LiFePO4 Load testing question.

Bunkysdad
Bunkysdad Registered Users Posts: 38 ✭✭
in Caravan, Recreational Vehicle, and Marine Power Systems #1
My question has to do with load-testing a battery…I tested my 200a/h LFP battery under a continuous load by using the air conditioner fan motor (fan only, compressor OFF) which uses about 24.5 amps (continuous) when on high fan speed…My expectation was that the fully charged 200a/hr battery should last nearly 8 hours at this steady rate, but it dropped to only 10% SOC in about 3.5hrs (as measured with built-in batt bluetooth and a separate Victron shunt meter), this with just 88.6 a/hrs consumed.

Does anybody see any flaw in this load-test methodology??

Thanks
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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #2
    Yeah, something isn't right.

    I would be suspicious of the battery, but the Victron Smart Shunt (which is the one I assume you have) does Coulomb counting, so if you had it configured for a 200Ah battery and sync'd it to full before you started your test, it wouldn't say 10% unless it thought you had taken out 90% of the Ah. 

    When you say built-in battery Bluetooth, I assume you mean the BMS? Did the BMS show all the cells at about full when you started? What did the individual cells show when you stopped the test?

    How were you measuring the load? Was the 24.5A what the Shunt reported? 
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #3
    Bunkysdad, you do not have to create a new thread/discussion with each question... For example, for Li Ion questions, just do the Q&A in your one thread--It will make it easier to see what has been asked and answered before...

    Same thing if/when you design your system. Starting a new/single thread about your system makes it much easier to follow.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #4
    Yes, the shunt based Victron BMV-12 was set at 200a/h capacity and configured for LFP…The battery has both a built-in BMS, and bluetooth SOC metering via smart phone…I started the test with the battery fully charged, and both meters were synchronized at a full 100% SOC…I don’t know the ‘end of test’ cell voltages, but they were all nearly identical at the start…The 24.5 amp (approx - with no substantial drift) load was uninterrupted-continuous throughout, as measured by both the Victron and by the battery’s internal bluetooth…The curious ‘end of test’ thing is that while the battery’s bluetooth was showing only 10% SOC and 180 total a/hrs consumed, the cumulative test amp totals came to only 88.68a/h per the bluetooth and 88.0a/h per the Victron?

    The bluetooth’s ‘displayed’ 180 ‘total amp hrs consumed’ (at 10% SOC) seems to correlate with it’s claimed 200a/h capacity, though in reality the actual amperage consumed was only about 88a/h, suggesting a battery capacity of just over 100a/h…
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #5
     Got it Bill…My second question emphasis had more to do with confirming my load testing methodology, thus my confusion.

    Many Thanks
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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #6
    Just to be clear: The "battery's internal Bluetooth" and the "BMS" are the same thing. The Bluetooth on your phone is connecting to the Bluetooth of the BMS. There almost certainly isn't anything else in the battery that is giving you information.

    What brand of battery is this? Do you have a link to it?

    At the end of the test, what does the BMV-712 say the SoC of the battery is? (again - I assume you mean the BMV-712 when you say BMV-12)
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #7
    You have two measuring tools that are off by 50%/factor of 2x.... Something is not right.

    Do you know what your load current was and time? It would be helpful to purchase an AC+DC Current Clamp DMM--If you have a 10 amp load and discharged for 10 hours--Then 10 amps * 10 hours = 100 AH... And see which is closer.

    https://www.amazon.com/gp/product/B019CY4FB4 (mid priced AC+DC CC DMM--Note there AC only CC DMMs... We need the AC+DC version here).

    You can also measure (with an accurate) DMM the voltage drop on the current shunt (if you can get to the Kelvin contacts) and figure out the current that way (example of math):
    • 50 mV @ 200 AH shunt resistor:
    • 0.010 volts measured * 1/0.050 volts full scale * 200 Amps = 400 Amp current flow
    That you had 24.5 amps (measured how?) @ 3.5 hours:
    • 24.5 a * 3.5 h = 85.75 AH
    ~300 Watts (24.5a*12v=294 Watts) is a good size fan as loading... But certainly within the range of possibility.

    Many Battery Monitors are programmable for shunt resistance/sizing (50 mV @ 100 Amp shunt, 50 mV @ 400 amps, 100 mV @ 20 amps, etc.)... Double checking the Victron BMS settings, and confirming the DC current measured by the Victron vs a second meter is needed.

    At this point, I would be going through the battery manual and making sure it was charged property--Somewhere around room temperature of 77F/25C, and that it was fully/correctly charged to 90%+ State of Charge...

    Lots of variables here--But a call to battery tech support is probably the next step after you have verified the basics.

    Having multiple "meters"--You are going to be at the mercy of inaccuracies... Some may be 2.5% accurate, but upwards of 10% accuracy is not unusual (especially for Watts where you are measuring both VAC and Amps DC so "two readings" and their errors).

    You can also run into other issues... I am guessing you have an 120 VAC inverter here? The power input for an AC inverter is actually a 120 Hz Sine Squared Wave... Not all measurement systems can accurately measure such a wave form. Looking for an RMS type meter for "debugging" and understanding your system can help with better accuracy on these measurements.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #8

    To clarify, the battery’s bluetooth displays both ‘total amps consumed’ and running ‘amps’ meaning current…

    Bluetooth display amps (cumulative time-based totals) agree with Victron, but ‘total amps consumed’ and SOC do not, as follows:

    TAC per Victron = 88a/h total consumed (agrees with cumulative measured totals)
    TAC per Bluetooth = 180a/hr total consumed 

    SOC per Victron = 54%
    SOC per Bluetooth = 10%

    Bunkysdad
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #9
    Yep... Understand... Just wanted to confirm the actual DC current draw on the battery... Knowing your AC loads (Watts) and/or DC load (Watts, Amps) to confirm the Victron is accurately reporting current.

    I believe that the Victron is correct... But still need to verify that. The BMS in the battery... Don't understand why it is so different. If they were within 10% of each other--That I would understand.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #10
    Yes Bill, the inverter is a Xantrex ProSine used to power up the camper’s test-load (fan), but shore power was not involved…I believe the results since they were all dc based, thus fairly straightforward - But since it’s under warranty it’ll require examination by the supplier-vendor… Testing was at about 73f deg…I’ve contacted the supplier’s tech-engineer and provided him with my test-results log - Having seen that, he seems most cooperative and well agrees that what appears to be a substantial ‘capacity discrepancy’ needs to be researched…

    Sorry Horsefly, correct Sir, I meant BMV-712 - to recap ‘end of’ 3hr 36 min, continuous 24.5a load-test (starting with battery FULLY charged and both meters reading 100% SOC):

    Note, Since any maker’s battery can sometimes be defective, from my perspective the brand name (at this juncture) does not seem all too relevant - the supplier has indicated his informed cooperation to achieve a willing resolution - obviously, it’s in their best interest to be advised of any issues..JMHO


    200a/h Battery Bluetooth Display:

    Volts = 13.0
    % SOC = 10%
    Consumed amps total = 180
    Amps (cumulative over time) = 88.68 (note, a 91a/h discrepancy)


    Victron BMV-712 (calibrated for a 200a/hr LFP):

    Volts = 13.0
    % SOC = 54%
    Consumed amps total = 88.0
    Amps (cumulative over time) = 87.65

    Hope this helps,

    Bunkysdad


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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #11
    Quick question @Bunkysdad - Was the 13V reading from both the BMS and from the BMV-712 while the battery was at rest? That is, was the current flowing out of the battery already stopped for a while at the point where you read 13V?

    13V for a 4S, 12V battery is not 10% SoC. Depending on the answer to my question, it is probably somewhere between 25% and 40%. For the battery to be at 10% SoC, I would expect the voltage of the pack to be more like 12.5V or less.

    I agree with @BB. that the Victron is more likely to have measured everything correctly.  It matches with your estimate of 24.5A for 3.5 hrs. It's estimate for SoC is of course dependent on the 200Ah being the correct capacity.

    I suspect two problems here. I think the battery probably is something less than 200Ah in capacity, and I suspect that the battery BMS is messed up. I understand you not wanting to identify the battery brand, so we're kind of stuck in terms of figuring out any more.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #12
    No, the 13.0v was once immediately having reached 10% SOC (per battery’s bluetooth) just before load terminated.
    I tend to agree with your assessment, and though accurately determining LFP SOC’s can sometimes be a bit dicey (considering potential meter drift), the Victron seemed to be ‘spot-on’…BTW, I just dropped the battery off at the supplier-vendor for bench testing…We’ll wait to see what develops, but since imported components ( - -) are involved, the sage lesson here seems to be “trust but verify”…

    Bunkysdad
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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #13
    Well, if the 13.0V was as - or soon after - there was a 24.5A load, the voltage would be sagging a bit, so after resting it probably came up to something more like 13.2V range. That would definitely be more in the 50% that the Victron was stating. In that case, the battery may be OK and really have a 200Ah capacity, or something close to 200Ah.  It must be a problem with the BMS.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #14
    Good point!
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #15
    To add to my previous, the battery weighs about 65 lbs, so that alone might imply a potential of 200a/h capacity…
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #16
    Depending on the Battery's BMS, it may still shut down when cell voltage (or whatever the battery electronics are programmed for) could shut down the battery at 50% SoC (via Victron) because it (wrongly or "bad cells") decides the battery is "empty" and it has a relay (or similar) to shut down at low cell voltage (to prevent cell damage).

    The victron with the shunt should be purely looking at Amps*Time for battery capacity (what you have programmed it for). The BMS battery capacity "SoC value" is a "side product" of the BMS job to make sure all cells are operating within correct voltage/temperature/current ranges (again, does vary by brand/model of battery/BMS). The battery's BMS will (as far as I can guess) not make a shutdown decision based just on calculated SoC. But if cell voltage falls below programmed value (brand/chemistry dependent), then BMS will alarm/shutdown.

    The battery capacity may be 200 AH "useful" capacity... Or if their limits are set to 95%-10% capacity usage for BMS--The you may see 200 AH marketing capacity * 0.85 actual derating = 170 AH useable capacity.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #17
    ALL Excellent points Bill, though since the battery is completely sealed, it’s not entirely clear to me how the can properly diagnose, or make repairs to the BMS?? Since we camp almost exclusively off-grid (this, why I chose a 200a/hr), I’m truly hoping for a workable resolution…I’ll be sure to keep the forum posted.

    Bunkysdad
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #18
    Sometimes "issues" can be fixed with programming/configuration changes, or even firmware update... Don't know for you batteries.

    Large/sealed/no user serviceable parts/expensive--The company had better have their ducks in a row to avoid warranty issues/costs. & unhappy customers.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #19
    Well, the idea of the BMS as a coulomb counter possibly gone wrong makes sense to me - that could be what’s driving an erroneous consumed amps total and SOC display…I can’t really understand why manufacturers of these ‘drop-ins’ design them so they are virtually unserviceable - seems there should at least be access to the BMS, truly makes little sense to me?…Either way, please know that I greatly appreciate all this sage input :)

    Bunkysdad
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #20
    Not sure where I found this, or if it helps, but it’s published for LiFePO4 chemistry:

    State-Of-Charge

    Voltage at rest (zero current)

    Voltage under load (0.25C)

    100%

    14.0 Volt

    13.6 Volt

    99%

    13.8 Volt

    13.4 Volt

    90%

    13.4 Volt

    13.3 Volt

    70%

    13.2 Volt

    13.2 Volt

    40%

    13.2 Volt

    13.1 Volt

    30%

    13.0 Volt

    13.0 Volt

    20%

    12.9 Volt

    12.9 Volt

    17%

    12.8 Volt

    12.8 Volt

    14%

    12.6 Volt

    12.5 Volt

    9%

    12.4 Volt

    12.0 Volt

    0%

    10.4 Volt

    10.0 Vol

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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #21
    Forget the voltage chart.  LFP's discharge curve has relatively small voltage drop slope vs state of charge.

    A three minute load test at 0.2 to 0.4 CA (40A to 80A for 200AH) is enough to tell condition of LFP battery.  For a new 200 AH battery each cell should have no more than 0.060 to 0.100 voltage slump at 0.2 to 0.4 CA load after 3 minutes of load from no load rested open circuit voltage reference point.

    You also have resistance of BMS and a little for internal wiring.  BMS is likely in range of couple of milliohms.  For 40 amp load, the BMS voltage drop should be in range of 0.080 volts.

    Assuming you are talking about a self contained 12v LFP battery, it is four series cells plus BMS in the case.  Total voltage slump from rested no-load voltage (no-load for five minutes) with 40 amp load should be 4x 0.06v plus 0.08v for BMS or about 0.32 volt slump after three minutes of 40 amp load.

    This all assumes battery is at room temp around 25 degs C.  If battery is below +10 degs C the voltage slump will be greater and capacity will be less.

    A lot of the cheap Chinese LFP 12v batteries are using used or reject cells.   The load test is a good way to test them.  A used LFP cell will have a greater voltage slump with load test.

    Capacity testing at low CA discharge current is not a good test.
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #22
    Ok, the method that you suggest sounds like a reasonable way to validate discrepancies between individual cell (actually, individual cell banks I believe?? - it IS a sealed drop-in) thus, one can thereby impute the general condition of the battery…Not to be picky here (your idea truly is helpful!), but this seems to fall short of explaining why the discrepancy between the Victron and the battery’s bluetooth ‘total consumed amps’ and the SOC’s, nor does it explain why a continuous 24.5a consumption load test is not a viable methodology to validate (approximate) real-world amp/hr capacity…Note too, that this rather mild load test (not so heavily reactive on what’s a new 200a/hr rated battery) might reasonably be seen (from my perspective) as a valid, simulated real-world load test…

    Another possibility (of several, although maybe not all too likely…) could be that the batt’s Bluetooth was improperly  calibrated ‘to display’ say a higher than what was ‘actual’ consumed amps (correlated with SOC?) to imply display authenticity the battery’s rating??…(Understand, Just a last-ditch mental exercise here only - not an accusation!)

    Either way, the battery is still being evaluated by the competent vendor, and so far the vendor appears to be addressing this issue in the best of Faith…
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #23
    FINALLY got resolution on the LiFePO4 :smile:…Vendor load-tested using a steady 500w lamp load and repeated again with a 250w load and measured almost 220 available a/hrs!!…Said the BMS had mis-calculated the SOC (as with the total consumed amps) - said that once they’d test cycled the battery a couple of times the BMS self-corrected and now all tracks good (this is a new, one time used 200a/hr battery)… I told him that I’m about a 96% off-grid desert camper (and Polaris RZR trail rider) and charge via 440w of solar, thus the 200a/hr capacity is vitally important to me…He said that they haven’t run into this issue before, but if I find it occurs again he’d be happy to replace it without an issue :smiley: !!

    Can’t beat that for Customer Service - Happy trails Again!!

    (will keep you posted…..)

    Bunkysdad


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