (okay, let's say 30 adjusted for inefficiencies). If I have 1000ah capacity I should be able to go a lot longer than 5 hours!

    And since when is 8 amps "a hefty load?" It a kW! That's nothing... My $30,000+ investment should do a lot more than that!
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #6
    Re: Battery Bank mysteriously losing voltage at night

    Plan on a night of candles & flashlights (before it get's really cold) get a block of dry ice for your freezer. Charge your batts fully, and then at dusk, disconnect the battery cable.
    Measure the voltage every hour till you go to bed, and then measure in AM when you get up. You may have some sort of sneak discharge path, and this would prove it.
    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 ,

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  • System2
    System2 Posts: 6,290 admin
    #7
    Re: Battery Bank mysteriously losing voltage at night

    Yeah, I think that is my only solution. The good news is that I don't expect snow any time soon here in the Caribbean...

    I do have an amp meter connected between the battery and the power panel, so I will know if there is energy flow after I shut down the inverters (which there should not be). I'll also check the voltage so I can rule out anything internal to the batts.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #8
    Re: Battery Bank mysteriously losing voltage at night

    Another possibiltiy that you might be missing is how you are measuring your voltage. Voltage under load will be significantly less than "the real" voltage.
    In order to get an accurate measurment of state of charge, you must wait at least 1 hour after all charging has stopped, and all loads have been shut off. Only then will you get a fairly accurate state of charge measurment. A temprature compensated hydrometer on the other hand cares not if the battery is being charged of discharged. Sometimes the best technology is the old tried and true.


    My battery bank routinely (12vdc) drops to 11.75 vdc under load. It comes right back up after the loads are turned off.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #9
    Re: Battery Bank mysteriously losing voltage at night

    Ps to my previous,

    A kilowatt is a big load, no mater how you cut it. Second, with your considerable investment in your system, you should, in my opinion invest a few more buck in a good metering system. I have a tri-metric meter and I love it. It gives me more info than I really need, but I can tell at any hour what is going on with my system.

    A meter at the charge control is only useful for that function, similarly, one at the inverter only tells what the inverter is using. A full system meter will tell you exactly what is going in/out all the time. Great for finding phantom loads. I had a persistant .05amp draw for no apparent reason, turns out it was a dying cell in the bank. Without the meter I wouldn't have found it as early.

    Icarus
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  • System2
    System2 Posts: 6,290 admin
    #10
    Re: Battery Bank mysteriously losing voltage at night
    icarus wrote: »
    Another possibiltiy that you might be missing is how you are measuring your voltage. Voltage under load will be significantly less than "the real" voltage.

    Thank for your input, but this is not the problem. The difference in my case is when the inverters cut out, the voltage does not come up after resting...
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  • System2
    System2 Posts: 6,290 admin
    #11
    Re: Battery Bank mysteriously losing voltage at night
    icarus wrote: »
    Second, with your considerable investment in your system, you should, in my opinion invest a few more buck in a good metering system. I have a tri-metric meter and I love it.

    Yeah - I did that. My Penta-metric died after 4 months of use.I haven't gotten around to sending it for repair (not so easy from down here).

    That was when I had my old battery bank, which I suspected was at the end of its useful life, so I did not pay much attention...
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