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Date:         Fri, 20 Sep 2013 19:21:12 -0500
Reply-To:     tom ring <taring@TARING.ORG>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         tom ring <taring@TARING.ORG>
Organization: Tippen Ringware
Subject:      Re: solar charge controller
In-Reply-To:  <523CBE7C.30901@colorado.edu>
Content-type: text/plain; charset=US-ASCII

One danger on the MPPT controllers is that the rating is usually for the output current, not the input current. I have 10 amps worth of panels and had a 10 amp controller. Went to a VW bug and bus event in Eau Claire WI last year. I was anxious to show my new setup. Walking around the area I came back with someone interested to see it. Surprisingly there was zero charge current as the 10 amp _output_ MPPT controller had blown. Shame on me.

I now have a 2000E controller, similar to the one mentioned, which has a 25 amp output limit. It's wonderful and shows a 30%+ current boost under the right conditions. The other good thing is that it has a very smart multistage charge charge control program, as well as a cheap temperature compensator addon. The downside, if you're an amateur radio operator, is that it is somewhat RF noisy, although there are ways to get it down to a reasonable level.

MPPT controllers really are the way to go. Switched relay or simple solid state controllers run maybe 30 or 40% efficient. Switchers run 50% or so. MPPTs can run around 97%, so efficient the 2000E has no real heat sink to handle 25 amps.

Consider. You spend $400 for 10 amps of panels. You buy a switched controller. You get an effective power of 60-70 watts. Same panels on a PWM switcher would give you 100-120. On MPPT you would get over 190.

The reason is that solar panels make the most power at roughly 17.7 volts and oput batteries want 11-14. The MPPT controllers do a kind of electronic magic and deliver almost all the power while letting each end run at the voltage it likes.

The basic thing to understand is power in times efficiency is equal to power out. In a simple DC circuit power = volts times amps. Lets simplify things and change the voltages to 20 from the panels and 10 to the battery.

We have 200 watts of panel on a sunny day, so we get 10 amps at 20 volts.

20 volts times 10 amps is 200. Times 97% efficiency gives 194 watts to be delivered.

194 watts down converted is 10 volts times something. The something is 19.4 amps. A miracle? Nope, a modern switching power conversion system.

The numbers are different when you put the voltages at 17.7+- and 13.8+-, but the concept is the same.

I paid $250 for the 2000E, but would have had to spend an extra $400 or so to get the same charging capacity for the additional panels if I hadn't used the MPPT controller. And they wouldn't have fit on the poptop.

tom

On 20 Sep 2013 at 15:30, Richard A Jones wrote:

> FWIW--the manual for the Blue Sky 2000 has a good > explanation of when MPPT helps the most. It is > on page 5 of this manual: > http://www.blueskyenergyinc.com/uploads/pdf/Manual_BSE_SB2000.pdf > > Briefly, the colder the panel and the lower the battery > voltage, the more MPPT helps. For hot temps and batteries > that are not deeply discharged, MPPT might not be worth > the extra price over simple PWM. > > Of course, you can buy a 12V 10 A MPPT controller on ebay > for $12.57! > > Richard >

----------------------------------------- Tom Ring K0TAR EN34hx

Never attribute to engineers that into which politicians, lawyers, accountants, and marketeers have poked their fingers. -----------------------------------------


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