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 > Inverter in repurposed generator locatoin with agm batteries

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dave17352

Ne

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Joined: 09/20/2004

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Posted: 06/28/19 07:18pm Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

With PWM you get Isc value for amps to the battery. Flat on roof will not get as many as aimed.

With MPPT it is all about watts. Let's say you have 300w aimed (series or parallel makes no diff) You lose 10% of your watts due to panel heating. So now you are at your 270 watts. Lose say 2% wiring loss from array to controller (which is pretty good) Now at 264.6 watts. Controller efficiency say 97% (it could be more like 95%) so now at 256.7 watts output.

Amps to battery is output watts/ battery voltage (assuming no wiring loss on that leg)

258/13 = 19.8
258/13.5 =19.1
258/14 = 18.4

With panels flat on roof you start with less than 300 then lose the 10% for heat and go from there.

The PWM does not lose amps from heat, in fact amps go up slightly, so you don't lose. Still get same as whatever Isc is at the time.

Only good reason to get MPPT instead of PWM for that sort of work is if you get one 300w panel instead of three 100s. Now you must have MPPT to get the buck converter in it so you can go from 24v panel to 12v battery.

And, in the above calculation where controller efficiency is mentioned, don't forget the efficiency is lower when dropping voltage, so 24-12 loses more than 12-12. That might be a good reason to stay in parallel with the three 100s and MPPT.

However, it can be wiring gauge that makes a practical difference to your installation plan. Just don't expect a bunch of amps that won't ever happen in real life.


Well we will see. The higher voltage will have less resistance. I payed 89 bucks for the mppt and I can go from series to parallel if I want to get to the nitty gritty. But knowing that I was seeing 10 amps before if I see 18 to 20 now I will call it a win! [emoticon] It is not always extremly hot when I am camping. Many times it is nice and cool which would negate the panel heating loss. All in all in will be a interesting experiment. Solar is fun that I know for sure.


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BFL13

Victoria, BC

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Posted: 06/28/19 07:41pm Link  |  Quote  |  Print  |  Notify Moderator

With 6.2 Isc, you would get 12.4 aimed at high sun. Flat on roof is less due not being aimed. How much less depends on how high the sun is anyway, where you are at that time of year.

In May at 49N with a single 130w panel (8.2 Isc) and PWM I got 8.1 aimed and 5.1 flat at 10 am, and 8.3 aimed and 7.2 flat at high noon.

If you were there then, your 12.4 would be 7.2/8.3 x 12.4 = 10.75 at high noon.

Flat vs aimed less diff going South so at 23N on 21 June sun is overhead no diff.

Don't confuse ambient cool temps with panel heating temps. The panel gets hot in the sun even when it is not that hot out ambient. Just put your hand on the panel! I took some temps with my IR gun on the back of my tilted up panel aimed at the sun at different ambients, so I have seen how that works.

Yes, solar is interesting, plus it is even useful! [emoticon]


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dave17352

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Posted: 06/28/19 08:48pm Link  |  Quote  |  Print  |  Notify Moderator

Ambient cooler and or cloudy days will still make for a cooler panel. Which will make for higher voltage. But of course if it is cloudy we won't be getting much current [emoticon]

marcsbigfoot20b27

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Posted: 06/28/19 09:31pm Link  |  Quote  |  Print  |  Notify Moderator

The best part of MPPT and series panels is that 3 100 watt panels in series will be around ~57volts.
In parallel around ~18 volts.

Now throw in major overcast skies.

MPPT maybe half panel output......let’s say 27 volts.
PWM parallel half output .... maybe 9 volts.

MPPT will at least charge some amps.
PWM at 9 volts = nothing.

Even 3 amps is better than nothing for X hours.

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