Why a 130W solar module does not produce 130 W when connected to battery?

[Sunking]

Who is going to pay $200 for a $40 10 amp controller? To do what you suggest has already been done in decades past in the 8-Track Tape days. A relay with a voltage comparator circuit to operate a relay. DOH! That is exactly what Coleman controllers did.

[inetdog]

All PWM controllers use FET's. All FET's have a voltage drop between Drain and Source of approx 1 volt when operated as a DC Switch or relay.

It all depends on the type of FET and the available Gate to Source voltage. Take a look at the upper left graph on page 3.

The Source to Drain voltage shown there has a purely resistive characteristic as long as a high Gate to Source voltage is applied.
It may be that design economics result in a working configuration in which a 1 volt drop under full current load it tolerated, but it is by no means a "diode drop" which produces a constant voltage offset independent of current.
If you use a big enough FET (or enough in parallel) you can reduce the offset to as small a number as you want.

If you want to make the statement that all known PWM CCs are designed around a one volt drop, I do not have any information to contradict that. But that would be an engineering decision rather than a requirement imposed by the technology.

Since an MPPT controller uses either rectifier diodes or synchronously switched FETs on its output, the losses there would be the same as the losses in a PWM CC, and you would then have the additional input stage and transformer losses to contend with. What the MPPT controller does always have available is a source for high Gate voltages.

[Sunking]

With a MOSFET, there is no need for a forward bias. This is one reason that circuits which must have polarity protection but cannot afford the voltage drop use active circuitry with FETs instead of a diode. Not that any particular PWM CC does that, but that is the theory.

All PWM controllers use FET's. All FET's have a voltage drop between Drain and Source of approx 1 volt when operated as a DC Switch or relay.

[RageFury]

Well, yes and no... You said "Though it only has the Voltage and amps the Panel(s) provide." - this is incorrect in that it doesn't have the constituent voltage and current to work with (ie- only 12v and only 8amps), but rather any combination of the two that provides the maximum power to the battery. It's a technicality, but an important one when comparing MPPT vs PWM controllers...

Understood.

[ckhorne]

Thanks for the clarification, but I thought that is what I said. Did I articulate it incorrectly? Would not surprise me if I did lol...

Well, yes and no... You said "Though it only has the Voltage and amps the Panel(s) provide." - this is incorrect in that it doesn't have the constituent voltage and current to work with (ie- only 12v and only 8amps), but rather any combination of the two that provides the maximum power to the battery. It's a technicality, but an important one when comparing MPPT vs PWM controllers...

[inetdog]

Regardless a PWM will always have a forward bias voltage drop of approx 1 volt in the FET junction.

With a MOSFET, there is no need for a forward bias. This is one reason that circuits which must have polarity protection but cannot afford the voltage drop use active circuitry with FETs instead of a diode. Not that any particular PWM CC does that, but that is the theory.

[Sunking]

Exactly right! Very similar to what a transformer does for AC, just harder to implement.

Not exactly as there will always be power losses in the conversion process. Transformers can be up to 99% efficient and MPPT controllers as high as 98%. It a moot point though so the concept is correct.

Now if the output voltage of the panels were exactly right at any given moment during charging, a PWM CC could be slightly more efficient than an MPPT controller simply because there is less electronics between the panels and the batteries.

Nah. If the panel voltage were always correct there would be no need for a controller. Regardless a PWM will always have a forward bias voltage drop of approx 1 volt in the FET junction. So say 13 volts @ 7.69 amps (100 watts) comes out at 12 volts @ 7.69 amps (92 watts) 0r 92% efficiency which is lower than MPPT 95 to 98%.

This is what makes MPPT controllers so much better. When using a PWM controller on a 12 volt battery you are forced to use panels made for 12 volt batteries that have a Vmp of 17 to 18 volts. It has to be to be able to supply a charging voltage up to 16 volts + 1 volt for forward bias voltage.

MPPT allows you to use much less expensive Grid Tied Panels and much smaller less expensive wiring between panel and controller. There are some MPPT controllers that you can use up to 100 volts on a 12 volt battery. In that case you input 100 volts @ 1 amp and out with 12 volts at 8.16 amps.

[RageFury]

Thank you all for your responses. Still learning.

[RageFury]

Incorrect. It can use the total power (watts) that the panels provide. It can reconfigure the volts and amps up or down as needed, within the power (volt*amps) sent to it. 2volts @ 12 amps is the same as 12v @ 2 amps in terms of power (both are 24 watts).

Contrast this to a PWM controller that is limited to the amount of input current (amps) given to it. It can not reconfigure the total current to deliver more current than the panel provides.

Thanks for the clarification, but I thought that is what I said. Did I articulate it incorrectly? Would not surprise me if I did lol...

[inetdog]

So as I understand it the MPPT Controller changes the voltage and the Amps as the situation requires. Though it only has the Voltage and amps the Panel(s) provide. No more and no less. Thus if the MPPT Controller steps the voltage down you get higher Amps as a consequence and when it steps the voltage up you get lower Amps as a consequence. In both situations the watts should remain the same no?

Exactly right! Very similar to what a transformer does for AC, just harder to implement.

Now if the output voltage of the panels were exactly right at any given moment during charging, a PWM CC could be slightly more efficient than an MPPT controller simply because there is less electronics between the panels and the batteries. But in practice that never happens.

So the tradeoff is really just between the added cost of the MPPT controller versus the potential savings in panel cost from their greater efficiency and the lower cost per watt panel selections.


There have been statements that all other things being equal a PWM controller can be expected to last longer than an MPPT controller just because it has fewer active electronic components. But I have no statistical results to either verify or refute that. You can infer a relationship from the manufacturer's warranty period.

[Sunking]

So as I understand it the MPPT Controller changes the voltage and the Amps as the situation requires. Though it only has the Voltage and amps the Panel(s) provide. No more and no less. Thus if the MPPT Controller steps the voltage down you get higher Amps as a consequence and when it steps the voltage up you get lower Amps as a consequence. In both situations the watts should remain the same no?

You almost have it correct. There is some power loss. Again, Power In = Power Out - Efficiency Loss

As an example lets use 100 watt input with 95 % efficiency in the MPPT controller or 95 watts output

Input = 100
Output = 95 watts.

Input = 5.55 amps @ 18 volts = 100 watts
Output 7.9 amps @ 12 volts = 95 watts

Nothing magical happened. We just use a True DC to DC converter instead of a voltage regulator to drop the voltage down. Very simple math of conversions minus losses.

Works much like a Transformer Power transfer. Current and voltage are inversely proportional. Electrical Power or Watts is a product of Voltage x Current. So at 100 watts all the following statements are TRUE:

100 watts = 1 volt x 100 amps
100 watts = 10 volts x 10 amps
100 watts = 100 volts x 1 amp
100 watts = 1000 volts x .1 amps
100 watts = 1,000,000 volts x .0001 amps

Now here is the Eye Opener lets use the same panel and battery, except this time we use a PWM Controller where Input Current = Output Current.

Input = 18 volts @ 5.55 amps = 100 watts
Output = 12 volts @ 5.55 amps = 67 watts.

[russ]

Volts x amps = watts

No magic watts appearing or disappearing though with PWM there can be watts lost.

As someone pointed out - a 130 watt panel would only put out 130 watts for a few minutes at solar noon on possibly a great summer day or possibly a sunny cold winter day.

[ckhorne]

So as I understand it the MPPT Controller changes the voltage and the Amps as the situation requires. Though it only has the Voltage and amps the Panel(s) provide. No more and no less.

Incorrect. It can use the total power (watts) that the panels provide. It can reconfigure the volts and amps up or down as needed, within the power (volt*amps) sent to it. 2volts @ 12 amps is the same as 12v @ 2 amps in terms of power (both are 24 watts).

Contrast this to a PWM controller that is limited to the amount of input current (amps) given to it. It can not reconfigure the total current to deliver more current than the panel provides.

Thus if the MPPT Controller steps the voltage down you get higher Amps as a consequence and when it steps the voltage up you get lower Amps as a consequence. In both situations the watts should remain the same no?

Correct.

[RageFury]

Read up on MPPT charge controllers. From

"The power point tracker (and all DC to DC converters) operates by taking the DC input current, changing it to AC, running through a transformer (usually a toroid, a doughnut looking transformer), and then rectifying it back to DC, followed by the output regulator."


What the MPPT does is vary the current so that voltage is within the range needed for charge. If your panel was generating 100w under normal conditions, your current charge current at 12v would be 100w/12v = 8.3amps. However, if your panel was in ideal conditions (such as crotalus's example), then it may produce 130 watts; the mppt controller would maintain the same 12v, buy yield 10.8 amps. (These are all assuming no inefficiencies in the charge controller).

In other words, with an mppt, incoming current has little bearing on output current. That's the whole point...

So as I understand it the MPPT Controller changes the voltage and the Amps as the situation requires. Though it only has the Voltage and amps the Panel(s) provide. No more and no less. Thus if the MPPT Controller steps the voltage down you get higher Amps as a consequence and when it steps the voltage up you get lower Amps as a consequence. In both situations the watts should remain the same no?

[ckhorne]

So what are you saying is that if proper MPPT charge controller is connected, the incoming current to the battery will exceed the short circuit current (Isc)???? How is that feasible? Please explain

Read up on MPPT charge controllers. From xxx: Mod note - don't include links to the competition of our sponsor - bad form.

"The power point tracker (and all DC to DC converters) operates by taking the DC input current, changing it to AC, running through a transformer (usually a toroid, a doughnut looking transformer), and then rectifying it back to DC, followed by the output regulator."


What the MPPT does is vary the current so that voltage is within the range needed for charge. If your panel was generating 100w under normal conditions, your current charge current at 12v would be 100w/12v = 8.3amps. However, if your panel was in ideal conditions (such as crotalus's example), then it may produce 130 watts; the mppt controller would maintain the same 12v, buy yield 10.8 amps. (These are all assuming no inefficiencies in the charge controller).

In other words, with an mppt, incoming current has little bearing on output current. That's the whole point...