BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

backroad
backroad Solar Expert Posts: 185 ✭✭
in Advanced Solar Electric Technical Forum #1
I can get my battery voltage up so far but my hydrometer tells me my volt meter lies.  I can get battery voltage up to about 25 volts but the hydrometer still says low.  How long does it take to get them on the same page?

my generator is out in the dark, happily causing the inverter to charge. The MX60 says that the battery voltage, while charging and running the household stuff, is 28.8 volts.
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #2
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE
    I can get my battery voltage up so far but my hydrometer tells me my volt meter lies.  I can get battery voltage up to about 25 volts but the hydrometer still says low.  How long does it take to get them on the same page? ...The MX60 says that the battery voltage, while charging and running the household stuff, is 28.8 volts.

    Dennis,

    The resting voltage for your fully charged battery bank is ~25.4 V, so don't be mislead by the charging voltage, which is displaying a false high "surface charge" on the battery plates. Like tea steeping, this surface charge (and the thin layer high of SG electrolyte in immediate cotact with the positive plates) will dissipate after the charging source is removed.

    The trick is to use a temperature compensated hydrometer, to take the reading after the charging source and loads been removed and the batteries have been allowed to rest for several hours, and to then pull and replace several electrolyte samples from each test cell before recording a reading. As mentioned above, the resting period allows the electrolyte SG to stabilize, and the pull and replace action helps further by mixing up the electrolyte in its cell.

    HTH,
    Jim / crewzer
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  • System2
    System2 Posts: 6,290 admin
    #3
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    Denis

    If you have a good Hydrometer, believe it. Meters are notorious battery killers. When my meter said 50% SOC, my electrolyte told me 25%...oops!

    Ralph
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  • backroad
    backroad Solar Expert Posts: 185 ✭✭
    #4
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    I've been doing a little bit of generator charging as well as the PV/inverter charging. As of this evening my non temperature adjusted hydrometer tells me my batteries are now UP to 1225-1250, which to me is awesome because it is at least moving in right direction. I was down in the 1125 range for quite a while and that really concerned me because of the cost of the Crown batteries. My battery voltage (resting with a draw) has been down as low as 23.4 and tonight, after the sun went down and I got home from work, was 25.0-25.1. My trying to run the new freezer almost created a disaster in the battery bank. More panels! My original thought was to go with 16 of the batteries, but the help at NAWS got me down to 12 batteries, and now I know they were probably correct. I probably could not have charged the 16 properly.
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  • System2
    System2 Posts: 6,290 admin
    #5
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    Hi Denis

    if you've the time and fuel, run you genset until the SG readings are where they're supposed to be, forget the voltage (but don't cook yourself). When i did a deeeep discharge it took a while to bring up the SG, almost 9;hours charging with about 70 amps dc (48v) to bring the reading up. I did'nt expect that, thought at couple of hours genset, then let the solar and wind carry the load. Good thing i was watching the acid because it didn't move off 1.125 for about 3 hours, and slowly crept up over the next 6 to 1.265 or so. Yes, i should have watched the SG on the downslope...forgot my own rule about not believing the meter! :oops:

    Once the SG is up where it's supposed to be you'll have less worries, getting it there is the hard part. Just run the genset for as long as it takes. BTW, i never use the Auto generator run settings on my X 4048. The times at voltage (absorb) are very innacurate and really don't reflect the state of charge. Now if someone would develop a hydrometer that was accurate, easy to read and interfaced with the charge controllers...

    ralph

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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #6
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    Dennis,

    Another consideration is to make sure your MX60's voltmeter accurately correlates to the voltage as measured at the battery terminals. I tend to verify my system's calibration during the last 1/2 of the absorb stage by comparing the MX' voltmeter with readings taken directly the MX's terminals and at my battery bank's main terminals. Considering that the primary operating range of a 24 V battery system is between ~24.4 V and 25.4 V, an error of just 0.1 V or 0.2 V can make a significant difference in battery performance and health.

    The MX' "Optimization" (aka "Advanced") menu includes a "Vbatt Calibration" function that allows users to adjust the voltmeter display.

    Another use for the Mate is to use it to set your inverter's charger setpoints for absorb voltage, absorb time, float voltage, etc. per the battery manufacturer's recommendations. See FX manual "Advanced" screen, Charger" menu. Again, you may need to tinker with the charger's settings in order to achieve the correct voltages as measured at the battery terminals.

    Sounds like you're making progress!

    HTH,
    Jim / crewzer
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #7
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    Also, "it has been reported" that the MX60 reports 30% more power than it actually generates (on several units tested). Another reason not to believe meters.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #8
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE
    Also, "it has been reported" that the MX60 reports 30% more power than it actually generates (on several units tested). Another reason not to believe meters.

    YIKES!! The Mate apparently has some accuracy problems, but, I'm fairly active on the OutBack forum, and I've not heard of any problem like this with the MX' built-in meter. Although the MX' PV input current is calculated, the PV Vin, the Batt Vout and the Batt A out values are all measured. According to these old tests, my MX' meter is fairly accurate. And, using my Fluke 189 as a reference, the MX in my system today is operation with a "Vbatt Calibration" offset of 0.0 V.

    There's no doubt that meters can be bad, and some go bad. But, the ones I've tested over the past five years (three MX60's and one Morningstar PS-30M w/ battery sense) all seemed to be pretty good. And, since I can't perform an SG test on my sealed VRLA batteries, I have to relay on the meters.

    Regards,
    Jim / crewzer
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #9
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    Found the post I was looking for:

    http://www.wind-sun.com/smf/index.php?topic=2199.msg16350#msg16350
    The MX60 does not have current measurement for the Array current and hence Array Power Values are not measured but estimates. Also, in my testing of multiple units, the DC watts accuracy was in the 10-35% range, always reporting high in DC watts ( meaning unit is reporting more watts than are actually being produced ). I've done some very extensive testing with lab quality power meters and have discussed with now "former" Outback staff, their take was it didn't effect the performance and the customers weren't complaining, which I agree.

    IMHO, I guess its the engineer part in me that gets ruffled when I see post after post of some outrageous system performance knowing its just digits on the LCD display, not watts going into the load rolleyes The MX60 is a good charger, its not a good watt meter ...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    #10
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    Don’t know what to say. :? Here are the results of my two tests:

    1) 547.5 W out indicated, and 545.33 W out measured and calculated, so indicated power was 0.4% high
    2) 546.5 W out indicated, and 543.72 W out measured and calculated, so indicated power was 0.5% high

    I have no reason to doubt SG’s posted results, but they certainly seem to be unique. I would think that there would have been plenty of chatter on OutBack’s forum is there was a consistent “10%-35%” problem.

    Regards,
    Jim / crewzer
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #11
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    I don't know either--I do not have a MX 60 or lab gear to measure this. That is why I used "reported" in quotes.

    Glad to hear yours is pretty close to dead on accurate.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • backroad
    backroad Solar Expert Posts: 185 ✭✭
    #12
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    This is just a repeat from another thread I've be working on, but.........

    By getting my generator to work with the inverter and doing just a bit of consistent charging(a couple of hours a day for a couple of days), I've been able to get my hydrometer readings up to 1225 fairly consistently and occasionally up to 1250. I'm wishing for more, and It's off to the learning process for me.

    Thanx a bunch to all of U...........
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  • Windsun
    Windsun Solar Expert Posts: 1,164 ✭✭
    #13
    Re: BATTERY VOLTAGE VS. HYDROMETER & ELECTROLITE

    We are not in the habit of measuring every MX60 setup we do, but of the dozen or so that we have, the meter on the mx60 has never been more than 1.5% off at max, and usually less.
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