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LifePO4 4S Ri (Internal Resistance) testing using Delta Voltage / Delta Current

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  • LifePO4 4S Ri (Internal Resistance) testing using Delta Voltage / Delta Current

    How do you know your LifePO4 cells are healthy? Use this procedure to test Ri (Internal Resistance) using the delta voltage / delta current method.



    Click through to description of video for links to spreadsheet, pl8 charge graphs, etc.

  • #2
    Hey Jesse one observation I made might save you some grief down the road regarding your Balance Plug. Your PL8 uses 3 amps to Balance Batteries with and your wire gauge might be a bit to small and too long. In the video you mentioned 22 AWG? Look into maybe 16 to 18 AWG. If you ever Top Balance, you will need it to be able to safely handle the current.

    Other than that good job 2 to 5 milli-ohms is what you should read for that battery, Just note come winter when say the batteries are 10 degrees cooler, your reading go up. So what else does this tell you? Tells you how the battery is going to perform charging and discharging.. One more factoid and food for thought. Fully charge the battery up and measure Ri. Something will happen you will not expect. Ri will shoot way up. This is what makes lithium different from all other batteries. All other types, Ri goes down as the battery charges up. Not lithium, it goes up. It goes down as it discharges and reverse polarity when over-discharged. Once you get your brain wrapped around that, it becomes very clear how to charge them up, and discharge them. That is what makes those sharp knee bends at the ends of the Charge/Discharge curve. Now you know why. Stay away from those curves. They are like a blonde with great big (.)(.) Fun to play with, but deadly stupid and will suck you dry.

    Example wire them all together and connect to an Inverter. With wire, crimping and connection resistance you are staring at 12 to 14 milli-ohms of resistance. Does not sound like a lot until you measure performance or do the math. If means for every 10 amps of load current you loose 0.12 volts or 1% of your voltage/power. In any battery system you limit voltage loss between points to 2% or less at max load current. At just 10 amps you are at 1% and stil have not accounted for any wire and connector losses.

    Think back to what I said about GBS and EV's are not a good match because their Ri is so high. It is as high as some 12 volt lead acid batteries with 100 AH capacity, Load up to 20 amps, and you have a .25 volt drop or 2% loss. You have a C/5 max load battery. Imagine if it were in a EV that takes up to 500 amps to accelerate and 50 amps to cruise. It would be unusable burning up most of its energy as waste heat. Resistance is important.

    That resistance also comes into play while charging. If your battery is resting at say 12 volts and you charge with 20 amps, what voltage will you see on the battery? Forget what you set the charger to because it means nothing. 20 amps x .012 Ohms = .24 volts. You only raise the battery voltage from 12 to 12.24 volts. Remove the current and the voltage drops back to 12 volts.

    Anyway this exercise will help you understand a bit more about what is going on and that voltage is just a number without a lot of weight behind it.

    Anyway what you have measured and learned is your 12 volt 100 AH battery has 10 to 12 milli-ohms of resistance. Just about exactly equal to a 12 volt lead acid battery @ 100 AH and will behave almost exactly the same.

    Now think about this. What would be the use of say a 100 AH cell with 1/4 to 1/100 the internal resistance. What can those cells do yours cannot? Don't get your shorts in a knot thinking I am criticizing your batteries, I am not. I just want you to know the limitations. Ri is a really bad thing to have in a battery. You are just now discovering why. It limits what you can do. Imagine the same battery 12 volt @ 100 AH if it only had say .0001 Ohms or .1 milli-ohm. You could easily charge or discharge at 2000 amps with the same 1% losses. That is 1000% more efficient. If you were to discharge your battery at 20 amps for 5 hours you loose 25 watt hours as waste heat and energy. Use a Boston Power 12 volt 100 AH battery Lithium battery and you only loose .025 watt hours. 25 watt hours out of 1200 may not sound like a lot, but it adds up to 2% loss. What it means is your battery is only good for low power applications of C/5 or less.

    Think about this. I have a hundred or more LiPo 3S 11.1 volt 2.2 AH batteries that out perform your battery. My LiPo's are 1/50 the size in capacity, but can easily supply 100 amps with 2% loss. I can run a much larger Inverter than you can with a battery the size of a pack of smokes and weighs less than 5 ounces. That is what Ri does.

    FWIW in your video, you did not give the Math Formulas to find resistance. Or I missed it.
    Last edited by Sunking; 07-30-2016, 08:18 PM.
    MSEE, PE

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    • #3
      Originally posted by Sunking View Post
      Hey Jesse one observation I made might save you some grief down the road regarding your Balance Plug. Your PL8 uses 3 amps to Balance Batteries with and your wire gauge might be a bit to small and too long. In the video you mentioned 22 AWG? Look into maybe 16 to 18 AWG. If you ever Top Balance, you will need it to be able to safely handle the current.
      This is a point of frustration for me. The balance connector uses very small pins. I guess my best option is to cut back to as close to the balance connector as I can safely get (1/2" or 1") and do my soldering there using 12 or 10 gauge wire. The connector is still the bottleneck, but at least the rest of my wire would be capable.

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      • #4
        Originally posted by Sunking View Post
        FWIW in your video, you did not give the Math Formulas to find resistance. Or I missed it.
        I didn't. I provided the formulas in the spreadsheet, if people are into that sort of thing. If not, there's a link in the description of the video to your post where you list the formulas.

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