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Old 10-06-2014, 08:36 PM   #1
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Why use MPPT charge controllers

One thing I keep running into is folks who don't like MPPT. I always struggle to explain the advantages but know they are many. I ran into this article which articulates the plus side of MPPT charge controllers. It si you choice as to using them or not, currently the only draw back is cost. But with increase charge that is usable to the batteries I feel its an acceptable cost. Read and enjoy.

http://www.midnitesolar.com/articles/whyMPPT.php

Chuck
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Old 10-08-2014, 01:47 PM   #2
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Re: Why use MPPT charge controllers

If I had a big solar system, I would def. use one
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Old 10-08-2014, 02:57 PM   #3
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Re: Why use MPPT charge controllers

Yeah It does take a fairly large setup. There are some new type of controllers coming out for smaller systems limited to 30 amps max that are showing promise for Skoolies and RV's.

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Old 10-08-2014, 10:42 PM   #4
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Re: Why use MPPT charge controllers

I just commented on this in another thread, but I'll repeat myself:

The big benefit to MPPT charge controllers is that the solar panel voltage can be higher than the battery bank voltage. This can result in some serious "bang for your buck". For instance, I was able to purchase 2x 300w @ 36 volt monocrystalline panels locally for around $1 per watt. Considering where I'm located this is a very good deal. Even with the extra cost of an MPPT charge controller, it made sense to go this route.
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Old 10-18-2014, 09:48 PM   #5
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Re: Why use MPPT charge controllers

Quote:
Originally Posted by cschlessman
One thing I keep running into is folks who don't like MPPT. I always struggle to explain the advantages but know they are many. I ran into this article which articulates the plus side of MPPT charge controllers. It si you choice as to using them or not, currently the only draw back is cost. But with increase charge that is usable to the batteries I feel its an acceptable cost. Read and enjoy.

Why MPPT

Chuck
Cost is not the only disadvantage, there are also conversion losses, but they can be offset by the greater power applied when the batteries are low.

The advantages in the article in the link about higher voltages at lower current and less "IR" losses in the wire don't really apply on a Skoolie with an average of 1 to 4 PV panels. In big array with 8 to 24 panels, wiring them in series makes a big difference. A Skoolie should have a proper installation with either MPPT or direct connect: short, heavy wires from the panels to the controller to the batteries.

HandyBob preaches only drawing down a small percentage of the battery capacity, and eliminating "IR" losses in the wires from the direct-connect PV by keeping them short and oversized. This works for him. But if you actually deep-discharge your battery bank, an MPPT charger will give you more charging output.

The power output of a photovoltaic panel is rated on a curve, from maximum current when shorted out to zero voltage, to maximum voltage at no current. The maximum power point is less than maximum current and slightly less than maximum voltage.

Let's take a theoretical 180-watt panel for 12 volts. The short-circuit current might be 12 amps at zero volts, which would be flat-line consistent at lower loadings and might begin to taper off at say 14.5 volts. The open-circuit voltage might be 22 volts at zero amps. And the rated output might be 10 amps loaded down to 18 volts, for the 180-watt specification. (These are round numbers to make the math easier, not an actual panel spec.)

If the batteries are run down to "zero capacity," which is specified as 10.5 volts, the direct-connect charging amperage in full sunlight is the 12 amps, and the wattage is 126 watts. Assuming the MPPT converter has a 5% loss, the MPPT charge with the panels running at 18 volts would be 180 watts less 5%, or 171 watts, giving 16.3 amps charging current at 10.5 volts. This is a 35.7% increase in charging current to fully discharged batteries.

(If you over-discharged your batteries to 6 volts, the direct-connect still provides 12 amps but only 72 watts, the MPPT would put out about 28.5 amps.)

When the batteries come up to 11.5 volts, the direct-connect's 12 amps is now providing 138 watts of charging. The MPPT is still 171 watts, but now that provides less than 15 amps. At 12.5 volts, the 12 amps of direct connect is now 150 watts, and the advantage of MPPT has dropped to 14%. At about 14.25 volts, there is now zero MPPT advantage, as the 12 amps (which is beginning to taper off) makes the same 171 watts as our MPPT example with the 5% conversion loss. Any battery charging voltage above that and the MPPT conversion losses actually cost available charging power.

The bottom line is that large arrays with panels in series can be efficient with MPPT as per the article. With small arrays, batteries that are never deeply discharged may not see any advantage due to conversion losses. But batteries that are actually deep-cycled will see probably 30% more charging without adding more panels when using MPPT conversion. With direct-connect, the more you need a charge, the less output power you will receive from your loaded-down panels.
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Old 10-20-2014, 11:13 AM   #6
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Re: Why use MPPT charge controllers

I disagree with your large array best value. If you look at some of the smaller MPPT charge controllers you will find they are designed for small arrays. Several makes that have improved their cost per watt are Blue Sky. Midnight Solar. There is never a reason to direct connect solar panels in my opinion. The main purpose of the controller is to control the charge. While Bob has lots of good information most of his complaints are about sales folks and poor installers and I agree with him on those points. As of today I m 28 years of grid counting my original skoolie cabin and have lived all the nightmares of sales people overstating the ability of their systems. MPPT is a positive advancement in solar. As I said many times on other forums each to his own. A direct connect system is looking for trouble any way you skin that cat. While that may have been accepted in the past (and yes Im guilty) with the low cost high quality PWM charge controllers out there, there is no reason to direct connect anymore. Just my 2 cents worthevryone does as they please.

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Old 10-20-2014, 12:52 PM   #7
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Re: Why use MPPT charge controllers

Hers another article on MPPT for your enjoyment

http://www.homepower.com/articles/so...ge-controllers

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Old 10-20-2014, 09:43 PM   #8
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Re: Why use MPPT charge controllers

I don't think I said that MPPT were not good for small systems. That was not my intention. I said the advantages claimed in the Midnight Solar article as I read it, were primarily lower IR losses in low-current/high voltage series systems up to 600 volts, did not (usually) apply to a couple of panels on a bus roof.

The other advantage they claim, overcoming partial shading, well they should have known better. You would think they would have seen on a meter what one leaf covering one 0.6 volt cell in a PV panel can do to a panel's or an array's output. A chain with a broken link is a broken chain.
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Old 10-21-2014, 01:10 PM   #9
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Re: Why use MPPT charge controllers

Just misread stuff no big deal. Sorry for the gray brain. Main thing was trying to help folks understand the new MPPT controllers that are out there. With manufacturers going away from 12 and 24 volt panels MPPT maybe a big help for 12 volt systems and even some 24 volt although in skoolies that is a rare system. Lots of good info out there. Im not sure who wrote the article at midnight but they usually are fairly reliable. Those guys have been around for a long time making some awesome products.

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