Confused about battery voltage vs. inverter voltage...is there a problem?

GhostHawk
GhostHawk Registered Users Posts: 5 ✭
in Advanced Solar Electric Technical Forum #1
Hi All,

First time poster. I have looked all over several books, manuals and internet to try to find an answer to this question.

My off-grid PV system is running on a Trace C40 charge controller, a Trace SW4048 inverter and 32 Trojan L16RE-B batteries at 48V.

While studying the inverter manual and other sources, I have become concerned about setting up the battery maintenance portions of my inverter. My confusion began when I began seeing battery voltages on the low side of 48VDC before I really expected to, based on use. So, out of curiosity, I took my VOM and measured the battery voltage at the connection to the inverter. I was surprised to find that voltage reading approximately 3VDC HIGHER than my inverter and controller were reporting.

So now I'm very confused. Since it appears that the battery voltage display on the inverter and controller agree with each other...the fact that the batteries themselves are running 3VDC higher leaves me wondering how to interpret things. I would like to get my system set up so that the inverter conditions the batteries properly and at the right time, while avoiding overdischarge. But, my confusion now involves which voltage I need to pay attention to.

I happen to have another SW4048 in my possession and thinking that it might be possible that the inverter was malfunctioning, I swapped them out yesterday and have been monitoring them for the past 24 hours. The only real difference I have seen between the two inverters, is that the recently installed one appears to be reading the battery voltage at about +.4VDC higher than the previous unit. Otherwise nothing has changed.

So, is this a normal situation? Should I simply use the inverter displayed voltage to configure battery conditioning sequences, or should I read the actual battery bank voltage and work from that?

I am hoping that one of the more experience moderators knows the simple answer to this. I can fill in more details if you have questions.

Thanks for any input...

Hawk
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Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    You may be seeing voltage drop in the wire leads from the battery - DC BUSS ?

    With no loads & no charging, measure voltages at the battery, Charge Controller terminals, and inverter terminals.

    Then fire up the inverter with a beefy load, and measure the 3 locations again. Shut down inverter, and enable the charge controller, and measure 3 locations again.

    With those 3 readings, 3 conditions, 1 meter, we can look at results, and maybe see what is going on.
    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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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #3
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    So when you check the voltage with your meter, it's showing 3 volts higher than the built in meter on your inverter? And the voltage your inverter displays, matches what the controller is showing? I'd be questioning the meter you're using. Either it isn't reading correctly, or both the inverter and CC are not reading properly, even though they agree with each other.
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  • Fullpower
    Fullpower Solar Expert Posts: 69 ✭✭
    #4
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    Ignoring the voltage display issue for the moment, I would like to point out a serious problem with your system specifications.
    You note approximate battery storage cpacity around 65 KilowattHours, and the charge controller rating of 40 Amps maximum.
    You will never achieve full charge on a bank of (32 ) L-16 batteries with 40 amps.
    Hopefully, you have additional battery charging current sources employed.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #5
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    Fullpower wrote: »
    You will never achieve full charge on a bank of (32 ) L-16 batteries with 40 amps.

    Yow ! I didn't see that, good catch - 4 parallel banks of ~400Ah batteries. Fuses somewhere i hope. 1,600Ah on hand, if you could ever fully charge it. 40A is hardly more than a trickle charge for that
    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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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #6
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    Fullpower wrote: »
    Ignoring the voltage display issue for the moment, I would like to point out a serious problem with your system specifications.
    You note approximate battery storage cpacity around 65 KilowattHours, and the charge controller rating of 40 Amps maximum.
    You will never achieve full charge on a bank of (32 ) L-16 batteries with 40 amps.
    Hopefully, you have additional battery charging current sources employed.

    Excellent point! Would be a shame to have to cart thirty-two L-16 batteries to the recyclers! Hope it's not already too late for them :(
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  • GhostHawk
    GhostHawk Registered Users Posts: 5 ✭
    #7
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    mike90045 wrote: »
    You may be seeing voltage drop in the wire leads from the battery - DC BUSS ?

    With no loads & no charging, measure voltages at the battery, Charge Controller terminals, and inverter terminals.

    Then fire up the inverter with a beefy load, and measure the 3 locations again. Shut down inverter, and enable the charge controller, and measure 3 locations again.

    With those 3 readings, 3 conditions, 1 meter, we can look at results, and maybe see what is going on.

    Thanks for the response!!

    I have gone over everything you recommended and am now seriously questioning my VOM. I am getting even wilder readings now...as much as 10V difference! So, before I drag anyone further through this, I am going to replace my VOM and take readings again before pursuing this.

    My system is generally performing OK, but I'm looking into why it doesn't seem to give me as much battery time as I expect. Am working the numbers today for answers. Thanks for your responses and will follow up when I can trust my numbers.

    Hawk
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #8
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    Check the battery (if DVM) and try measuring your car battery (around 12.7 volts if car off and charged, and round 14.2 if engine is running/battery charging).

    DVM/DMM's seem to fail in ways that almost convince you that they are right and the circuit is the problem.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • GhostHawk
    GhostHawk Registered Users Posts: 5 ✭
    #9
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    waynefromnscanada wrote: »
    Excellent point! Would be a shame to have to cart thirty-two L-16 batteries to the recyclers! Hope it's not already too late for them :(

    Hmm...I'm looking a bit ignorant now aren't I? Thanks for pointing out this issue. I will hook up my second charge controller and break the battery bank up. As a matter of fact, this would also mean that I am over-batterying (is that a real word?) my inverter too...since it is only rated to 40 amps. I have a second inverter that matches this one with a connector cable to join them up. Sounds like I need to double down the system and split the battery bank between the two inverters.

    I appreciate all you posters pointing out what should be the obvious. I, as a lot of people, am learning as I go and appreciate all of your input tremendously. I am replacing my VOM and will get to work on this.
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  • GhostHawk
    GhostHawk Registered Users Posts: 5 ✭
    #10
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    Fullpower wrote: »
    Ignoring the voltage display issue for the moment, I would like to point out a serious problem with your system specifications.
    You note approximate battery storage cpacity around 65 KilowattHours, and the charge controller rating of 40 Amps maximum.
    You will never achieve full charge on a bank of (32 ) L-16 batteries with 40 amps.
    Hopefully, you have additional battery charging current sources employed.

    Heya,

    I wanted to thank you for pointing this out. With all the reading I have done, I still overlooked that part of it when sizing my system. Now that I have more of a clue than before, I am taking immediate action to remedy this situation. I have divided the battery bank in half and will be installing a second SW4048 and charge controller to handle the other half of the bank. Now all I have to do it get these beasts fed properly with PV.

    Thanks again...GREATLY appreciated!

    Hawk
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #11
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    If you are near a Sears (or are OK with ordering over the Internet), this DC Current Clamp Meter does pretty much everything you need a DMM to do, plus it can measure AC or DC current up to 400 amps by just placing the Current Clamp over the wire. You can certainly pay >$300 for a Fluke DC current clamp meter... But at $60.00, this is about the best deal I have seen for a meter that will meet your basic debugging needs (the thermal couple probe failed on me almost immediately--but the rest of the meter seems to work fine).

    Otherwise, logging (once a month or so) the specific gravity with a good hydrometer and even using a volt meter across each cell (or 6 volt battery) can show you things too (as well as the DC Current Clamp Meter) when the bank is under heavy charging, loading, and resting.

    You can also use the clamp meter to measure the current sharing (or not) among your four strings (bad connections, shorted or open cells, etc.).

    Regarding your very large battery bank, have you worked out your load (AH/WH, peak, average watts, etc.) load requirements?

    Also, will this be a pure off grid system, or do you have Utility Power and/or a Genset available?

    I like to spec. a "balanced" system (loads, battery bank, solar array, etc.). A very large bank can be very expensive to maintain (and replace every 8 years or so).

    You should also have a fuse (rated for wiring/max current you expect) on each string in the bank. 1,300 AH @ 48 volts can put a world of hurt on a dropped wrench or a shorted cable.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #12
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    You need to develop about 200 amps of battery charging current. You can just about do it with two SW4048's and at least a 12kW generator (or grid).

    There are some quirks when you charge with two SW's. Make sure the temp sensor are put together in same spot. You with get some thrashing of current between the two SW's and you won't get fully twice the charging current capability of a single unit.
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  • GhostHawk
    GhostHawk Registered Users Posts: 5 ✭
    #13
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    BB. wrote: »
    If you are near a Sears (or are OK with ordering over the Internet), this DC Current Clamp Meter does pretty much everything you need a DMM to do, plus it can measure AC or DC current up to 400 amps by just placing the Current Clamp over the wire. You can certainly pay >$300 for a Fluke DC current clamp meter... But at $60.00, this is about the best deal I have seen for a meter that will meet your basic debugging needs (the thermal couple probe failed on me almost immediately--but the rest of the meter seems to work fine).

    Otherwise, logging (once a month or so) the specific gravity with a good hydrometer and even using a volt meter across each cell (or 6 volt battery) can show you things too (as well as the DC Current Clamp Meter) when the bank is under heavy charging, loading, and resting.

    You can also use the clamp meter to measure the current sharing (or not) among your four strings (bad connections, shorted or open cells, etc.).

    Regarding your very large battery bank, have you worked out your load (AH/WH, peak, average watts, etc.) load requirements?

    Also, will this be a pure off grid system, or do you have Utility Power and/or a Genset available?

    I like to spec. a "balanced" system (loads, battery bank, solar array, etc.). A very large bank can be very expensive to maintain (and replace every 8 years or so).

    You should also have a fuse (rated for wiring/max current you expect) on each string in the bank. 1,300 AH @ 48 volts can put a world of hurt on a dropped wrench or a shorted cable.

    -Bill

    Thank you very much for the information Bill!

    I believe I will buy the meter you suggested and just be done with it.

    Now, regarding your questions... I have worked out my average daily load requirements and they run about 1300ah, and as I have about 1500ah in the battery bank, that seemed pretty good.

    This is a purely off grid system that is already up and running. I have been converting this from a 24V mod-sine to a 48V sinewave system for a couple of months now. Weather is slowing down getting the PV array up, so I am supplementing with generator power for now.

    Each of my battery banks is on its own breaker. I actually have one of the 240V cabinet setups for the SW4048's, complete. They were set up very nicely for portable use, but fit very nicely in my 1700 sf home.

    Thanks also for the safety advice.

    Hawk
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #14
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    GhostHawk wrote: »
    Now, regarding your questions... I have worked out my average daily load requirements and they run about 1300ah, and as I have about 1500ah in the battery bank, that seemed pretty good.

    Hmmm...
    • 1,300 AH * 48 volts * 0.85 efficient inverter = 53,040 WH = 53 kWH per day or ~1,600 kWH per month
    That is a lot of power, and would require a lot of solar panels/generator fuel to keep up with.

    Is that the real loads? Can you address some conservation issues to perhaps get some of the usage reduced (A/C, electric hot water, cooking, computer server farm?).

    That battery bank is actually small for those size of daily loads (depending on your load profile such as daytime charging vs daytime load usage).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #15
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    WOW!!! Over 50 Kwh/day for an off grid system! Holy smokes, that's a HUGE load! I run my house on about 4 Kwh/day and many others on here run a lot less than that. It's far, far cheaper and easier to conserve, conserve, conserve than to build and maintain an off grid system capable of supporting what may be excessive consumption.
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  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    #16
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?

    How are you able to draw 1300 ah out of a 1500 ah battery bank without killing it,, nearly the first time?

    Next question,, if you are drawing 1300 ah, that translates to ~54 amps averaged over 24 hours. Probably closer to 75 amps on average during the day,, what is the peak load?

    Something is not making a lot of sense here! For example,, how are you able to deliver ~ 50-75 amps from a 40 amp inverter? (40*24=960ah,, how do you get to 1300?)

    Tony
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  • rplarry
    rplarry Solar Expert Posts: 203 ✭✭
    #17
    Re: Confused about battery voltage vs. inverter voltage...is there a problem?
    BB. wrote: »
    If you are near a Sears (or are OK with ordering over the Internet), this DC Current Clamp Meter does pretty much everything you need a DMM to do, plus it can measure AC or DC current up to 400 amps by just placing the Current Clamp over the wire. You can certainly pay >$300 for a Fluke DC current clamp meter... But at $60.00, this is about the best deal I have seen for a meter that will meet your basic debugging needs (the thermal couple probe failed on me almost immediately--but the rest of the meter seems to work fine).

    Here is another meter that also measures 400 AC and DC amps but is a little cheaper: http://www.ebay.com/itm/New-UNI-T-UT203-LCD-Digital-Clamp-Multimeter-DMM-DC-AC-Current-Voltmeter-400A-/280794332022?pt=LH_DefaultDomain_0&hash=item4160a57b76

    I have one and it works well,
    HTH
    Larry
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