Re: Looks like solar cells are better !!
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I didn’t see a link, but looking at Amazon, I think you’re looking at something like this?
Right up front: These are NOT 24v panels---their output is likely optimal only to charge 12v of lead acid per panel. More on that below.
The way that ad is written, it’s hard to know for sure that you actually get a solar panel at all---could be just a solar controller.
Having said that, their photos reveal it includes BOTH. And in the minimal specs they show for the panels themselves, except for the dimensions, they only indicate 24v and ~20% efficiency plus construction images validating it as a monocrystalline high efficiency solar panel.
But without V-I characteristics, you really have no idea how this would work as you intend. Let’s say you get 2 panels (and 2 charge controllers with them). First, that charge controller (if used) is a PWM controller and not MPPT. This means that it will not be useful at all to boost the voltage and will not optimize its operating point and give you the best efficiency all the time. Still, a charge controller is nice (and free with this deal). But let’s ignore it.
I’m guessing your figuring is to use 1 panel to charge 24v of lead acid. Is that correct?
Let’s say you connect the panel simply thru a fuse for charging.
Do you just expect 100watts (or 4amps) charging whenever the sun is shining? It’s not that simple. For one thing, that 100watts is the rated output, and likely for high noon at the equator. Mount your panel horizontal and even at the equator, the output will vary thru the day, maybe getting 5.5hours avg at 100watts. Now, move to 45deg latitude (Salem, Oregon) and mounted horizontally, these are now 70watt panels, max. And depending on the time of year and the weather, you might get 1 hour of son or 4hours of sun, so from Spring to Fall, you might figure 3-hours, but not 100watts---just 70, or 210w-h per-panel per-day.
Now, we haven’t at all talked about the solar panel’s V-I curve, AND the ad mentions nowhere what that curve looks like. We have no clue what the Voc, Isc are, let alone what the Vmpp, Impp are. Those are key! Now, let’s take a WAG and assume these cells are 0.45vpc (at mpp) and you have 32 of them in series---we might then expect a panel MPP of 14.4v.
But why guess? Looking for similar looking panels, here’s one that looks like it uses the same panels:
And there we see this:
And of course, the specs are for 25degC, and 1 full equator sun
Even at the equator, if you took one of these panels and hooked it up to 24v of PbA batteries, you will get ZERO charge current unless the batteries are nearly dead (21.6v). And only then will you get any charge current at all. And MPP is at 18v (@5.55a) and only pointed directly at the son (tilted near 45deg here in the NW, at noon, mid-summer)
So their add is misleading: One of these panels is sized to charge a single 12v battery bank. And that PWM controller won’t do DCDC conversion to get the panel to operate at MPP, so you may as well use it as a boat anchor. Still, it’s a decent deal if the quality is there. But what should we then expect?
For mid-summer, with horizontal mounting, at high noon and direct connected to one 12v battery that is nom. 12v, we would expect roughly 6amps * 70% (latitude adjust) = 4.2amps.
Total power from panel ~ 50watts.
And you would need 4 of these. Cost: $280 for 200watts of PEAK power, high noon, mid-summer.
So, you lose 30% due to horizontal mounting at 45deg latitude and another 20% due to an operating voltage of 12v instead of at Vmp.
Now, what could you do to improve things?
· Invest in a single nom. 48v MPPT controller
· Explore mounting options to get better sun
A MPPT controller is effectively a DCDC that is programmed to maximize charge current to the battery (respecting battery limits) by adjusting the V-I operating point of the solar panel outputs. With a MPPT controller, you could get close to recovering all of that 20% loss.
Mounting options---now this gets fun for boats, especially on sunny late Fall thru early Spring days with calm, highly reflective water. I visualize having 4 panels like these that I could attach (small bungies to the eyelets) on one side of my boat, electrically connected in series, then plugged into a socket on the side of the boat (that goes to a MPPT) controller. If I’m heading upstream (East) for hours, mount them on the starboard side vertically.
Else, mount them on the other side.
Advantage: Low sun, panels pointed directly at it AND getting nearly a 2nd sun’s energy off the water. I imagine that in this configuration on a nice late Fall, early Spring day one could effectively get close to rated power out of the panel. But maybe not.
From: email@example.com [mailto:firstname.lastname@example.org] On Behalf Of Electri-Cal
With the newer solar panels on Amazon, I see 10 amp at 24 volt in a size that fits Surprises aft deck. In fact 2 panels ( 21 by 41 in. ) will fit easy and still open as designed for access. My cruise speed uses about 10 amp on the battery, so two panels ought to do, even on slightly cloudy days. With my two 24 volt batt. packs, that could equate to running as long as I could stand to sit and cruise. The panel cost is now down to $ 70. 00 each -- which is just about what a battery costs at bi Mart. With my heavier duty fork lift # 27 size, that's about enough to fill in the gap even better, for safety's sake on the water.