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groingo
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# Posted: 15 Sep 2016 01:34pm
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Regarding PV input voltage or power coming from panels to charge controller.
Does MPPT reduce amount of voltage coming directly from panels as batteries charge?
Example: When my PWM is showing 22.50 volts from PV the MPPT shows only 18.40 volts.
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MtnDon
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# Posted: 15 Sep 2016 02:19pm
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And if you disconnect the controllers altogether the voltage increases above either of those numbers.
I think what is happening is the MPPT can make better use of the total watts coming from the panels. That can cause the apparent voltage from the PV to drop when the system is working. at extracting as many watts as possible and pushing as much as possible into the batteries. The voltage can also change by the time you switch from one to the other.
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groingo
Member
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# Posted: 15 Sep 2016 04:22pm
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Yes, that was disconnected from the battery.
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MtnDon
Member
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# Posted: 15 Sep 2016 04:38pm
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If the readings were with the PV connected to the controller but with the batteries not being connected to the controller I don't think the numbers mean anything at all. My theory if you wish to call it that, was dependent on the batteries being connected to the controller and PV.
All the charge controllers I am familiar with need to be first connected to the batteries before the PV is connected. I know with one of my Midnite they even mention that one can have problems with the PV system running and the batteries are disconnected and then reconnected. Therefore I have never connected PV without also having the batteries connected first.
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creeky
Member
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# Posted: 15 Sep 2016 07:04pm
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groingo. mppt controllers use pwm to charge. but what they also do is take excess voltage and turn it into amps to charge your battery.
while a plain pwm controller cannot use the excess voltage.
as MtnDon points out. Connect your batteries before turning on any pv to a controller. your manual should have the proper implentation order.
Also: it's good practice to never turn on PV breakers while the voltage is high. Put a blanket or some'at over your panels. Turn on the PV breakers. Go remove the blanket.
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MtnDon
Member
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# Posted: 16 Sep 2016 10:22am - Edited by: MtnDon
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Quoting: creeky Also: it's good practice to never turn on PV breakers while the voltage is high. Put a blanket or some'at over your panels. Turn on the PV breakers. Go remove the blanket.
I have to ask, why?
Assuming the breakers have been sized according to a minimum of Voc x 1.25 and that similarly their amperage is correct what is the problem you perceive?
If the sun is up, what you don't want to do is make or break a panel connection unless the final breakers or disconnects are Open. No current flow possible.
The 250 or so panel array a mile from my home was not covered up before activating the system. I was their for the event. And it was during the day at Noon.
And none of the nearly dozen roof top residential solar arrays that are in my neighborhood have had any covers used during installation and placing into service... All done by commercial installers...
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creeky
Member
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# Posted: 17 Sep 2016 05:07pm
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Well. Amazing how things progress over the years.
Back when i built my system you saw great photos of burned out dc switches where folks had turned on the pv while the voltage was high.
I'm sure. with the growth in the industry. the dc switch arc control has improved.
I have 6 year old switches. I'll stick with my blanket over the pv panels. I'm oversized by 40%.
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MtnDon
Member
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# Posted: 17 Sep 2016 06:47pm - Edited by: MtnDon
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OK. But since it was something seen on the internet there is also a reasonable chance that the parts used were not up to the voltage or the amps. Ratings are important. And we only have the word of the author of what was posted that the switches were real DC rated switches. DC is not new but it is more prolific with the increase in PV availability. And many folks do not understand how/why DC is so different than AC, and hence why the equipment is different.
I know this was switch / breaker oriented but I think is a good place for a reminder... There is also a difference between using switches/disconnects or breakers that are designed for a particular DC load and using fuses. Again there are some differences in fuses, not all AC fuses are DC rated. And you can have some truly marvelous fires occur when a DC rated fuse is pulled out of a fuse holder while under load. Talking about cartridge type fuses. Pulling one under load can create a flame arc that melts components, while possibly causing serious burns to people.
A good reason to avoid DC fuses and use Midnite Solar MN-EPV breakers.
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