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MPPT

Discussion in 'Mathematics and Physics' started by PG1995, Dec 24, 2013.

  1. steveB

    steveB Well-Known Member Most Helpful Member

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    There are a lot of questions there. I'll make an attempt at most of them.

    It's not clear what PWM charging is exactly, as compared to MPPT charging, when solar cells are considered. That reference is talking about battery charging in general, which is a separate subject from MPPT and solar cells.

    I can't claim to know this for sure, or based on any direct knowledge. But, common sense tells me this is a cost issue. If you need to put batteries in outer-space, light weight and small volume are critical. If you need to put them in aircraft, light weight and small volume are very important. If you need to put them in cars, light weight and small volume are very desirable. But, for solar, where you are on the ground typically and already used a wide area for the solar cells, light weight and small volume are no where near as important than cost.

    First of all, you would never do this. The voltage difference is just too much for any practical battery to operate under these conditions. Fire, explosion, outgassing, overheating and lawsuits will result for doing this.

    However, let's pretend you could do it with a new technology. Remember that the most simple model you can make for a battery is an ideal voltage source in series with a resistance. Without the resistance, you can't even do what you say within theory because an ideal 36V source in parallel with an ideal 12V source is an indeterminate case, or a case where an "immovable object meets an irresistible force", so to speak. With the resistors in place, you can do it in theory, and if you want to understand it, just draw the circuit and analyze it. You'll pretty much get what NorthGuy described.

    Don't know, but the battery manufacturer would tell you. Probably you can get a basic idea by the state of charge you are at. Knowing state of charge for a battery is not always easy, but there are various ways to estimate it. It's easier for some batteries. For example, the unloaded voltage of a lithium-ion battery is a good indicator of state of charge, I believe. Battery aging is a very difficult thing to factor in to these estimates.

    Yes you make sense. Basically, you typically want to condition power in particular ways for various devices. Solar panels are a type of unregulated power source. A converter makes a very effective regulator and is very adaptable to various circumstances.
     
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  2. NorthGuy

    NorthGuy Well-Known Member

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    So called PWM chargers are simply constant voltage (CV) chargers - simply a mosfet, which turns on when battery's voltage is lower and off when it is higher, or some algorithm of that sort. They can do the full profile charging. First stage - constant current (CC) - with solar panels this is practically unlimited current - you would do MPPT search, the PWM controller simply stays at 100% duty cycle. Second stage - consant voltage (CV), at which you don't do MPPT any more, so PWM is doing the same as MPPT would. Third stage - float - CV but at lower voltage level. Again both MPPT and PWM are the same here.
     
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  3. PG1995

    PG1995 Active Member

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    Thank you, NG, Steve.

    These two links, link #1 and link #2, explain PWM charger controller. In simple terms, PWM charger is mainly concerned about safeguarding the battery against any damage and regulating the current flowing into the battery being charge. As you decrease or increase the duty, the value of average voltage changes and so does the average current flowing into the battery.

    I'm still very much confused about that scenario where 12v battery is being charged using 36v battery. In my humble opinion, there isn't really a need to include resistors for proper analysis as Steve has pointed out because we are just considering a hypothetical situation in general terms. But let's modify the scenario a bit. Let's use a 24v DC source to charge 12v battery which is sitting at 10v. I won't repeat the details from my previous post. A voltmeter reads the difference between potential of current carriers between two points. If it reads 4v around a resistor then it means that the current carriers at its positive terminal have 4 joules of more energy than the carriers at its negative terminal (assuming positive charge carrier in terms of conventional current). In other words, the resistor is consuming that 4 joules of energy. Now return to that battery charging scenario. When that 12v battery is connected to 24v DC source for charging, the voltmeter will read 10v at the start. Why? Shouldn't the voltmeter read 14v instead of 10v? Could you please help me? Thanks.

    My other query was about sensing different states of a battery being charged. Steve has pointed out that knowing or sensing the state of a battery isn't easy. Do such battery charger ICs employ some mechanism to sense the state? It has to sense the state because the battery isn't going to tell the regulator to limit the current flowing into the battery and battery itself is indefensible against the excess current.

    Regards
    PG
     
  4. dave

    Dave New Member

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  5. NorthGuy

    NorthGuy Well-Known Member

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    Regular charger is connected to a stable source, so, in theory, it can produce unlimited current. Solar panel can't. Therefore, you shouldn't worry about too much current flowing into the battery. The only thing you need to be concerned about is voltage.

    As the voltage rise you limit it at a constant level, which produces the current curve that you see on stage 2.

    Once you decide your battery is fully charged, you drop voltage to the float level enering stage 3. You cannot "sense" when to do that. Common practice is either to give a fixed time to stage 2, or to do it until current drops to a certain level.

    If you want to sense the battery state, the best way is to count amper-hours in and out, but I don't think you should worry about the state.
     
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  6. steveB

    steveB Well-Known Member Most Helpful Member

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    As I pointed out, a hypothetical (or theoretical) case does not allow ideal voltage sources to be put in parallel if the voltage values are different. Even a 12.0 V and 12.1 V ideal source can not be theoretically connected. Parallel connections require both voltages to be the same, and it is impossible with ideal sources. However, add a series resistance to each source (which is a good approximation to reality very often) and there is no problem. The resistors drop the necessary voltages to allow the parallel combination to have equal voltages at the terminals. THeory is satisfied, even if a practical example might blow up in your face.

    However, perhaps I misunderstood what you are saying. Maybe you are included a resistor between the batteries, in which case ideal sources can be considered hypothetically.

    I've lost you here. Can you provide a schematic?

    I think chargers often try to sense the state of the battery, unless the battery can be charged by an automatic self correcting method. If a battery can be changed with a voltage source and resistor, then the process may be automatic and the current automatically goes to zero as the battery gets near the voltage source voltage. However, if you need to change between voltage and current charging, or do trickle charging or some other more sophisticated method, then you have to estimate the state of charge. Sometimes, the battery voltage tells you enough to make a safe charging system, and sometimes you need to be more sophisticated method. It all depends on the battery type, and people are always trying to figure out better and better methods, especially with the newer battery technologies.

    The best thing to do is identify the battery type/technology you are using, and read up on how charging is done for that type. There are hundreds of different battery types, so it's not easy to answer generally.
     
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