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Salvo's avatar
Salvo
Explorer
Dec 10, 2015

Some cool battery graphs

I'm checking out the Costco GC2 batteries and Iota 55A converter as well as the new 16-bit a/d for Arduino.

Prior to discharging the 208AH GC2 batteries, they were top charged with 16V for about an hour.

They are then discharge to about 50% (actually 99.8AH) with a load current of 14A for 7 hours. The load current consists of a 100W light connected to a msw inverter and some lighting inside the MH.

Data is acquired using an Arduino Uno board, and a 16-bit a/d converter. Battery voltage and current (measured across a 100A shunt) is recorded every second.

The first graph shows battery voltage (in blue) and current (in pink) over the entire 7 hour discharge period. Voltage scale is on the left y-axis and current scale is on the right y-axis. Current is defined as negative when going out of the battery. Top of the graph is 0A and the bottom is -16A. The current oscillates due to inverter operation. It does not pull a constant current.



The second graph is same as first, but just looking at the first 30 min of discharge.



Like wise the third graph shows the first 5 min of discharge. The inverter is switched on at about the 3/4 minute mark to get -8A, and a little later the add lights were turned on to drop down to -14A. Battery voltage is naturally decaying (from time 0 to 3/4 min) because the 16V charging power supply was just removed.

What's surprising is that there are two different voltage discharge slopes. The first slope is fast, going from 14V to 12.4V in about 6 minutes. This may be the top charge. The second slope is very gradual, going from 12.4V to 11.8V in about 7 hours.



The next graph shows SOC with respect to battery voltage. The weird looking top I believe is due to the top charge.



The last graph is same as above, but the voltage is corrected for a no-load current. The internal resistance of the battery is 12 mohm. The load current multiplied by the resistance is added to the voltage measurement. I also started the plot before the weird looking top comes into play.



And here's the test set-up. The 100A shunt is connected directly the the negative battery post.

Charge plots still to come.


40 Replies

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  • BFL13's avatar
    BFL13
    Explorer II
    Dec 11, 2015
    Salvo wrote:
    I think one can assume 100% AH efficiency below the gassing voltage. And efficiency drops when greater than 75% SOC. Efficiency should be close to 0% at 100% SOC.

    Of course the energy efficiency is much lower.

    BFL13 wrote:
    So you are claiming 100% charging efficiency????



    Trimetric user's manual says Trojan wet cells were tested, and showed 95-98% charging efficiency below gassing voltage. Above gassing voltage of course efficiency is less. Trimetric chose 94% setting for their default average, but you can adjust that if you find it isn't quite right for your own batteries.

    I did not change my Tri setting of 94% so I may be getting a tad higher in SOC earlier on, than my graphs show, which explains the 5% I think your SOCs are too high. It would seem that they are too high but not by that much. Maybe 3% too high? Something anyway.

    Gassing voltage at 20C is about 14.4 ISTR, ( It is lower at 25C) so you should apply some heat loss towards the end of even the bulk stage going to 14.6, and then more after that.
  • Salvo's avatar
    Salvo
    Explorer
    Dec 11, 2015
    It's amazing how cheap this set-up is. The Aduino Uno computer only costs $7. Add a memory card module for $6 and 16-bit a/d for $10 and you got it. All from China.

    I have a few other attachments like acclerometer, more sram memory (for very high speed measurements) and radio control on/off program control.

    MEXICOWANDERER wrote:


    What can be done for many hundreds of dollars these days, ...
  • pianotuna's avatar
    pianotuna
    Nomad III
    Dec 11, 2015
    Hi Salvo,

    100% efficiency below gassing voltage--but what about internal resistance especially after reaching 85% of fully charged?
  • Salvo's avatar
    Salvo
    Explorer
    Dec 11, 2015
    I think one can assume 100% AH efficiency below the gassing voltage. And efficiency drops when greater than 75% SOC. Efficiency should be close to 0% at 100% SOC.

    Of course the energy efficiency is much lower.

    BFL13 wrote:
    So you are claiming 100% charging efficiency????

  • BFL13's avatar
    BFL13
    Explorer II
    Dec 11, 2015
    So you are claiming 100% charging efficiency????

    Anyway that does explain why the SOC plot is showing SOC too high compared with my results. I think if you stopped at your "90%" and then did another discharge to 50%, counting the AH, you would see fewer AH than had you actually reached 90.

    But that is hard to compare results too, because of "progressive capacity loss" with successive 50-90s.
  • MEXICOWANDERER's avatar
    MEXICOWANDERER
    Explorer
    Dec 11, 2015
    Nope.
    The data acquisition is in my humble opinion, flawless. Two ears and a tail.

    My inability to produce graphing here left me frustrated in showing the relevance of my Vmax charging regimen and value of 2nd stage 14.0 vdc charging with generator. Your graph illustrates this point beautifully.

    Observation and reaction to trends & tendencies data are worth their weight in gold. All my A to D data disappeared when I was hospitalized 26-years ago. It was fun. Configuring a reactive charging regimen to the data was an obsession. Brings back fond memories.

    What can be done for many hundreds of dollars these days, took many tens of thousands of dollars and hundreds of hours of untallied labor back then. But it was worth it.
  • Salvo's avatar
    Salvo
    Explorer
    Dec 11, 2015
    It's hard to say there's something wrong with my methodology.

    I measure current every second. During discharge, I have almost 26,000 measurements. All 26,000 current measurements are added together and divided by 60 sec/min * 60 min/hr. The total is 100AH removed during discharge.

    During charge, my SOC starting point is 108 AH / 208 AH = 51.9%

    When charging, each one-second measurement adds to the initial 108 AH capacity. There are no losses involved. In other words, when in bulk, if 10A are charging the battery for one hour then battery capacity increases by 10 AH.
  • BFL13's avatar
    BFL13
    Explorer II
    Dec 11, 2015
    Very interesting graphs. (Not at all ugly either! :) )

    I do wonder about the charging graph's SOC plot though. It seems to be showing about 5% too high an SOC at that 65 minute point, compared with what I have seen with my recharges.

    Perhaps there is a different allowance for heat loss?

    I also can't reconcile your battery acceptance rate of 20 amps at about 95 minutes at "90%" SOC at 14.2 volts with my "marker" of 5 amps per 110AH at 14.5v, which would be 9.5 amps for 208AH, and less than that at a lower voltage (14.2 vs 14.5)

    That would also make your 50-90 take about an hour and a half, which is about a half hour too soon, based on what I have seen.

    So either the SOC plot there has something wrong with it, or there is more to be learned here about this. (My "marker" works out right cross-checking with the Trimetric AH counter, so it is not "wrong"---the thing to figure out here is what is making the difference)

    "The AH returned" plot on this graph is linear, which would keep the SOC percentage lower than the SOC plot in the above graph.

    http://www.engineersedge.com/battery/specific_gravity_battery.htm

    For sure the drop from 14.6 battery to 14.2 has less of a drop in amps than dropping from 14.6 to 13.6 does. 13.6 means twice the amps drop.
  • Salvo's avatar
    Salvo
    Explorer
    Dec 11, 2015
    The 55A Iota IQ4 converter is in boost for 80 minutes. SOC at that point is 87%. At the 65 min mark the battery is at 14.5V and the converter at 14.8V. This triggers a timer within the Iota and it remains in boost mode for an additional 15 min, or to the 80 min mark.

    Current slopes downward at 65 min because the Iota output voltage reached the limit of 14.8V. In other words, tapering begins.

    In boost, battery voltage rises fairly linearly, until about 50 min into charging. Voltage begins to increase faster an that point. That's due to the increase in battery resistance starting around 75% SOC.

    The right side y-axis scale is for current and SOC.



    Another interesting point is when converter voltage drops from 14.8V to 14.2V. Battery surface charge and IR drop reduces, causing it to accept about 8A more current. In other words, dropping from 14.8V to 14.2V is not that huge.

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