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Estimated charge current at different terminal voltages?

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  • #16
    An OCV 12.0V battery connected to another more charged battery OCV 12.6V

    .6V / .01Ohm = 60A... Terminal voltage goes to 12.5xV and the batteries balance their charge, current tapers off as voltage difference tapers off?

    OCV 14.6V alternator hooked to OCV 12V battery

    2.6V / .01Ohm = 260A... If the alternator is really big say 400A then 260A flows and battery boils like crazy and maybe explodes? 60A alternator gets maxed out with say 20A going to electronics and lights and maybe 40A going to the battery? That's a fast battery charge, 100Ah starting batteries are designed to handle high .4/C rate and just be full all the time? Starting the engine make battery drop slightly on charge and then gets topped right off seconds or minutes after alternator is spinning and the alternator controller lowers the field pulse duty cycle to produce 13.6V float voltage?

    Ohm's law is simple math, but me and I'm sure alot of other people don't realize the .01 Ohm typical resistance of a lead acid and the voltage difference between the charger and battery divided by the internal resistance = the potential amperage flow if the charger has enough current?

    A year ago I was charging a lead acid starting battery for a Jeep with a spare 90W 19.5V laptop PSU (cause I didn't have a dedicated lead acid battery charger). I disconnected the vehicle clamp (to remove any vehicle drains from my unorthodox charging to make things simple) and I hooked up the PSU to the battery and the PSU was overheating like I thought it would, so I plugged the PSU into a Kill-a-Watt and saw it was taking 180W, then I used a piece of steel wire as a resistor between the battery and PSU to slow it down (and waste a few watts as heat) and adjusted the resistor till the PSU was taking in 100W. I left the PSU hooked up till the specific gravity stopped rising, I was able to turn down the resistance less and less and did not need the resistor towards the end, without overheating the PSU, 15V OCV battery lets say, 19.5V - 15V = 4.5V, 4.5V / .01Ohm = 450A, should have overloaded the 4.6A PSU right? This observation I had leaves me confused, unless the battery Ri was at least 1.022 Ohm? I swear I saw 15V terminal voltage with the PSU hooked up without the resistor. Or the PSU was unable to push more than it's 4.6A thru the battery for some reason once it was charged?

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    • #17
      Originally posted by copymepls View Post
      An OCV 12.0V battery connected to another more charged battery OCV 12.6V

      .6V / .01Ohm = 60A... Terminal voltage goes to 12.5xV and the batteries balance their charge, current tapers off as voltage difference tapers off?
      Pretty close except you did not account the Ri in the other battery, and cable resistance. .

      Originally posted by copymepls View Post
      OCV 14.6V alternator hooked to OCV 12V battery

      2.6V / .01Ohm = 260A... If the alternator is really big say 400A then 260A flows and battery boils like crazy and maybe explodes?
      OK here is where I think you might be getting lost. Alternators and Chargers have Current Limiters built into so they do not self destruct and/or start fires. In your example let's say 100 amps. Initially when you connect the alternator the voltage falls to 100 Amps x .01 Ohms + 12 volts = 13 volts.

      I used .01 Ohms as an example because it is easy to work with and do the math in your head. In reality a GC battery Ri is not that low. I cannot tell you with any accuracy what the Ri of any battery truly is as it varies quite a bit. I can tell you as a rule, the larger the battery is, the lower the Ri is going to be. But Ri is going to be dependent mainly on how the battery is constructed and what it is designed for.

      SLI aka Starting Lighting Ignition batteries are constructed to deliver extremely high discharge currents and them be charged extremely fast. To do that they use thin spongy textured like plates to increase surface are which lowers Ri. Great for cranking engines, but really poor for cycle applications. Thin plates cannot be deep cycled.

      Deep Cycle batteries on the other hand haver much thicker heavier plates that can be deeply discharged and cycled many times. Downside is Ri is not as low. That means charged slower and cannot be discharged at high rates without voltage loss.

      Then we have the hybrids. A cross between a SLI and Deep Cycle that can do both, but just not quite as good as a true SLI or Deep Cycle. Hybrids have many marketing names like GOLF CART, Marine, Traction, RV, Liesure. Trolling, and the list goes on and on.

      All the above refers to Flooded Lead Acid batteries. But there is another type of lead acid that is a thoroughbred called AGM or Absorbed Glass Mat. I think of them kike hybrids on steroids. AGM packs a punch and can take a punch. Very low Ri that can charge and discharge at very high rates. The automotive industry is switching to them and those types can easily be charged at 1C.

      Lastly there is one other source of resistance you have overlooked and not accounted for which makes the battery Ri a moot point. Wire/Cable, fuse, and connection resistances. That is where most of the voltage loss occurs and why you are worrying about nothing. Couple that with using GC batteries, and no worries my friend. All you have to worry about is if the wire you use can handle the expected charge and load currents. Use an automotive type cable which uses a high temperature cable insulation, and you are good to go. Example a #6 AWG can easily handle 100 amps. At 100 amps with just 10-feet of cable 1-way you are going to loose 1 volt because the 6 AWG has .01 Ohms resistance, plus roughly .03 Ohms for your pair of GC batteries wired in series. So now your charge circuit realistically is .04 to .06 Ohms. Now look what happens to your example of a 100 Amp Alternator. At best only 50 amps of charge current from a source capable of 100 amps.

      So relax. You are using GC batteries that can take high currents and with the wiring makes all your worries go away. Worry about the wiring and minimizing resistance. That has to do with workmanship and tooling.
      Last edited by Sunking; 02-04-2017, 02:04 PM.
      MSEE, PE

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      • #18
        Right, I was just pretending we had wires with no resistance so we could just talk about the battery and chargers.

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        • #19
          I took apart a few Jeep alternators before, just a 3 phase coil arrangement with a rectifier, the field on the rotor is connected to some brushes and leads to the ECU it looks like. I was picturing the ECU used some voltage monitoring circuit and used PWM/duty cycle etc to control the field to control the alternator output by rapidly flipping it on and off to control voltage. Is that what you mean is the current limiter?

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          • #20
            Some other alternators I am not too familiar with seem to have a built in board that does something????

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            • #21
              Or maybe I didn't notice they maybe had current limiting diodes in the rectifier? That makes sense.

              Or just a fuse.
              Last edited by copymepls; 02-04-2017, 04:12 PM.

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              • #22
                Originally posted by Sunking View Post

                OK here is where I think you might be getting lost. Alternators and Chargers have Current Limiters built into so they do not self destruct and/or start fires. In your example let's say 100 amps. Initially when you connect the alternator the voltage falls to 100 Amps x .01 Ohms + 12 volts = 13 volts.
                I was picturing a huge 400A rated alternator from a big rig or some machine except 12V not 24V

                A 120A jeep alternator is limited to 120A? Is it limited to the rated current stamped on the alternator?

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                • #23
                  Originally posted by copymepls View Post

                  I was picturing a huge 400A rated alternator from a big rig or some machine except 12V not 24V

                  A 120A jeep alternator is limited to 120A? Is it limited to the rated current stamped on the alternator?
                  Should be. Relax.

                  MSEE, PE

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                  • #24
                    They never seem to put the AH on cranking batteries,and I know they are not supposed to be cycled deeply, but do they have the same energy capacity per mass as deep cycles? Like a 50lb deep cycle and a 50lb crank battery both drained to 80% SoC, would you get similar watthour outputs from them?

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                    • #25
                      Originally posted by copymepls View Post
                      They never seem to put the AH on cranking batteries,and I know they are not supposed to be cycled deeply, but do they have the same energy capacity per mass as deep cycles?
                      SLI (Starting Lighting & Ignition) batteries aka cranking do not have AH rating because they are not made to be deeply discharged. However that is changing with new vehicles. Most new vehicles sold today are equipped with AGM hybrid batteries to meet EPA mileage requirements. To do that your vehicle engine shuts off while stopped like at a traffic light. Stil not something you want to use for cycle applications.

                      Originally posted by copymepls View Post
                      Like a 50lb deep cycle and a 50lb crank battery both drained to 80% SoC, would you get similar watthour outputs from them?
                      Initially yes, but after just a few cycles the SLI battery capacity would drop off quickly. SLI batteries are constructed with thin spongy like plates to increase surface area and that lowers the internal resistance to give you Cranking Amps. With each cycle you loose plate material. SLI batteries use stronger acid and higher voltages. That eats away at the plates. Not a big problem for short burst of power than quickly recharged. Deep discharges requires much longer recharge time, thus accelerates plate corrosion.

                      Hybrids and deep cycle use heavier thicker plates, lower acid concentration and slightly lower voltages. That gives them the deep discharge capability. A SLI battery would have to be significantly larger for a given weight. Think of steel wool compared to solid steel block of equal weight.

                      MSEE, PE

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