MPPT

[steveB]

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

I have read that mppt chargers are recent addition to category of solar chargers and before that PWM chargers were very popular which are less efficient than mppt ones. I have tried to find some good source which describes how pulse width modulation charger works but I couldn't find any good source. Could you please point me to some good source, or if possible, could you briefly describe it yourself? I think it would be good thing to have overall understanding of its operation and knowing shortcomings of PWM chargers will highlight the importance of using mppt. You might find this text useful.

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.

It has been mentioned that lead acid batteries are used for solar energy. Any special reason for this choice? Why not lithium-ion batteries? One of the reasons for not using Li-ion might be their high price. .When it comes to solar energy, do both lead acid and sealed lead acid batteries share the same place?

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.

I understand that when a battery is connected to a charger then the voltage, Vout, at the output terminals of charger is same as that of the battery, as you have also told me. I'm still little confused so let's look at it from a different point of view. Let's suppose we have two batteries; the one is rated at 36 V and is fully charged, and the other is rated 12V and is sitting at 10 V. The positive terminal of one battery is connected to positive terminal of the other battery and the same goes for negative terminals. The 36V battery should be able to push current through the 12V battery because it sits at higher voltage. We have also connected a voltmeter which will read 10V at the start. But why? We know that resistance of one ohm means drop of one volt per ampere, i.e. 1Ω=1V/A where one volt is one joule per coulomb and ampere is flow of charge of one coulomb per unit time through a certain point. Think of 12 volt battery which sits at 10v as a resistance. Please have a look here. The 36 volt battery is pushing the current clockwise and 12volt battery counterclockwise and the winner is going to be 36 volt battery. We can see that 36-10=26volt. Where are these 26 volts going? They are being used to charge up the battery but then why isn't the voltmeter reading 26 volts? How do we calculate the resistance of 12V battery? I think that I know the answer but for some reason I don't know how to put it together. Please guide me. Thanks.

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.

This is the charging profile for a lead acid battery cell. I don't think a battery internally can control the amount of current during its charging. As we can see that the current needs to be closely monitored and gradually decreased in Stage 2. There should be some external mechanism or device which regulates the current going into the battery. Obviously, such a device called battery charger or regulator. But how would a regulator determine that battery has entered Stage 2 in order to tightly control the current?

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.

Although I have made this query before, I'm still confused so I will give it another try. Why don't we connect a load, such as electric heater, directly to the panel? We need to consider two important points about a solar panel first. First a solar panel behaves like a current source. A current source is always concerned about passing a fixed amount of current through a resistance and in order to do that it can adjust the voltage (its pressure) around the resistance. Secondly, a solar panel's maximum power is rated for Vmpp and Impp which is mostly calculated at 1000W/m^2 at 25C. Suppose the heater resistance is 20. We know that V=IR=(1.5)(20)=30V, and Power=VI=(30)(1.5)=45W. But from the figure we can see that the panel is still willing to provide more voltage (strong push or more pressure) if it is offered a suitable resistance. It appears that that suitable resistance is 33Ω. If we had used resistance or heather of 33Ω then power obtained would be almost (50)(1.5)=75W. We would only be able to utilize this extra power of 30W (75-45) if the heater's resistance can be adjusted. Or, the easier way would be to use an mppt tracker. Suppose, the tracker steps down Vmpp to 30V on its output terminal and steps up the current to 2.5A then you would be able to get maximum power even with your 33Ω heater. We should also consider another issue. If we were using a light bulb instead of heater then there would be other issues. First, if we had connected light bulb directly to the panel then in the first place we would be wasting some power and on the other hand light from the bulb would continuously vary. If we had connected light bulb using an mppt even then brightness will vary during different times of the day. So this tells us another advantage of using a battery an intermediate energy storage device. Do I make sense?

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.

 

[NorthGuy]

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.

 

[PG1995]

Thank you, NG, Steve.

These two links, link #1 and **broken link removed**, 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

 

[NorthGuy]

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.

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.

 

[steveB]

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.

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.

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.

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

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.

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.