Volts vs SOC under load?

2twisty2twisty Solar Expert Posts: 199 ✭✭✭
I'm a bit concerned:

My Classic (with WbJr) reports that my SOC is 75%.

Yet it also says that my batteries are at 47.3V (48V Nominal).

That doesn't seem to jive with all the SOC charts I've seen, but all of those are VOC, and not under load.

I can't take SG readings at the moment, so I can't confirm actual SG vs Voltage vs SOC right now.

Is there a good resource for SOC vs SG charts? If I determine that my Classic is reporting SOC incorrectly, what can be done about it?

Comments

  • 2twisty2twisty Solar Expert Posts: 199 ✭✭✭
    Re: Volts vs SOC under load?

    Found it. Apparently, when I did a factory reset on the Classic recently after a firmware upgrade, I missed one of my settings when putting them back in: the size of the battery bank.

    The classic thought that the bank was 400Ah instead of 208Ah. As soon as I corrected that figure, the SOC dropped to 51%, and my generator kicked on.

    This is precisely why I asked the folks at Midnite Solar to add a feature that allows for the backup/restore of settings!

    Ugh. Thankfully, it was only a few days, and each day I was able to get all the way to float, so even though I touched 50% SOC a couple times, I recharged right away.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Volts vs SOC under load?

    Don't even try to make a correlation.

    First off, the charts showing SOC as a function of V are resting Voltage (not Voc which is a term applied only to PV). Not completely accurate even on new batteries fully charged and sitting for a day.

    Second, a battery monitor estimates SOC based on current in/out and programmed capacity & efficiency of the battery. It too is somewhat inaccurate (and gets worse over time).

    Third, the load the batteries are under will vary the Voltage but not effect the SOC at the moment. The load does affect the actual capacity at the moment.

    If your batteries are at 75% SOC and your Voltage is reading <48 Volts under load then that load is a heavy one, or the Classic & WBjr. are programmed incorrectly or something has gone wrong with batteries and/or wiring.

    To test this, take the load off and see how much the Voltage rebounds.
  • 2twisty2twisty Solar Expert Posts: 199 ✭✭✭
    Re: Volts vs SOC under load?

    http://www.rolls-battery.com/content/technical-downloads?q=node/51

    Is this chart of SOC vs SG valid for all lead-acid batteries? Does it vary based on type (L16, GC2, etc) or by manufacturer?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Volts vs SOC under load?
    2twisty wrote: »
    http://www.rolls-battery.com/content/technical-downloads?q=node/51

    Is this chart of SOC vs SG valid for all lead-acid batteries? Does it vary based on type (L16, GC2, etc) or by manufacturer?

    It is valid for any lead-acid battery whose 100% SOC is an SG of 1.255-1.275. :p

    Frankly that's a pretty broad range.

    It's best to get information from the maker of a specific battery, and even then it may not be right (outdated data for batteries that have changed design for example).

    If you charge them completely to begin with to where the SG will not rise any further (this includes needing to do commission cycles) and write down what you get with your hydrometer under the temperatures you experience ... then you have a fairly reliable baseline SG.

    This is one of the reasons I don't like battery monitors: program them wrong and it's all wrong from then on out.
  • boBboB Solar Expert Posts: 965 ✭✭✭✭
    Re: Volts vs SOC under load?

    The ACTUAL capacity of the battery can also make a difference.

    If the battery is labeled as a 400 Amp-Hour battery, it may or may not actually be 400 amp hours when
    fully charged.

    The only way to find this out is to charge the living daylights out of it (Maybe a long Absorb charge as well),
    and then count amp-hours out of it with a moderate load. Then when the voltage gets low enough to
    at least make you think it is at a certain SOC, measure the specific gravity to see what it really is
    at compared with the manufacturer's charts. It doesn't have to be completely discharged, but will
    be a better test if you can draw more amp-hours out of the bank... Say, 75% or so.

    Does that sound about right, guys ?

    Also, the battery temperature variance from 25 degrees C (or the batteries' reference temperature)
    will affect capacity as you know. Also make sure that the reference temperature in the Classic
    is set to 25 degrees C in the WB Jr., Setup, MORE menu. It should be, but a few of them got
    out set to 10 C. That lower number has been limited to 20C now.


    boB
Sign In or Register to comment.