Sunday, Apr 04, 2010 at 23:16
If you have the specifications for the solar panel and the effiency of the MPPT charger you can do your own maths to work out what you may and may not gain by replacing the common switching regulator woth an mppt.
for the example i've chosen a BP380 panel
panel spec here and a mppt12-1
mppt spec
I've derated the typical mptt effiency from the manufacturer from 97% to 95% to allow for real life.
I'm also using 14.5V as the charging voltage ( because it fits with the mppt spec and this isn't a discussion about charging voltage ).
Panel max pwr = 80W
Voc = 22.1V
Isc = 4.8A
Vmaxpwr = 17.6V
Imaxpwr = 4.55A
Battery charge Voltage = 14.5V
MPPT effiency = 95%
You need to understand the difference between a switching regulator and an MPPT.
Basically a switching regulator will connect then disconnect the power source to the load to maintain an average output voltage for a series regulator. Or for a shunt regulator, it will switch a short circuit on and off across the power source. To prevent the load being shorted out it is typically isolated by an electronic device that only allows current to flow from the source to the load.
In this case the solar panel is the power source and the battery is our load.
What this means is that for a switching regulator the maximum current we can possibly get from the solar panel is Isc ( 4.8A ).
At 14.5V ( the load voltage ) and 4.8A we get ( P = V * I ) 69.6W, not 80W.
We've lost some power during the period that the panel isn't connected to the load.
A MPPT uses some electronic smarts to adjust the load on the solar panel to try and maintain it at it's maximum power point to extract the maximum amount of power from the panel as it can.
The circuitry of the mppt transers power by adjusting both the voltage and the current as it passes through the regulator. There is a lot of information available on the web if you are interested in the theory of how an mppt works.
Bottom line is with an mppt we operate the panel at it's max power point and have the full 80W going into the mppt. The mppt effiency ( power transfer ratio ) is 95% so we get 95% of our 80W at the output ( 76W ). A gain of 6.4W or just over 9% increase in power.
With our power law ( P = V * I ) we have 76W = 14.5V * 5.25A available to charge our battery.
Note the increase in current, greater than both the max pwr current and the Isc.
At lower illumination levels you will get a greater increase in power output with the mppt than a switching regulator. You'll need to get some solar panel data to see this.
Most manufacturers of mppt's seem to specify an effiency gain over a std regulator over a period somewhere around 25-30%, in the fine print they suggest that ~20% is average. My feeling, not backed up by any real data, is that i'd expect 10% and any more is a bonus. ymmv
Is it worth getting a mppt instead of another panel? With the theory above, the price of a solar panel and the price of a mppt you can determine this yourself.
AnswerID:
411665
Follow Up By: Maîneÿ . . .- Monday, Apr 05, 2010 at 01:39
Monday, Apr 05, 2010 at 01:39
That is a lot of assumptions
Typical 80 Watt panel has an operating voltage of ~17.6 volts, delivers 4.55 amps
( Volts x Amps = Watts - 17.6 x 4.55 = ~80 )
To compare a MPPT reg with an on/off switching reg is like comparing today’s technology with a T model Ford.
You correctly state you "loose some power during the period that the panel isn't connected to the load" because the on/off reg is not connected some of the time, that’s why you should compare a reg with constant charging capabilities, they are not disconnected from the battery and your numbers are then irrelevant.
The MPPT versus series regulated question is not always straight forward as it very much depends on the conditions under which charging is taking place.
Be aware that the % gains claimed for MPPT are not likely to be ‘real world’ because the hotter the panels become, the maximum power point voltage de-creases and gets closer to a typical battery voltage, therefore wiping out most if not all, of any gain an MPPT unit will give.
MPPT's are most effective under COLD conditions, so when used in WARM weather you definitely don't get the claimed benefits at all.
Maybe you can gain 25% by adding a MPPT in cool conditions, that does not help you much if your MPPT is only 85% efficient because you've only gained about 5 watts, which nothing at all to get excited about, because an efficient regulator would have done that anyway.
Maîneÿ . . .
FollowupID:
681689
Follow Up By: greybeard - Monday, Apr 05, 2010 at 10:11
Monday, Apr 05, 2010 at 10:11
i disagree with the incorrect rewording and technical inaccuracies in the reply above.
All data quoted can be found in the manufacturer's source documents that are available by following the links provided in the original reply.
I'd strongly suggest that anyone with any vague interest in this subject perform their own research and reach their own conclusions.
Please note that it is possible to exclude words as well as include them when you use most search engines. for example you could search for 'mppt regulator' -waffle.
This would give you results that contained the words mppt and regulator and exclude results that include waffle.
FollowupID:
681706
Follow Up By: Maîneÿ . . .- Monday, Apr 05, 2010 at 11:37
Monday, Apr 05, 2010 at 11:37
greybeard,
If you disagree, how about you simply *copy/paste* what you believe to be the “incorrect rewording and technical inaccuracies” the bits and parts that you don't understand or agree with, and I'm positive it can be clarified just for you :-)
As you say some of the information you have posted is available from the "manufacturers source documents"... so using your very own analogy, the information available from every single snakefish oil web site is also then correct too?
That implies HyClones and the 'fuel pill' works work 100%, simply because the manufacturers web site says they do, so it must be true and correct (not)
Do you think the manufacturer will really print the fact that MPPT's are most effective under C00L conditions, that they will then in fact then loose some of their claimed benefits under H0T conditions, no, they only tell you what they want you to read that will SELL their product.
I’m not saying they don’t work at all, I’m saying you have to put up ALL the facts, not just the obvious benefits, please tell it as it is and state the negative points also.
Maîneÿ . . .
FollowupID:
681718