Looking for insight regarding 250watt solar panels

Fredricksen

Member
disclaimer: Excuse the ignorance of my questions.. I am not yet well versed in solar and power integration.

I have an opportunity to get some 'residential' solar panels (250 watt / 30V) very, very cheap.

Is the thought of putting 4 of these panels on a vehicle roof worth pursing?
Or is this a dog that won't hunt??

I have been looking at this in terms of automotive/rv equipment, given that I already have a Tripp Lite inverter.
But, given the panels' output.. should I be looking at this as an off-grid install with a different type of charge controller and inverter?
Or is there a controller configuration that can be done to use these panels.. (panels in series-parallel, multiple controllers, etc)

Thoughts?
Particular equipment to look at?


The (evolving) vehicle is an ambulance with a 2000 watt Tripp Lite inverter
and currently, two flooded lead acid batteries "house batteries". (don't laugh)




 

dreadlocks

Well-known member
if you can fit all 4 of em on the roof, and are fine with that much weight up high, then go for it.. look at a Victron SmartSolar controller.. you'll size the Solar Charger up to your desired battery bank size.. which is about 0.15-0.2C, and C = battery in AH.. probably have two parallel series of two, but it would depend on voltage specs of the panels.

overbuilding solar is a good choice for boon-docking, conditions and solar conditions can be so variable its best to build for the worst and hope for the best.. with 1000W of solar on the roof you could still be outputting ~100W of power on a overcast day w/no direct sun.
 
Last edited:

Joe917

Explorer
Nothing wrong with using residential panels, the cheapest option. You will need an MPPT solar controller to make the most of the higher voltage panel, hard to beat MorningStar.
Flooded lead acid is the cheapest most robust choice for deep cycle house batteries, so no problem there!
The inverter has no bearing on the solar or charge controller.
 

Recommended books for Overlanding

Long Way Round: Chasing Shadows Across the World
by Ewan McGregor, Charley Boorman
From $14.59
Into Africa
by Sam Manicom
From $24.65
Long Way Down: An Epic Journey by Motorcycle from Scotlan...
by Ewan McGregor, Charley Boorman
From $5.69

shade

Well-known member
Nothing wrong with the idea, but the specifications and condition of the panels are important. Are they new? Do you have a link to the exact model?

One of the reasons I chose this panel was for the high mechanical load capacity.
 

john61ct

Adventurer
Such panels are great, you just want MPPT rather than PMW.

Victron SmartSolar is a great series, lots of flexibility, good value.

One 75/15 SC per panel would give maximum efficiency in a partial shading scenario, maybe $130 each.
 

Fredricksen

Member
[B]@john61ct, [/B] would the MPPT's outputs then be wired together in parallel then connect to the batteries?
(and the 30 volt panel output is not a problem?)

This is actually sounding pretty straight forward.. :)
 
So I have a solar powered workshop/cabin out on some property and use 270 watt panels. They are quite large. If your Ambulance has a nice square top roof you may be able to fit them up there.

Since they are 30v panels you will either need to run a 24 volt system (which in a vehicle setting usually isn't super practical), or get a charge controller that can handle the input and decrease the voltage. You will probably not want them in series due to how much voltage that would create and also with shading issues. Running them in parallel would likely be better for your scenario. You may honestly find that you only need two of them. 500 watts is quite a bit of power when coupled with charging from the alternator as well.
 
Last edited:

john61ct

Adventurer
would the MPPT's outputs then be wired together in parallel then connect to the batteries?
(and the 30 volt panel output is not a problem?)
As long as panel voltage is within the MPPT SC, say 10-15% headroom, yes straightforward, the first number "75, 100, 150" on the Victrons.

Yes outputs are paralleled to the bank, do not listen to malarkey about the SCs "needing" coordination.
 

dreadlocks

Well-known member
SmartSolars need to coordinate if you use their dynamic absorb for lead batteries.. the way it works is, in the morning when a charge current starts coming in.. it takes the battery voltage and uses that as a multiplier for absorb time.. a lower voltage will have a longer absorb, higher voltage a shorter absorb.. If you have multiple SC's this starts getting wonkey because they all wont bootup at exact same time, so the one that sees sun first.. starts up and changes the battery voltage, then the next one starts up and thinks its more charged than it really was and calculates a shorter absorb time, if you got more this just gets worse as the string wakes up in the morning.

Coordination between SmartSolars takes the first one that wakes up and makes it the master, and all the rest will switch between bulk/absorb/float in sync with this master controller... is this coordination completely nessicary? no, but it does allow them to all behave like one big charger, so you can harvest every drop of energy in the most efficient way and not having solar chargers cut out to float too early before the battery is full.
 

shade

Well-known member
SmartSolars need to coordinate if you use their dynamic absorb for lead batteries.. the way it works is, in the morning when a charge current starts coming in.. it takes the battery voltage and uses that as a multiplier for absorb time.. a lower voltage will have a longer absorb, higher voltage a shorter absorb.. If you have multiple SC's this starts getting wonkey because they all wont bootup at exact same time, so the one that sees sun first.. starts up and changes the battery voltage, then the next one starts up and thinks its more charged than it really was and calculates a shorter absorb time, if you got more this just gets worse as the string wakes up in the morning.

Coordination between SmartSolars takes the first one that wakes up and makes it the master, and all the rest will switch between bulk/absorb/float in sync with this master controller... is this coordination completely nessicary? no, but it does allow them to all behave like one big charger, so you can harvest every drop of energy in the most efficient way and not having solar chargers cut out to float too early before the battery is full.
I haven't been following this closely. Has Victron already implemented this, or is it due in a software update?
 

dreadlocks

Well-known member
They've implemented a work around on stable releases, the fixed absorb time.. but the syncing I think its still in beta channel AFIK.
 

Recommended books for Overlanding

Fredricksen

Member
So let me ask this..

Would it be better to use a single larger capacity mppt like a SmartSolar MPPT 150/35, with the 4 panels in series (120v @ 8 amps)?

The trade off would be the effectiveness of the array as a whole, (impact of shade, etc. )
OR.. from what I read,
when using multiple mppts, one unit becomes the high pole in the tent,
while the others prematurely switch to float, and the batteries don't get a full charge.

(I found this post on victrons Q&A
https://community.victronenergy.com/questions/8178/why-doesnt-the-documentation-warn-that-multiple-mp.html )
 

dreadlocks

Well-known member
I would do two series of 2, for 60v with two in parallel for ~17A.. you only need a 100/30, I got an open box for like $180 off amazon.

separate SC's is more advantageous when your panels are not in same location, like one fixed and one portable would be better suited by separate controllers because one panel is gonna be at like 80% output and the other at like 20%, the one at 20% would basically be cut out of the loop, so that panel would be effectively dead.. where with its own SC, it could be contributing still..

When all are butted up right next to each other in same fixed space, its a little less relevant.. and besides, your gonna have major overkill so squeaking out a few more percent wont be worthwhile.
 
Last edited:
Top