Topping up 48v system using solar panels?

[Electric Ray]

Originally Posted by socaldmax

I'm not sure I see what the problem is. It's a 350w controller, and you're only connecting 160w of panels, so...

it doesn't look to me like you're exceeding any current limit. 8A @ 57.6v = 460.8 watts max. output. So you're about 300 watts short of blowing it up. LOL You will never see 160w out of the panels, even under perfect conditions. Depending on your proximity to the equator, TOD, which month it is, you'll see the sun lower in the sky with a corresponding lower panel output.

I also agree that connecting the panels 2x2 is the best configuration. I'm not a fan of low power panels, I prefer 230w - 300w+, but if 4 x 40w will do what you need, then by all means.

Don't skimp on the cable size, and keep the runs as short as possible to minimize voltage drop (power loss.)

Initially I thought the same thing... BUT those little Genasun MPPTs are limited to 8A on the incoming (Panel) side, so 40W/17v = 2.35A per panel in parallel. the MPPT is also limited to 43 v so you cant have more than two small panels in series. That is why 2 parallel strings of two is the only option that uses all 4 panels. We get very close to the theoretical Max wattage from our 3500W array on partially cloudy days around midday (called the edge of cloud effect) so designing a system so close to the maximum capacity of the MPPT would not be best practice

[finnarne]

Originally Posted by Electric Ray

Initially I thought the same thing... BUT those little Genasun MPPTs are limited to 8A on the incoming (Panel) side, so 40W/17v = 2.35A per panel in parallel. the MPPT is also limited to 43 v so you cant have more than two small panels in series. That is why 2 parallel strings of two is the only option that uses all 4 panels. We get very close to the theoretical Max wattage from our 3500W array on partially cloudy days around midday (called the edge of cloud effect) so designing a system so close to the maximum capacity of the MPPT would not be best practice

Absolute Maximum Panel Open-Circuit Voltage (Voc) is 63 volt.
Maximum Recommended Panel Vmp (for the 48 volt version) is 43 volt

Normally I would have had the highest possible voltage and as low current as possible, as voltage drop in the cables and connections is constant, and because a normal mppt can take a higher voltage and a given current into a given voltage and a higher current.

Then you have the possible problem of shades, which could limit the throughput of panels connected in series. On a normal controller, a panel voltage of 5 volt would not give much charging, but since the Boost controller are capable of boosting the power, 3 or 4 panels in parallel giving 5 volt each, could actually charge your batteries, but I doubt that it would charge that much. I plan to test during summer to see what gives best charging, parallel or serial connection, as I have 3x30 watts panels.

For you, I would also set the pannels in a 2x2 configuration. If you want more power/larger pannels, I guess you have the choice of either more 350watts Genasun Boost controller, or a more expensive one.

Also, there is an interesting footnote on the specification tab of the Genasun page, stating

Panel ratings have increased since we designed the GVB. Although we don't believe in changing specifications without a corresponding engineering change, based on both our customers' experiences over the years as well as the headroom we designed into the GVB, we feel comfortable recommending the GVB for panels with Imp up to 9A.

[grjfield]

I appreciate the feedback. Sounds like 2 sets of 2 - 40W panels is viable and within the 'range' of the controller. I may start there and build once I get a few KW-Hr's under my belt. ...perhaps buy an addl 2 - 40W panels.. upgrading to 2 sets of 3-40W panels to eek out 34V @ 7A for ~238W.

As I mentioned in an earlier post I do have 1 - 100W panel. Would my math be correct if I paralleled 3 - 40W panels (17V @ 7A), and then paralleled 1 -40W with 1 - 100W (17V@ 8.2A)... Then series both together for 34V @ 8.2A. Or is it a bad idea to have an 8.2A in series with a 7A. Perhaps being at the 49th parallel, I wont be hitting the 8.2A in any case.

Finally I have read blocking diodes should be used when running parallel solar panels. Is that correct?

Thanks
Gerard

[Dsanduril]

For connecting panels in series or parallel the rules are pretty simple:

Series Connections

Panels should have the same current profile. The lowest current panel in the string will set the maximum current for the string. In your case your string would be limited to 7A because that is the lowest current panel. And it will be limited all the time because the 7A panel, when operating at only 3.5A will still limit the other panel that would want to be putting out 4.1A under the same conditions.

If the current mismatch is too great you can cause problems for the higher current panel, so best to get reasonably close (your 7A vs. 8.2A is probably close enough), but you will still be driving to the lowest rated panel.

Parallel Connections

Parallel connections should be made between panels of like voltage profile. Here current really doesn't matter, but especially if you are using an MPPT controller the controller will not do a good job if the voltage profiles of the panels in parallel differ by too much. Parallel configurations are a little more forgiving in this regard than series (say you have an 18V and a 17V panel, you will be operating 1V off of best point for one of those, but 1V in a 40W panel is only about 2.2W of output lost - vs 1A at 17V lost in series is 17W lost).

-----------------

In the grand scheme of things you plan to add the 100W as described will probably get you more total power than having it sitting in a closet not connected to anything, but the installation won't be 'ideal'. Do you care if you already own the panel and the option is leaving it sitting in the dark somewhere?

As to diodes, I'll leave that to someone else, don't think I agree with general consensus on that one.

[socaldmax]

For the amount of effort expended, I'd sell the 40w panels and get 3 more 100w panels. Remember, these numbers are ratings based on lab conditions, you'll be lucky to see 75% of that, especially at this time of the year.

Dsanduril did an excellent job explaining why you should have matched sets of panels for parallel or series connections. Sometimes it's hard to find panels that match the output profile of older/lower output panels.


As for blocking diodes, I believe they all have them already. I don't believe I've ever seen a solar panel that didn't have them.

[Dsanduril]

One other thing just for general information - shading changes both the current and voltage profiles of the shaded panel. This is why you should try to get similar exposure for all panels; doesn't matter if you are series or parallel connected, shade on one panel will change both its current and voltage output compared with other unshaded panels.

In parallel a shaded panel will likely stop production, as its voltage will drop by at least 25-33% (depending on the number of bypass diodes) and thus simply can't keep up with the other panels at full voltage.

In series connection voltage also drops, and all of the current from other panels in the string has to go through the bypass diode(s). This leads to hot diodes, and diode failures (newer panels less susceptible than older designs, but none are infallible).

For these reasons the industry is heading toward MPPT at each panel (with micro-inverters being the most prevalent approach) so that you can avoid the mis-matches caused by shading. One of our own (search for the threads) has been working in developing a low-cost micro MPPT controller that could be attached to the back of each panel. Since it is very rare on boats to get an entirely unshaded array, having MPPT at each panel stands to offer great benefit (if you can afford all the pieces and parts).

[grjfield]

Thanks for all the great feedback.

My worst case is to need the solar as a range extender for my electric motor where the wind has died. As long as the wind blows, I can regen as I sail.

My PLAN A is to go 'safe' initially with the 2 series set of 2 - 40W panels in parallel, giving 34V @ 4.7A. ...boosted by the Genasun to 48V for my 235AHr bank.

As noted by several, I will likely get somewatt (lol) less than expected but I can hopefully get a feel for how it pans out and get some good measurements along the way.
I'll use the 100W to charge my 12V house bank if needed.

Later on, if PLAN A is underpowered, perhaps I can buy another 2-100W panels and set up a 3 series set of 2 - 40W/100W panels in parallel, giving 51V @ 8.2A....pushing both the Volt limit of 43V (but still under the 64V max limit) and the Amp limit of 8 (or 9 as noted). Also as noted, with shade, my latitude, and the constant movement, I may be lucky to achieve even 75% efficiency.

Many thanks to all who assisted!
Gerard

PLAN A
|--40W---| |--40W--|
| |----| |
|--40W---| |--40W--|
| |
| |
_| 34V @ 4.7A |___

PLAN B
|--40W---| |--40W--| |--40W--|
| |----| |----| |
|_-100W-| |-100W--| |-100W--|
| |
| |
_| 51V @ 8.1A |___