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  • What Is Your Internal Resistance?

    OK you might be saying to yourself "huh"? But it is a number you can put to great use, especially as a barometer to tell you are on borrowed time and won't be long till you are replacing batteries. But this thread is all about me.

    Basically a call to post your batteries internal resistance for me to crunch data. It will not take long measure it, nor is it difficult requiring a lot of test equipment, just a couple of simple steps. What you get out of it is priceless.

    So here is how you do this.

    1. First you want the batteries to be at or near 100% charged up, anything above 90% is good and rested a bit if possible.

    2. Turn off all loads so there is ZERO current flowing and panels are off like just after sunset. Once you are sure the system is dead, measure and record the battery voltage right on the battery positive and negative post. Monitor for a minute or so until it stabilizes. Label this voltage as V1

    3. Now leaving your meter connected to the battery turn on all the load devices you can without over loading anything.

    4. This maybe the show stopper for most of you, but note and record the current with everything turned on.

    6. Simultaneously while you record current, note loaded voltage on battery and record that voltage. Label current as Il, and voltage as V2

    7. Test concluded, put everything back to normal.

    OK to find your battery or batteries internal resistance we use the data you collected. Ri = [V1-V2] / Il

    So let's run through an example. 12 volt system loaded with 40 amps. Open circuit battery voltage or V1 = 12.5 volts, Loaded voltage or V2 = 11.86 volts. So Ri = [12.5 - 11.86] / 40 = .016 Ohms.

    How do you use that number? It works best on brand new batteries to establish a Base Line, but still useful on older batteries. When you start noting the Ri is growing means you have lost capacity and the batteries are on there way out.
    MSEE, PE

  • #2
    Originally posted by Sunking View Post
    OK you might be saying to yourself "huh"? But it is a number you can put to great use, especially as a barometer to tell you are on borrowed time and won't be long till you are replacing batteries. But this thread is all about me.

    Basically a call to post your batteries internal resistance for me to crunch data. It will not take long measure it, nor is it difficult requiring a lot of test equipment, just a couple of simple steps. What you get out of it is priceless.

    So here is how you do this.

    1. First you want the batteries to be at or near 100% charged up, anything above 90% is good and rested a bit if possible.

    2. Turn off all loads so there is ZERO current flowing and panels are off like just after sunset. Once you are sure the system is dead, measure and record the battery voltage right on the battery positive and negative post. Monitor for a minute or so until it stabilizes. Label this voltage as V1

    3. Now leaving your meter connected to the battery turn on all the load devices you can without over loading anything.

    4. This maybe the show stopper for most of you, but note and record the current with everything turned on.

    6. Simultaneously while you record current, note loaded voltage on battery and record that voltage. Label current as Il, and voltage as V2

    7. Test concluded, put everything back to normal.

    OK to find your battery or batteries internal resistance we use the data you collected. Ri = [V1-V2] / Il

    So let's run through an example. 12 volt system loaded with 40 amps. Open circuit battery voltage or V1 = 12.5 volts, Loaded voltage or V2 = 11.86 volts. So Ri = [12.5 - 11.86] / 40 = .016 Ohms.

    How do you use that number? It works best on brand new batteries to establish a Base Line, but still useful on older batteries. When you start noting the Ri is growing means you have lost capacity and the batteries are on there way out.
    Sunking

    Those are really easy to understand and simple steps to take to understand the health of your batteries. Thanks.

    Now on the side of levity, I thought this posting was related to your personal resistance to alcohol consumption.

    Comment


    • #3
      My personal resistance to aclohol consumption would be a negitive number.

      WWW

      Comment


      • #4
        This is GREAT!

        Ok, here are two quick ones:

        Optima D34M 55ah Blue-Top AGM. (light gray case). 8 months old. Approx 25 cycles not exceeding 60% DOD. Typical storage / operation temps 75F - 100F, average would be in the middle. Test was performed at 80F. Manufacturer lists 0.0028 ohms. No abusive charging, and typical discharge ranging from 0.05C to 0.15C for most cycles. Prior to test, it had a 48-hour rest after a weeks worth of maintenance at 13.6v float. Fluke 87V used for voltage test.

        V1 = 13.07
        V2 = 12.90
        Constant current load = 3.5A

        So that makes what, [13.07-12.90] / 3.5 = 0.0485 ohms for Optima Blue Top.

        Sidenote: Test taken with inexpensive Centech battery analyzer reports 2.64milliohm. BUT this uses a different method for detection, however THAT kind of detection may be what Optima uses for it's own rating. Not sure why my voltmeter results are so different, although maybe I should try again RIGHT after a charge period with a 5 minute rest?

        What about lifepo4?

        Unfortunately I didn't have time to fully charge the lifepo4, so this test was done at 50% DOD. This might be a good candidate since the extreme voltage drop from 100% SOC using this method might skew results. I forget actually if resistance goes higher or lower at top of charge. Same conditions as above.

        40AH GBS lifepo4.

        V1 = 13.27
        V2 = 13.21
        Constant current load 3.5A

        [13.27-13.21] / 3.5 = 0.0171 ohms

        But remember I'm at 50% DOD, so this may not be accurate. I am NOT going to put the lead-based Centech analyzer on it!

        That was cool. I'm just wondering how accurate the measurement is if you don't rest for 12 or more hours though. Still, if one follows a repetetive procedure, this is a great way to get a baseline.

        Comment


        • #5
          Originally posted by PNjunction View Post
          40AH GBS lifepo4.

          V1 = 13.27
          V2 = 13.21
          Constant current load 3.5A

          [13.27-13.21] / 3.5 = 0.0171 ohms

          But remember I'm at 50% DOD, so this may not be accurate. I am NOT going to put the lead-based Centech analyzer on it!

          That was cool. I'm just wondering how accurate the measurement is if you don't rest for 12 or more hours though. Still, if one follows a repetetive procedure, this is a great way to get a baseline.
          PN and others following you might be interested to know the battery Ri is directly related to the battery SOC and temperature. There are EV's they use the battery Ri to determine SOC and they are very accurate especially when combined with Coulomb Counting (current in/current out). As the battery SOC goes lower, Ri increases. Now to make it work is the trick as you have to build a data table to reference SOC percentage.

          The LSV I am building uses an AC motor and a Curtis Controller with built in SOC metering. It use Coulomb Counting and Weighted SOC voltages. The controller then sends out a 0-20 ma signal to a fuel gauge meter. Works great if using LFP or other lithium battery type. Completely worthless if using Lead Acid due to the greedy SOB Mr Peukert.

          EDIT NOTE:

          PN the test becomes more accurate with a heavier load. If you have the equipment to load it down in excess of 1C even works. If you want to know the CCA, freeze the battery down to 0 degrees, and load the battery down until the voltage reaches 7.2 volts on a 12 volt battery. Example if you had dealer specs on say the Trojan T-105 you will find CCA, CA, MCA, and RC spec hidden from the public. CCA on a T-105 is rated at 720 amps, and CA is 805 amps. Do the math for CA which is at 32 degrees. [6.3 - 3.6] / 805 amps = .0033 Ohms @ 32 degrees. At room temp it would go down.

          So don't be afraid to load it down, just keep the current below what your wiring can safely handle without melting the insulation. In the above example would pull roughly 5.7 Kw from a battery designed to deliver 200 watts without excessive voltage sag. Heck a golf cart with a performance controller can pull up to 600 amps @ 48 vdc. With voltage sag works out to around 17.2 Kw or roughly 23 horse power.
          MSEE, PE

          Comment


          • #6
            How about my fully charged Optima Blue Top as measured by a Junsi iCharger 306B hobby-charger?

            It's own Ri mode reports 35 millohm on the screw-down terminals. Only 29 milliohm directly on the lead terminal posts. Kind of close to my earlier measurement with your method. I'd like to compare that to a Revolectrix Powerlab 8, but don't own one of those.

            Heck, if one wanted baseline measurements, they might want to look into one of the hobby chargers JUST for taking an internal resistance measurement. Figuring out how to get to that mode might prove challenging for those that don't like clicking through a lot of menus though.

            Comment


            • #7
              Originally posted by PNjunction View Post
              How about my fully charged Optima Blue Top as measured by a Junsi iCharger 306B hobby-charger?

              It's own Ri mode reports 35 millohm on the screw-down terminals. Only 29 milliohm directly on the lead terminal posts. Kind of close to my earlier measurement with your method. I'd like to compare that to a Revolectrix Powerlab 8, but don't own one of those.

              Heck, if one wanted baseline measurements, they might want to look into one of the hobby chargers JUST for taking an internal resistance measurement. Figuring out how to get to that mode might prove challenging for those that don't like clicking through a lot of menus though.
              Revolectrix does make some nice chargers that come preloaded with a lot of battery recipes. I own the smaller Multi4 which I use for my RC batteries.

              Comment


              • #8
                Unfortunately the multi-4 doesn't measure internal resistance afaik.

                So here is the perfect justification to upgrade to the 10xp, powerlab 6/8 as you can do your RC chores in addition to using it for measuring your Pb battery internal resistance.

                Comment


                • #9
                  Originally posted by PNjunction View Post
                  It's own Ri mode reports 35 millohm on the screw-down terminals. Only 29 milliohm directly on the lead terminal posts.
                  6 milli-ohms connection resistance? That is not good.
                  MSEE, PE

                  Comment


                  • #10
                    I was surprised by the difference - then again, Optima states that the screw-terminals are not intended for high-amperage loads.

                    That is why I kind of cringe when I see those connecting these together in series / parallel strings using the screw-terminals rather than the larger lead terminal posts. It may be convenient, and perhaps not noticeable for those who are charging and discharging lightly.

                    Also not to get too sidetracked into RC modeling, but apparently the Revolectrix chargers need to have the battery very well discharged before they can do internal resistance measurements. This may not be ideal for those using them to determine the Ri of their large lead-acid banks. Then again, perhaps they act differently with large lead acid batteries - I don't know. I do know that the Junsi iChargers don't require a deep discharge to measure Ri. Food for thought for those who want to use these to baseline their Ri.

                    Comment


                    • #11
                      PN connection resistance are measured in micro-ohms, not milli-ohms. 6 milli-ohms = 6000 micro. Small connectors will be 500 micro or less. In telecom they specify and test 15 micro-ohms for battery term post. You cannot get that low with your battery but .006 ohm connection resistance just don't sound right to me. That alone will sag battery voltage 1% at 20 amps and drop 2.4 watts on the connector warming it up. .006 ohms is what I expect to see on a 12 volt 50 AH battery.

                      Just curious how are you measuring it?
                      MSEE, PE

                      Comment


                      • #12
                        Bump!
                        OK, I eventually got round to doing this, been meaning to do it for months...

                        I have 5 12V 7Ah batteries in parallel, all secondhand freebies (there's another dozen waiting for me to collect)...

                        "Battery A" V1=12.55V , 4W bulb (V2)=11.20V , 35W bulb (V2)=9.7V

                        "Battery B" V1=12.63V , 4W bulb (V2)=12.37V , 35W bulb (V2)=0.23V

                        "Battery C" V1=12.67V , 4W bulb (V2)=12.50V , 35W bulb (V2)=12.03V

                        "Battery D" V1=12.79V , 4W bulb (V2)=12.25V , 35W bulb (V2)=0.45V

                        "Battery E" V1=12.80V , 4W bulb (V2)=12.55V , 35W bulb (V2)=12.14V

                        yup, forgot to measure current...

                        They deliver, between them, around 9A for 5 hours during daylight, any daylight "left over" goes into charging.
                        The load is disconnected most of the time, but I might forget to disconnect it for a few days at a time.
                        My panel will deliver almost 150W, but 75 is more or less guaranteed...
                        Dem

                        Comment


                        • #13
                          Originally posted by FunGas View Post
                          Bump!
                          OK, I eventually got round to doing this, been meaning to do it for months...

                          I have 5 12V 7Ah batteries in parallel, all secondhand freebies (there's another dozen waiting for me to collect)...

                          "Battery A" V1=12.55V , 4W bulb (V2)=11.20V , 35W bulb (V2)=9.7V

                          "Battery B" V1=12.63V , 4W bulb (V2)=12.37V , 35W bulb (V2)=0.23V

                          "Battery C" V1=12.67V , 4W bulb (V2)=12.50V , 35W bulb (V2)=12.03V

                          "Battery D" V1=12.79V , 4W bulb (V2)=12.25V , 35W bulb (V2)=0.45V

                          "Battery E" V1=12.80V , 4W bulb (V2)=12.55V , 35W bulb (V2)=12.14V

                          yup, forgot to measure current...
                          Well even though you did not get the current, based on wattage the news is not good. Battery E is the only good one you have (Roughly .2 Ohm's) and it is not great by any means. B and D if not a typo are boat anchors, and battery A is ready to be used as a boat anchor. Battery c is so so.
                          MSEE, PE

                          Comment


                          • #14
                            Battery C was *new* in June, just goes to show how a bad one in the stack can kill off the other good ones.

                            I found 3 more with a standing charge of over 12V, just wired them up and discarded the 2 bad ones, lets give it a few days on charge an I'll repeat the test, with a current value this time

                            [edit]
                            OK, here's the results from the second test after a couple of days charging, off load.

                            A 12.85V, 9.6V, 2.75A
                            B 13.0V, 12.70V, 3.25A
                            C 13.07V, 12.38V, 3.25A
                            D 13.09V, 12.34V, 3.25A
                            E 13.02V, 12.33V, 3.25A
                            F 13.07V, 12.23V, 3.20A

                            So, the approximate internal resistances are:-

                            A 9.35Ω
                            B 9.09Ω
                            C 9.26Ω
                            D 9.29Ω
                            E 9.23Ω
                            F 9.25Ω

                            I say approximate, because my current meter is ex-surplus, probably about 50 years old.

                            My question is, what's the ditch point? I guess battery A is knackered, but what about battery E?
                            Dem

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                            • #15
                              bump! (my [edit] didn't bump the thread)
                              I'd appreciate any input...
                              Dem

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