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Motorhome Build Thread
#11
(10-01-2020, 01:39 PM)ajw22 Wrote: Solar side 37.5v for 24V system sounds a bit too low.  Have to take into account that the panel voltage drops with temperature, and a 24V system actually has a full charge voltage of closer to 28V.  And many solar chargers need a bit more voltage difference to operate efficiently and at full power.

Which panels specifically are you thinking of using?

I have some new Dimplex 230w panels at my disposal, so, how to work out the current value for the two MPPT controllers then...

Is it 920(watts) / 75v = 12.26amps ...???
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#12
(10-01-2020, 01:39 PM)ajw22 Wrote: Solar side 37.5v for 24V system sounds a bit too low.  Have to take into account that the panel voltage drops with temperature, and a 24V system actually has a full charge voltage of closer to 28V.  And many solar chargers need a bit more voltage difference to operate efficiently and at full power.

Which panels specifically are you thinking of using?
Agree with @ajw22 here.   I'd series them as you suggest "....should I link them in series/pairs to give 75v a...."  and get a controller that can accept up to 100v.   
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#13
(10-01-2020, 01:44 PM)Mezbatt Wrote:
(10-01-2020, 01:39 PM)ajw22 Wrote: Solar side 37.5v for 24V system sounds a bit too low.  Have to take into account that the panel voltage drops with temperature, and a 24V system actually has a full charge voltage of closer to 28V.  And many solar chargers need a bit more voltage difference to operate efficiently and at full power.

Which panels specifically are you thinking of using?

I have some new Dimplex 230w panels at my disposal, so, how to work out the current value for the two MPPT controllers then...

Is it 920(watts) / 75v = 12.26amps ...???

+1 to posts above, two in series is likely to to be better here.
Your diagram looks OK. You need a suitable DC breaker at the input to each MPPT & another at each output to the battery.

You need the full spec sheet for your panels. It should show STC & NOCT values for Vmp & Imp & also Voc & Isc.
The MPPT controllers input voltage should be rated for Voc STC with a bit of margin, eg 10-15V to cover for highest voltage on cold days ie the max voltage input.
The current input & output should also be rated to cope with the Isc of the panels in and the total max power out current.
In actual use, in the middle of a clear sunny day, the panels should produce near the NOC Vmp & Imp values, less current if not facing the sun eg installed flat vs tilted, etc
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#14
(10-01-2020, 01:44 PM)Mezbatt Wrote: I have some new Dimplex 230w panels at my disposal, so, how to work out the current value for the two MPPT controllers then...

Is it 920(watts) / 75v = 12.26amps ...???

I'm guessing it's the DimplexRenewables dxpvm230p6-30 , spec sheet:
https://db.photovoltaikforum.com/PvForum...0_EN_09_10

It's a common misunderstanding of the different voltage/current specification with PV panels:
"Open circuit voltage" (Voc ; 36.65V in this case) is the maximum voltage the panel can output if _nothing_ is connected to it.  This value is just used for safety calculations, and does not occur under normal running operation.
"Maximum power operating voltage" (Vmp ; 29.0V in this case) is the voltage the panel is most efficient at.  This is approximately the voltage under normal running operation.

Same pattern with "Short circuit current" (Isc ; 8.59A) for safety calc, and "Maximum power operating current" (Imp ; 7.99A) in actual use.


So with a 2x series arrangement, the charger needs to support at _least_ 73.3V (36.65V * 2), but do add some safety buffer here.
And with 2x in parallel, the potential max current if something should short circuit is 17.18A (8.59A * 2), so the cables have to be sized for that, and the breaker sized to trip before the cables start to overheat.

Actual operation values for this 2p2s panel arrangement will be (Vmp * 2) * (Imp * 2) = (29V*2) * (7.99A*2) = 58V * 15.98A = 927W
Of course only under ideal conditions.
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Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#15
Thank you again gents, this is the stuff i need......

So layout of panels reached, one MPPT controller or two..??? Just run this by me again please the need for two..?? 

I'll get a picture of the panel spec label and post up. 

Now my issue of "do i get a combined charger/inverter" or do i buy separate units.? Any pro's & cons appreciated.....

I have been seduced by Victron (Goddess of power control) so looking to use as much of it as i can afford but open to other product references as well. 

I will draft up a diagram using all the info gathered so far ready for approval and marking...!!!!

Thanks...

here's my truck that's getting a full revamp and solar install.
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#16
(10-01-2020, 11:01 PM)ajw22 Wrote: potential max current if something should short circuit is 17.18A (8.59A * 2), so the cables have to be sized for that, and the breaker sized to trip before the cables start to overheat.

Just wanted to clarify that the breaker is not really for the protection of the cables, since the panels can't magically produce more current to melt the wires.  But it is usually a regulation requirement, and a very useful safety switch for doing maintenance work.
Normal AC breakers may fail catastrophically with DC currents, so make sure to get DC rated breakers.  And with many DC breakers, current must only flow in one direction, so make sure to check the wiring specifications.
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https://secondlifestorage.com/showthread.php?tid=6458
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#17
(10-02-2020, 06:52 AM)ajw22 Wrote:
(10-01-2020, 11:01 PM)ajw22 Wrote: potential max current if something should short circuit is 17.18A (8.59A * 2), so the cables have to be sized for that, and the breaker sized to trip before the cables start to overheat.

Just wanted to clarify that the breaker is not really for the protection of the cables, since the panels can't magically produce more current to melt the wires.  But it is usually a regulation requirement, and a very useful safety switch for doing maintenance work.
Normal AC breakers may fail catastrophically with DC currents, so make sure to get DC rated breakers.  And with many DC breakers, current must only flow in one direction, so make sure to check the wiring specifications.

Bloomin typical, about 7 years ago I slung out a huge box of breakers, S,D&Tpole AC & DC rated from when I worked as a panel wireman. Now I got to buy them...!! I'm presuming and myself unprompted would use DP breakers for the solar panels.
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#18
(10-02-2020, 06:52 AM)ajw22 Wrote: Normal AC breakers may fail catastrophically with DC currents, so make sure to get DC rated breakers.  And with many DC breakers, current must only flow in one direction, so make sure to check the wiring specifications.
+1 look for the non-polarized ones (better).
Needed for safety & really useful for isolating gear.
The Voc from 2 panels gives a good bite if you can't isolate it!

Basically you need:
Panels > DC breaker > MPPT(s) > DC breaker > battery
battery > big DC breaker > inverter > AC breaker(s) + RCD(s) > AC loads
+ battery > DC breaker(s) > other DC loads
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#19
Quick question here..... I'm planning a setup whereby the automation will allow only one bank (2S2P x2banks in total) of panels to tilt at a time to pick up a more efficient angle to the sun. They will tilt in opposite directions (one way or the other) via a clever method we have come up with but this will give rise to different charging currents feeding the batteries almost all of the time. IE: A lead bank and a trailing bank with reference to charging potential. Any thoughts or input on this idea please..????
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#20
(10-20-2020, 04:01 AM)Mezbatt Wrote: Quick question here..... I'm planning a setup whereby the automation will allow only one bank (2S2P x2banks in total) of panels to tilt at a time to pick up a more efficient angle to the sun. They will tilt in opposite directions (one way or the other) via a clever method we have come up with but this will give rise to different charging currents feeding the batteries almost all of the time. IE: A lead bank and a trailing bank with reference to charging potential. Any thoughts or input on this idea please..????

Should work fine, probably should use two separate MPPT chargers for best results.
Maybe you could run this length-ways along the van with the hinge side down the centre line?
Lift drivers side row or passengers side row up from outside edge.
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